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The Chemistry, Physics and Manufacturing of Glaze Frits Section: Ceramic Tile, Subsection: General Description A detailed discussion of the oxides and their purposes, crystallization, phase separation, thermal expansion, solubility, opacity, matteness, batching, melting.
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Glassy structure of frits
A glass is an inorganic product of melting that has been cooled to a solid state without crystallization. For normal glasses and frits, solidification to the amorphous state is effected by rapid raising of the viscosity of the melt during cooling. When the viscosity is high enough, elements are forced to assume an irregular three-dimensional network. This is true even for opaque and matt frits, the rapid quenching in water freezes the structure of molten batch.
Influence of modifiers on the structure Frits cannot really be considered as glasses, rather, they are prevalently amorphous materials. In many cases the micro-matrix of the frit particles reveals that the identity of the original raw materials has not been lost, residuals of their crystal structure and relics of incompletely dissolved particles are evident (depending on melting conditions). Unlike glasses, where homogeneity is important in the melting and freezing process, for frits a homogeneous structure is of secondary importance. However, both frits and glasses have a disordered ultimate molecular structure, thus we can deduce that bonds between elemental atoms and oxygen have differing distances and strengths. Due to bonds strength differences frits and glasses donâ€™t have a definite melting point corresponding to a complete collapse of the entire structure. Rather a step-by-step breaking of bonds accompanies a gradual decrease in viscosity. The glassy structure described above suggests that chemical bonds among atoms must be partially covalent and partially ionic. This because covalent bonds have well defined angles and distances (incompatible with glassy structure) while ionic bonds are non-directional. In order to understand the behavior of each element and its role in the glassy network we need to consider the differences in electro negativity among different elements and oxygen.
ELECTRO NEGATIVITY OF ELEMENTS IN OXIDE GLASSES Group 1 Boron Silicon Phosphorus Arsenic Antimony
Group 2 2.0 1.8 2.1 2.0 1.8
Berylium Aluminum Titanium Zirconium Tin
Group 3 1.5 1.5 1.6 1.6 1.7
Magnesium Calcium Strontium Barium Lithium Sodium Potassium
1.2 1.0 1.0 0.9 1.0 0.9 0.8
Group 1: elements having higher electro-negativity, their oxides form glasses when melted alone.
Group 2: elements having oxides not able to form a glass when melted alone, but they will when melted with elements of group 1. Group 3: elements never able to form a glassy structure. While the quality of bonds and electro-negativity are good predictors of the behavior of elements in forming a glassy structure, there are other aspects not to be ignored. For instance the strength of the bonds and the number of electrons in the external shells of atoms (presenting the possibility of different coordination status) deserve consideration. In addition, oxides normally forming glasses can form crystalline structures if cooled too slowly, thus those prone to crystal formation can only be frozen to a glassy state if cooled more quickly. This kinetic aspect isnâ€™t important for the frit-making process because the melts are quenched in water, however when frits are subjected to thermal treatment as glaze materials glass bonds can break and be rebuilt to crystal phases. Crystallization reactions can partially evolve, depending on oxide ratios, to form crystals while excess components remain in as a glassy matrix. Considering electro-negativity, structural and kinetic approaches we have following table for main elements forming frits:
ELEMENTS OF GLASSES AND FRITS Glass Makers
Si, Silicon B, Boron Al, Aluminum Zr, Zirconium
Al, Aluminum Zr, Zirconium Ti, Titanium Pb, Lead
Mg, Magnesium Li, Lithium Pb, Lead Zn, Zinc
Ba, Barium Ca, Calcium Sr, Strontium Na, Sodium K, Potassium
Silicate frits Silica melts at 1710C and the glass melt is highly viscous, it is too refractory to be melted in a normal furnace. When we adjust the composition of a frit we are, in effect, modifying this hypothetic molten silica original glass by adding different elements in order to obtain suitable characteristics. Glassy silica is comprised of a network of tetrahedron SiO44- units and when we add modifiers or intermediate elements the continuous threedimensional network is broken (and weakened) and the tetrahedrons connect to form different structures like chains, rings layers, etc. The modifications we make to the original hypothetical silica glass structure (by adding different glass making, intermediate and modifier elements) enables us to control, among other things, the melting temperature and viscosity. Of course, frits and glazes must be available in a wide range of fusibilities in order to be used with different firing cycles. However, within each range we need to build glass bond networks having a range strengths in order to minimize variations of viscosity with temperature.
When we add intermediate or glass forming elements we have a more complicated evolution of glass (more detail later). The ionic field force F gives us an idea about how different ions participate in the formation of a glass network: F = z/r2 Where z is the oxidation status of ion and r is its ionic radius. For oxide glasses F is proportional to coordination power that each element exerts towards surrounding elements. The ionic field forces F of the main frit elements are listed below:
Cations having high charges and small radiuses (and thus high field forces like boron and silicon), are network formers. Network modifiers have small charges, large radiuses and a big coordination number for oxygen ions. While considering ions as rigid spheres is an over-simplified way to describe reality, it has still proven useful to describe characteristics of each. For instance lithium has a ionic radius smaller than sodium and so it can locate into smaller cavities. The ionic field force of lithium is also stronger than sodium and it is essentially non-directional, thus it more easily produces crystals of a separate phase. Alkaline earth elements locate into cavities of the network as well, but they have double charges and thus act like a bridges between two oxygen ions (preventing the three-dimensional network from being fully destroyed). Moreover bonds between alkaline earth ions and oxygen are stronger than alkaline so we observe neither a rapid decrease in viscosity or a significant increase of the thermal expansion coefficient. It is notable that for similar molar percentages, frits containing magnesium crystallize more easily than frits containing calcium. Aluminum, titanium and zirconium are classified as intermediate glass formers because they have a strong four-way coordination for oxygen ions, like silicon. Thus, for these oxides, we do not observe any interruption of the three-dimensional silicate-based network. For a better understanding consider more details about aluminum, boron, zirconium and titanium. Aluminum: Usually aluminum shows a four-way coordination when it acts as a glass former, tetrahedrons are linked to four oxygen atoms while the local excess of negative charge is counterbalanced by an alkaline cation placed close to the aluminum ion. Thus additions of aluminum to a glass help to stop alkaline ions from breaking the three dimensional network of the glass. This produces the characteristic lower melt fluidity and tendency to crystallize and also reduces the thermal expansion and thermal stability. One downside to alumina is that it contributes to a higher viscosity of frit batches during melting (in the furnace tank) making homogenization more difficult. Usually the percentage of aluminum oxide is in the range 4 â€“ 12%. Boron: Boron is a basic component of frits yet its characteristics are so peculiar that it cannot easilly be compared to other elements. Boron, like aluminum, exhibits a four-way coordination when forming a glass network (being in the center of a tetrahedron of oxygen ions). This is possible only when the molar alkaline percentage is less than 30-40% because above this limit boron has three-way coordination, forming triangles. Another peculiar characteristic is that boron is not just dispersed as tetrahedrons or triangles in the network of silica tetrahedrons. Rather it forms boric groups, containing from 3 to 5 boron atoms and the groups are randomly dispersed in the glassy matrix. However single BO3 triangles and BO4 tetrahedrons are always present. For quenched frits the presence of these groups is likely minimal but we can presume they form again when glazes containing the frit are fired (there are experimental evidences demonstrating this). Boron oxide is an important component of low melting frits because it increases fusibility without a proportional increase in thermal expansion. Moreover boron oxide and sodium borate, due to their low melting point, are useful during smelting of frits because they form a glassy matrix early and act as catalysts in the melting and dissolving of other materials. Zirconium - Titanium: Their influence on surrounding oxygen ions is very strong and scarcely directional so their solubility in frits is poor. Their solubility in glass and actions as glass formers are proportional to temperature. In quenched frits they remain in the dissolved in the glassy matrix but when we fire them again (within a glaze), these oxides easily precipitate crystal compounds.
Crystallization It is the process leading to the formation of crystals from the disordered structure of a molten glass. As already discussed, we know that during the firing of a glaze the frit (or mixture of frits) can develop crystals like zircon, willemite or sphene giving opacity or characteristic surfaces. Generally speaking we can observe a crystallization when a liquid glass is saturated by an element forming a stable crystal. We can visualize the firing process of a frit like this: Starting from room temperature the milled frit reaches a temperature at which ions acquire sufficient mobility to form crystals. These grow until the temperature is so high that the forming liquid glass becomes like a solvent and subsequently dissolves the crystals. However, during cooling we observe an opposite process, from a fixed temperature crystals appear again and grow until the melt viscosity is so high that ion mobility is zero. Of course the quantity and dimensions of crystals depends on the cooling cycle of the kiln and on the particle size distribution of frit grains (because nucleation of crystals starts from the surface of grains). In other words,
surface aesthetic effects depend on firing and milling.
PHASES CRYSTALLIZING FROM FRITS Crystals listed here precipitate from frits if the concentration of constituent elements is high enough and the firing conditions are sympathetic.
PHASES CRYSTALLIZING FROM FRITS ANORTITE
2CaO• Al2O3• SiO2
CaO• TiO2• SiO2
Li2O• Al2O3• 4SiO2
CaO• MgO• 2SiO2
K2O •Al2O3 •4SiO2
There are oxides and compounds that act as nucleating agents. For example, titanium oxide, tungsten oxide, tin oxide and calcium fluoride promote nucleus formation and enhance crystal development. When we speak about frits and glazes we have to consider that a glaze, even when composed of only one frit, is in contact with a ceramic body. This proximity implies that the glaze composition will change because there is a migration of elements from body to glaze and vice versa. This phenomena, could for example, be responsible for a matt glaze that fires fully transparent or slightly opalescent.
Phase separation This the process leading to the formation of two or more glassy phases from a single glassy phase. Phase separation arises from tendency of some liquid glasses to separate and form two liquid different phases which are more stable than a single one (due to bond energies and geometric configurations of elements). Often, for example, we see transparent glazes become opalescent with an accompanying bluish shade (during firing the frit melt splits to form two different liquids having different compositions and different refractive indexes, the appearance of opacity is a result of small drops of one of them acting as diffusion centers for light. The effects of phase separation on the chemical and physical properties of glazes are not to be ignored, they can effect significant changes in thermal expansion, viscosity, and chemical resistance. Phase separation is an important characteristic of several glasses (for instance, the Vycor process is based on a phase separation to obtain pure silica glasses). In the case of ceramic frits we observe phase separation when they contain a large amounts of boron oxide and small amounts of alumina.
Phase Separation of a Glass
Types of Frits The starting point from which to derive the composition of a frit suitable for our purposes is hypothetical silica glass.
Main criteria for normal frits are: Negligible solubility Thermal expansion lower than ceramic the body Fusibility suitable for firing cycle of corresponding glaze Wide soaking range Lowest possible cost of raw materials. Consider the following theoretical process to create a transparent frit from silica: 1. Add sodium and potassium oxides to enhance fusibility and lithium oxide, more expensive, just in case it is needed for special effects. For the same purpose we can also add small amounts of calcium, barium and zinc oxides keeping percentages under certain limits to avoid crystallization. 2. At this point the original silica glass is more fusible but the resulting frit is similar to a window or bottle glass, perhaps it has some solubility and its viscosity changes a lot for small changes in temperature. 3. We can limit the use of alkaline and alkaline earth oxides and adjust fusibility using boron oxide (it is a network former and we can keep solubility negligible using it). 4. To reduce changes in melt viscosity with temperature and improve durability we add aluminum oxide (an added benefit is that aluminum oxide is sourced from low cost feldspars which can also supply alkaline oxides).
Transparent Frits So called “high temperature frits” are widely used for single firing and traditional frits are used for engobes or special glazes. I invented high temperature frits in 1982 based on two simple observations: 1. Calcium and zinc matt glazes are defect free in the single firing process, even when fired at high temperatures 2. A mixture 50/50 of these isn’t matt because the concentration of crystallizing elements is not enough to ensure crystal development during firing 3. By increasing the alumina percentage we also avoid crystallization and the resulting frit is also more stable for higher temperatures. The incentive for the initial research came from the necessity to prepare glossy glazes for single fired wall tiles, the so called “monoporosa” technology. In the process the frits crystallize on the surface forming a gas permeable layer of sintered grains (that survives to temperatures above 1000C). Under this the oxidation of organic matter in the body, the expulsion of crystallization water and the liberation of CO2 from carbonates can evolve without producing bubbles in glaze layer. Subsequent to this the glossy frits change from a solid form to a semi-solid one and then liquid with low viscosity within a narrow range of temperatures (for traditional frits viscosity changes take place over a broad range of temperature). These frits contain high percentages of calcium, magnesium, barium and zinc oxides so during pre-heating they also crystallize as alkaline earth compounds with silica and alumina. The cooling step in roller kilns is so fast that crystals cannot form again, thus the glazes appear glossy after firing. Consider following glossy frit
BaO CaO MgO ZnO K2O
4 14 2 5 2.5
The percentage of zinc oxide is kept as low as possible for cost reasons. The calcium and magnesium oxides contribute to formation of crystals
during pre-heating. Barium oxide enhances thermal expansion and brightness. The boron oxide percentage is small to keep the melting point high. The alumina keeps viscosity high at soaking temperature to avoid bubbling caused by over-firing and to avoid liquid phase separation.
Opaque Frits Zirconium silicate is the best opacifier for frits in terms of cost and properties. It also has glass forming abilities as an intermediate element, it is a network former with coordination 4. Zirconium can remain in the glass network when the frit is quenched, but this status is not stable. During firing of the glaze the coordination changes to 6 as a modifier and afterwards to 7 as an oxide (which crystallizes). The first step towards opacity is precipitation of crystals of zirconium oxide and second by crystals of zirconium silicate, both are present in opaque glazes contributing to opacity. However a large amount of zircon remains in the glassy matrix, perhaps more than 50% and opacity is directly proportional to amount of zircon in the batch, usually more than 8%.
Matt Frits Once quenched in water frits are amorphous and transparent but sometimes, during firing of the host glaze, we obtain a matt surface texture. We observe a crystallization process leading to the formation of one or more crystal phases as a consequence of thermal treatment. We obtain a matt frit when the composition is suitable but the quantity, shape and dimensions of the crystals depend on the thermal treatment of the glaze. Recently it has been customary to describe as ceramic-glass some glazes or grains of frit that have been strongly crystallized after firing. But, it is also evident that traditional matt glazes can be considered as ceramic-glasses. The biggest difference between traditional and more recent products is composition (recent products borrow their composition from true ceramic-glasses which are densely crystallized). In order to overcome problems of simple calcium or zinc matt frits (high expansion for calcium – cost and poor acid resistance for zinc) we prefer frits crystallizing different a phase (like anortite, spodumene, zircon or wollastonite). Matt Frit Example
CaO MgO ZnO K2O
37 1 4.6 0.2
During firing the above frit crystallizes wollastonite and its thermal expansion is about 76 x 10-7 C-1.
Frit Production Fritting is the process to produce a non-soluble and glassy product, starting from a batch composed of natural raw materials and chemicals, and quenching the melt in water in order to produce a crumbled and brittle glass. The process is triggered by low melting compounds (like boric acid, borax, alkaline carbonates, feldspars) and subsequently accelerated by them to melting and dissolve the more refractory materials like quartz sand, zircon flour, aluminium oxide, etc. Frits play an essential role in obtaining glassy glazes at normal firing temperature of ceramic tiles. Usually we describe frits by oxide composition, like glasses. But this method is not fully correct because a glass is absolutely homogeneous while a frit can be non-homogeneous (and thus characteristics can change depending on homogeneity status). For example, we can easily imagine a frit containing some undissolved quartz as being more fusible than a similar one that is completely homogeneous. Frits usually have a higher viscosity than glasses and so it is not as easy get a completely amorphous and homogeneous product (even at maximum running temperatures of tank furnaces and melting times). Materials like zircon or silica sand slowly dissolve and often we have relics of these materials in the quenched frit. During firing of the host glaze we can observe crystallization or liquid phase separation and the extension of these reactions strictly depends on the homogeneity of the frit itself. However we use oxide composition to describe a frit because it is a fast method and it also enables us to calculate characteristics like thermal expansion, surface tension, molar composition and Seger formula. However, to give a complete picture of frit it is a habit provide characteristic temperatures (using a heating microscope apparatus) and measured thermal expansion (or a calculated one).
Behavior of specific materials during the melting process Alkali carbonates: They melt at respective melting points and quickly enter the glassy matrix lowering the viscosity. Barium carbonate/Calcium carbonate/Dolomite: They loose carbon dioxide at respective temperatures and quickly react entering the glassy matrix. Boric acid: Above 180C it loses crystallization water and oxide B2O3 melts at 300C. Due to its low melting point boric oxide is the first material forming a liquid phase in tank furnace. Borax: Above 120C it loses crystallization water and it melts at 740C. Colemanite: It is a natural raw material having formula 2CaO •3B2O3 •5H2O and a content of boron oxide in the range 36–42%. It has a violent reaction during water elimination at 400C and melts around 1100C. Feldspar: At normal running temperatures of tank furnaces they quickly melt forming a viscous glass. Silica sand: It slowly dissolves in the melt when its viscosity decreases enough to enhance migration speed of silicon ions through the glassy network. Usually we have a good speed when temperature is 200–300C above the melting temperature of the frit (viscosity is low and migration speed of ions is good enough).
Zirconium silicate: We use milled zirconium silicate having average size of grains 25 microns (grains of original zircon sand are too big for a rapid dissolving of material in the liquid frit). Zircon starts to melt at 1380C forming a liquid phase and zirconium oxide, but its solubility in the liquid phase is poor and dissolving time quite long. Often it settles to the bottom of tank furnaces forming a deposit of zirconium silicate and zirconium oxide.
Batching and melting Batching is fully automatic: each material is extracted from its silo and weighed by a hopper, where all components are added. The batch is transferred to a mixer and then to a pneumatic conveyor transferring it to the silo feeding the tank furnace. Of course the procedure is controlled by a computer where all compositions are stored (it also records number of prepared batches for each tank furnace). The batch is transferred to the tank furnace by a cochlea or piston according to a fixed speed. This speed regulates the melting or staying time of the frit in tank furnace. Pressure and temperature in the tank furnace are controlled. The depth of the molten frit depends on tank furnace design. Frit is expelled from the exit hole and falls into the water.
Quality Control The main control is made by preparing and firing a tile glazed with the new frit and comparing it to one made with same thickness of the standard material. In case the frit is opaque we often add to each glaze a fixed amount of cobalt oxide in order to compare opacity. A further important control is executed by preparing a small cylinder of powdered frit, as shown in the picture, and firing it. We can then compare fusibility and opacity with a similar cylinder prepared from the standard frit. If more tight control is needed we can compare the characteristic temperatures derived from a heating microscope with similar data from the standard frit.
By Nilo Tozzi
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Glaze Slurry is Difficult to Use We often tend to put so much effort into adjusting our glazes to fine-tune fired properties that we tolerate poor application properties. Such glazes are not only frustrating to use, but they often produce poor fired results. When a slurry is right it should 'gel' and 'hang on'. You should be able to dip your finger in and pull it out with an even coverage andno drips. In fact, a thixotropic glaze will resist shedding off ware covered with wax emulsion! And it will not settle out hard in the container! Yes, there is no reason to put up with a glaze that drips and drips, cracks on drying, dusts and does not produce an even layer (yes, these problems are all related). In industry, maintaining the 'rheology' (flow properties) of the glaze slurry through material, water quality, seasonal, and personnel changes is often the most difficult challenge a factory faces. Maintenance of thespecific gravity in particular is a reference point, 'an anchor' around which all other adjustments hinge (if your glaze is working well, measure its specific gravity now). If your glazes application or drying properties are often problematic or they are always difficult to work with, read on, recipe change(s) might be most appropriate. Be careful about using glaze additives, try all the other approaches first.
Gel Your Glaze, Adjust its Recipe to Gel The glaze slurry must bethixotropic, it must "gel" so that the mechanism of its initial adherence to the ware is, to a considerable extent, a function of this property rather than absorption of water by porous bisque. While thixotropic behavior can be achieved by using glaze additives, most people lack the experience, knowledge, equipment and circumstances to use them properly. It is thus desirable to avoid additives if possible and try to select a kaolin or ball clay that contributes thixotropic properties. EPK (kaolin) is a good example. If your glaze does not contain adequate kaolin (15-20%) then use ceramic calculations (e.g. Digitalfire INSIGHT) to adjust it so that it does. How is this possible? Because ceramic chemistry sees materials as 'oxide contributors' and it is thus possible to supply a specific chemistry from different mixtures of materials. It is possible to have 20% kaolin in cone 04 glazes if you use low alumina boron frits. At higher temperatures glazes have significantly more Al2O3 and SiO2 and so it is usually easy to achieve a 25% kaolin content (because it contributes Al2O3 and SiO2). However there are many high temperature glazes that have large percentages of feldspar, sometimes 70%! In such the feldspar is supplying all of the needed Al2O3 and so there is little room for clay in the recipe. These glazes are evil and there is no need for this. The simplest way to fix this problem is use ceramic calculations to reduce the feldspar and supply the alkali oxides from other sources. This will enable you to increase the kaolin to supply the lost Al2O3 (from the feldspar reduction). Ferro Frit 3110 is a good example of a frit that is very similar to a feldspar in chemistry, but it has very low alumina.
Consider What Materials You Are Using Different clays produce slurries of differing properties. Bentonite-like materials have the ability to gel in water in small amounts, they will help suspend the other particles better than any other material. However bentonites gel the water and hold onto it so well that using any more than 5% will cause glazes dry too slowly and shrink too much. Ball clay is better, 20% of it in a recipe can produce a nice slurry, and many people prefer its characteristics. However ball clay glazes do not necessarily gel well (and ball clay introduces more iron than you might want). If you have a kaolin that suspends well, it is the ideal material. In North American, EPK, for example, produces very nice slurries that suspend well and gel to help them hold on immediately after the dip. Experiment with the kaolins and ball clays available to you to find the best one. Some materials are soluble or partially soluble, this is even the case with some frits (which are of course not intended for glazes). When materials dissolve in the glaze they introduce electrolytes into the water which in turn can affect the viscosity of the glaze. For example, high nepheline syenite glazes can thicken over time and each time you add water to re thin the glaze shrinks and cracks more during drying on the ware. High boron materials are often soluble.
Clays, especially raw and native clays, often contain soluble sulfates that can dramatically affect the slurry. These problems can be insidious because these materials often dissolve slowly overtime and thus the rheology will change accordingly. Admittedly, companies with a continuous production line can use slightly soluble materials since their glaze is used quickly and is not stored.
Dense Bisque Ware Bisque ware should not be too high in porosity. Variable porosity means variable thicknesses in the overlying glaze. Porous bisque ware demands that glaze slurries be thin and runny or the application will be too thick. If you are used to bisque firing from cone 010 to 06, go to 04 or higher if you can and use a more gelled glaze.
Use Additives Only if Absolutely Necessary Misuse of glaze additives is very common because they are not nearly as well understood as other materials. Often they are listed in recipes in which they are not really needed. They should be avoided if possible, because they often have detrimental side effects. Remember, although you might think your glaze needs them, does it really? Only all-fritted glazes with very low kaolin normally need additives in typical traditional ceramic applications (an exception is crystalline glazes that require a low alumina content). Many people use additives that actually worsen the application properties of their glazes. In these cases, often a recipe adjustment to increase clay content (by sourcing the same chemistry from a different set of materials) or a simple bentonite addition would be much better (i.e. gum additions may give a thinner applying, slower drying, 'drippy' glaze). Again, do not use an additive if it is not needed, additives are not a substitute for a good glaze recipe. Individual additives often defy easy classification because they claim to impart suspending, adhesive and flow properties. Thus picking the right one is a matter of discerning the need and using the additive that 'emphasizes' the needed slurry property and gives the fewest side effects (i.e. color change, slow drying rate, biodegradation, film formation). I might add that it is also common to use much more of an additive than is needed, normally completely ruining glaze slurry properties. While gum does form a gel to suspend particles, it is usually more useful in making the slurry 'sticky', and acts as a temporary glue to cement otherwise loosely adhered particles; thus it is referred to as a 'binder', 'hardener', 'adhesive'. Remember that the mechanism of glaze adherence is normally simply contact, it 'hangs on' to irregularities in the surface by virtue of its own strength. Thus a harder dry glaze layer will adhere better. Note also that clays can impart both dry hardness plus suspension and gelling properties to the slurry, whereas gums usually only harden it. Starches usually act as hardeners and may thicken the slurry (therefore suspending it better). Cellulose ethers are used like gum and starch to harden and thicken, they are said to be more consistent and easier to control. Claylike plasticizers (like Veegum) can impart similar claylike properties to a slurry, but remember that the beneficial properties of kaolin, for example, come largely from having alot of it in the recipe. Bentonite clay, likewise, can be beneficial but only in amounts small enough that prevent it from slowing down the drying significantly. So generally clay-like additives have these same limitations. Other additives include wetting agents, foam control agents and sealers. People who know how to use these materials can do things with glaze that others might think impossible. Likewise, those of us who do not know how to use them can create a real mess. Manufacturers usually have instructions so do not buy these materials without good instructions.
Flocculants, Deflocculants Electrolytes change the pH of the suspension and affect the charge of particles (this changing slurry viscosity); a few drops can make a thick slurry very runny and thin (deflocculating it), or make a thin one gel (flocculating it). Thus deflocculants/deflocculants can be used to adjust otherwise variable flow properties. But this cannot be done by a novice. The amounts required are generally extremely small and must be tuned to the specific batch by careful measurements (a few drops too much can literally turn your glaze into jelly or make it settle like a rock). It is amazing how much a small amount of a flocculant, such as calcium chloride, epsom salts, or
vinegar can gel a glaze (so it makes sense to test on a small amount before adding it to a whole batch). At the risk of being repetitive, please consider: if you need to use these materials is it possible that adjusting the recipe to increase the clay or remove soluble materials (e.g. boric acid, nepheline syenite, lithium carbonate) would be a better approach. One real downside of these materials is they can put a glaze batch on a roller coaster viscosity ride, even with powerful mixing equipment to try stabilize their action. Do you really want that?
Mixing If you are storing your glaze slurries it is very beneficial to have a mixer that can put alot of energy into the slurry to thoroughly wet the surfaces of all particles during primary mixing. After this, final adjustments with water content and possible additives can be done to establish the final rheological properties. When this is done the glaze slurry will be more stable for a longer period of time.
Conclusion My general advice is this: If your glaze is not suspending, hardening, gelling or applying properly, then, if possible reformulate it to have more clay, especially kaolin. If it still needs help then add bentonite (up to 3%). If the glaze still needs extra help, then use an additive, but beware. If there are still problems, then, heaven forbid, use a flocculant or deflocculant or study up on other more exotic additives!
Understanding the Terra Cotta Slip Casting Recipes In North America Section: Clay Bodies, Subsection: Formulation Description This article helps you understand a good recipe for a red casting body so that you will have control and adjustability.
Article In North America there is a raw material that makes the creation of a low fire red casting body very easy. It is called Redart. This clay is air floated, high in iron, low in soluble salts and matures at a low temperature (around cone 1). In addition Redart deflocculates very well. Adding iron oxide to a clay causes it to gel badly on dispersion however the natural form of iron in Redart does not do this at all. To use the casting process efficiently you must understand how deflocculation works. Believe me, if you are new to casting and have never mixed a heavy casting slip with low water content you will never go back to a simple clay water mix again after you learn the deflocculation process (see the link to an article on this). Although it is possible, there are a number reasons why you would not normally use a 100% Redart formula. Redart plasticity and dry strength are quite low so added ball clay is needed If you need to fit commercial glazes you need to increase the thermal expansion of Redart, this is done by adding talc Using a body of one ingredient only is asking for trouble. Materials do vary and it is best to have a body that is a mix so that variation in any one material is diluted by the others. There are many advantages to having some sort of non-plastic filler in the body, especially for casting since it speeds up the casting rate and improves drying performance.
Red Casting Powder Starting Recipe Redart
Recipe Notes IMCO makes a red low fire clay named Banta that is much more plastic, you could restore some color by using it in place of the ball clay. This is one time where you actually want to use a dirty talc and ball clay, that is, ones that contain some iron. Since the Redart has been cut to 50% of the recipe it stands to reason that the body will only fire half as red. However keep in mind that the iron color does not really begin to develop into a bright red until cone 03-02, so if you are firing lower than this the color is not as much lighter as you might think it would be. The talc does improve the casting performance and working properties of the body so it is a desirable ingredient. You can adapt this body to higher temperatures by changing the talc to feldspar and reducing the amount of Redart in favor of kaolin and silica. If you can find a non-plastic finely ground red burning filler (i.e. a shale, argilitic rock, etc), this could be the secret to the reddest possible color. Note that there are red burning ball clays on the market also. If you find that a white burning scum deposits on the surface after drying you may need to add about 0.3% barium carbonate to the dry mix, it will precipitate the soluble salts inside the body so
they do not come to the surface during drying.
Terra Cotta Process Considerations At cone 06 almost any mix not containing expensive fluxes is going to be non-vitreous and highly porous. However by 04 this figure drops dramatically. By cone 03-02 it is possible to have stoneware properties in most terra cottas. This is because natural red clays contain so much flux. Consider some of the considerations you must think about when formulating a terra cotta casting body. Body color is usually important, people want the deepest possible shade of red in their terra cotta. If you are using commercial glazes fit is a big issue and the body needs to have significant talc, up to 50%. This is not a problem for white bodies since you can couple it with white burning plastic ball clay. But for red bodies it is a challenge because there is not enough room in the recipe for flux given the amount of clay needed to achieve workability (the lower plasticity red plus the ball clay). This means you have a tug of war between color, fit of commercial glazes, maturity and workability. An obvious way to ease things is to formulate your own glazes, this enables removing the talc completely. As mentioned, you cannot just add iron to a white burning clay to get terra cotta color since iron causes slip to severely gel. In my experience you need to use at least 50% dark red clay and deal with the opposing issues of maturity and working properties in the remainder. In North America the primary red clay used in terra cotta bodies is Redart. It casts very well but it is just not plastic enough so you may need to augment it with ball clay to create a slip that has adequate dry strength and shrinkage to pull away from the mold. However to get a dark enough color you normally want a high proportion of red clay. If you can fit a glaze to the talcless version of the body, you may be able to work with a recipe of up to 80% red clay and 20% ball clay. However it is not typical to fire earthenware this high since color and resistance to warp change dramatically with only slight over firing and the red color is lost under transparent glazes. The beautiful red colors depend on stopping well short of vitrification. Many commercial wares are 6-8% porosity and yet they are very strong, so do not worry about higher-than-stoneware porosity that is common with earthenware. Just make sure the glaze fits (can survive a hot water:ice water test) and strengthens the ware. In terra cotta bodies the glaze is not nearly as well interfaced and adhered to the body as with stonewares. Glazes are dramatically more prone to crazing, shivering and chipping off. To make good quality earthenware requires much more technical glaze knowledge than for making stoneware.
Combining Casting a Thrown or Hand-build Elements This is a very practical aspect of terra cotta. Some commercial terra cotta throwing bodies will deflocculate. However you will find that elements cast from the recipe given here can be joined onto leather hard objects using the casting slip itself as a glue. You may need to adjust the recipe to match color it ware is not to be glazed.
Deflocculation Deflocculation refers to the magic process of creating a slurry with only half the amount of water that would normally be necessary (see the link to an article on this topic). Generally you measure the water and put it into a container, start your propeller mixer and add most but not all of the deflocculant. Then you add the powder body mix. The slurry becomes very heavy so you need a powerful mixer than can run for hours. Getting the last bit of powder to mix in can take some time so patience is required.
Powder Mix 100 Water Soda Ash Sodium Silicate or
36.0% of dry amt 0.05-.1% of dry amt about 0.2 to 0.3% of dry amt
36% of dry amt
0.5% of dry amt
Note that water and deflocculant amounts are examples, you need to know how to measure specific gravity and viscosity to adapt their amounts to your materials and water (see the article on deflocculation for more information).
Being Realistic Terra cotta ware is never going to be anywhere near as strong as stoneware. Terra cotta is a matter of tradition and getting a nice warm red colored body. Terra cotta is much stronger than white burning earthenware and aesthetically it goes well with brightly decorated light colored opacified glazes. If you can make fine quality earthenware then you can be proud, it is not an easy job.
Low Fire White Talc Casting Body Recipe Section: Clay Bodies, Subsection: Formulation Description The classic white ball clay talc casting and modelling recipe has been used for many years. It is a dream to use as long as you are aware of the problems and risks.
Article When you buy a low temperature white burning body from a ceramic supplier you are buying what the industry calls a 'talc body'. Talc bodies are basically 50% ball clay and 50% talc (variations are discussed below). If your supplier says the recipe is a secret, let it pass, everyone is entitled to their beliefs. On the other hand, some suppliers have gone through a lot of effort to choose the best ball clay and talc available in their area and they might be putting in a small amount of other things. The ball clay in this body gives it great dry strength for handling and the talc imparts the good casting, drying and glaze fit properties (more on that in a minute). The use of this type of body started in the hobby casting industry and it became a standard on which the prepared glaze industry could rely. They created all of their products to fit this type of body. This standardization was a big reason for the success of what the commercial ceramic manufacturing industry would regard as an unorthodox and problematic recipe. However hobby cast ware has even thicknesses and is fired relatively slowly in top loading kilns so the body does not suffer process related failures that large industrial users would encounter (firing cracks, glaze crazing). Almost anyone can cast it successfully and very difficult shapes can be made because of its high wet and drying strength. It has both a fast casting rate and good strength (qualities not normally found together). Talc bodies are also very stable when fired to cone 05-06, they do not warp, even on thin pieces. So again, hobbyists with no knowledge of ceramic manufacture can make complex overhung shapes and never even think about warping issues. Another factor is that talc bodies often fire amazingly white compared to stonewares and thus bright colored glazes work well on them. Manufacturers of modeling, throwing and sculpture bodies figured out that this 50:50 talc:ball clay recipe could be adapted to plastic forming by simply adding a little bentonite to improve plasticity. Such bodies have the best throwing properties, for example, of any clay available. The beauty of this adaptation is that the whole world of prepared glazes is then available to potters and sculptors.
The Down Side There has to be some downside to this. There is. Firing and fired properties had to be compromised to get the easy working properties. Firing is done at cone 06, that is a very low temperature, there is no getting around it. Pieces are weak, you can rip them apart with your bare hands easily. They are completely unsuitable for functional uses. You can fire higher to get more strength but you will not find commercial glazes to fit. And if you fire too high (e.g. cone 02) the ware can become brittle with some ball clay:talc combinations (others can survive to cone 6 amazingly). Talc bodies fire white simply because there is no glass development taking place, no 'glue' is forming to cement the matrix together (that is why pieces are weak). However if you fire a talc body higher the color darkens dramatically because glass development brings out the color of the iron. These bodies have 50% ball clay, there is no getting around it. Ball clay by itself will shiver almost any glaze on the planet. Ball clay contains lots of free quartz, each quartz particle 'throws tantrums' when heated and cooled through 1060F (sudden expansion and contraction of up to 5%). If you have enough of these particles they impose their character on the whole matrix. In addition, micro-cracks radiate outward from each grain of quartz and the degree of cracking varies geometrically with grain size. Micro-cracks become mega cracks if firing is too quick or pieces have too much variation in thickness.
Glaze fit: Glaze is just not 'glued on' very well. Therefore crazing and shivering will occur with much smaller differences in body and glaze thermal expansion than in stoneware where there is a highly develop clay/glaze interface. Food Safety: Low fire glazes are much more soluble and prone to leaching and they are not nearly as hard a strong. But they look good, right! Porosity: Low fire bodies are porous, up to 15% air space! Than means they will soak up water like a sponge. If a piece stays wet it can breed bacteria. If the water gets between two glazed walls it can explode a piece when it turns to steam during heating. So, low fire talc bodies are a bit of a king-with-no-clothes situation (complete with a crowd of people so absorbed with the whiteness that they overlook the problems with the whole process). These bodies pretend to be ceramic with fancy glossy glazes but underneath they are porous and weak and there unceramic.
The Materials Talc is used in low fire hobby bodies for several reasons: It increases the thermal expansion. Without the talc it is very difficult to create glazes with a low enough thermal expansion to prevent crazing on typical clay bodies made from clay, feldspar and quartz. So adding talc was a no-brainer. You must realize however that this creates possible problems. If the body expands more on heatup (and therefore contracts more on cool down) then you are going to have more firing cracks if there are gradients in the ware (variations in thickness that cause variations in temperature across the cross section during heat up/cool down). Talc is white and therefore a good major ingredient in a white burning body. I should correct that. Some talcs are white and some fire very yellowish or even brownish. Therefore, if you are formulating your own body, try many different talcs to find the whitest one. However take into consideration that the whitest one might not be the most consistent one. A good policy is to use two or more talcs in a recipe to dilute the effects of changes in one of them. Another thing to keep in mind is that there are a lot of differences in the mineralogies of different talcs. It is quite remarkable how different they can behave in terms of melting, expansion, color, working properties. One thing to be especially aware of: Some talcs are much more temperature volatile than others and will cause a body to liquefy if over fired too much, others will be more temperature stable. Ball clay imparts plasticity to clay bodies. It is much too plastic to use by itself, thus it is mixed with other materials. Talc is an ideal complement to ball clay to create a dryable body that still has excellent working properties (one of the primary reasons is its particle shape). People used to using porcelains and stoneware are often surprised at the feel of a talc body. High-ball clay bodies are not used in industry because of drying and firing cracks and glaze shivering (from the high quartz content), but in the hobby casting market it works because of the 50% talc to dilute the problematic effects of high ball clay. So if you use talc bodies, you just need to be aware that you are using a body that industry could not use because you are willing to shepherd it through the process slower and get a weaker more porous fired product than what they are willing to tolerate. Ball clays are dirty (high in iron) compared to kaolins. However they are so much more plastic that the iron is considered tolerable. However in talc bodies it is normal to use the whitest ball clays available, especially those that have low soluble salts content (which can produce an off-color scum on the fired surface). Since ball clays are a natural mined product they are usually just blended, ground and sold. That means there is variation in their chemistry and mineralogy (and also particle size if the manufacturer is not vigilant). Thus it is best if your talc body employs more than one ball clay to dilute the effects of changes in any one of them. Ask your supplier if this is the case (if you are using a prepared clay body). Be ntonite is ball clay on steroids! It adds a lot of plasticity for a little addition. Bentonite is also dirty (high in red burning iron), the more you use the darker the body will fire (white bentonites are not
plastic enough and they are too expensive, forget about them for this use). Do not put bentonite in the casting version of this body, the casting rate will slow down dramatically. Whiting or Calcium Carbonate: Many talc bodies have small additions of this (e.g. 5%). Many people think it is used to whiten the color, but not so. Whiting reduces a phenomenon called 'moisture expansion' that occurs in low firing porous bodies when they soak up water. This expansion causes a glaze that was otherwise fitting to be stretched and therefore craze.
Gradients The ideal firing circumstance is that every section of a piece be at the same temperature throughout the firing. When this is true you can fire very fast. However if the kiln is not heating evenly, for example, part of a piece will get ahead of the rest on heatup. As the piece goes through critical temperatures at which sudden expansion occurs the variation in temperatures across it will translate into waves of expansion moving across it. When these waves hit weak or thinner sections, something has to give. If a piece has widely varying thickness across its cross section, then on cool down of the kiln the thicker sections are going to lag behind. As above, waves of expansion change will travel across the piece and crack it. As if the above are not enough to worry about, remember that talc bodies are made from materials susceptible to this issue. Together they create a body designed have a high expansion. It sounds like a recipe for cracks for the unwary. If it were not for its open and porous nature to stop the majority of microcracks (at the nearest pore) talc bodies would be much more prone to cracking. So if you are forming ware using a technique that produces wide variations in thickness (e.g. throwing, pressing) be wary.
Safety Talc is a fibrous magnesium silicate. So is asbestos. They often occur together geologically. These types of mineral particles like to get stuck in your lungs. Ball clays contain lots of very small quartz particles that can be sharply angular and they like to get very deep into the lungs and get stuck there also. Millions of these stuck particles cause silicosis and asbestosic. Thus hobby casting bodies are a mixture of the two most potentially dangerous materials in ceramics. So caution is needed, be aware.
Testing and Adapting Can you test incoming body stocks for problems? Yes, you must. The low fire process has so many advantages but remember: it has costs. The biggest single issue is likely glaze fit, crazing and shivering are so much more common than with stoneware. You really should glaze test pieces and then stress them using a boiling water:ice water test. At the first sign of a problem adjust your glaze accordingly. If you are using a commercial glaze contact your supplier to make sure they have a strategy to do this (e.g. mixing two base glazes, using an additive). If you make your own glaze use a selection of high and low expansion frits so you can trade them off against each other to adjust expansion to adapt. Check incoming material for large quartz grains. You can do this by simply washing some of the material through a 325 screen. Larger quartz particles will be immediately evident. Their presence can give you an alert to call the supplier and to watch your firing. Do a test firing for color. If it is darker this could be an alert that your supplier has changed talcs. If it is lighter or darker they might have changed ball clays. This could mean that other working and firing characteristic could be affected.
The Recipe Use this recipe as a starting point. For the casting version start using the amount of water and deflocculant shown. Note that slip casting recipes that show water and deflocculant do so in terms of the total clay content. Thus 45% water means that for 100 pounds of clay you need 45 lbs of water.
A Typical cone 06-04 Ceramic Slip Talc
45.0% of dry amt
0.1% of dry amt
0.2-0.4% of dry amt
For the modeling version add bentonite (up to 5%). Use the most plastic bentonite you can find, do not worry about the fired color, 5% of even a dark brown firing bentonite should not change the color of the body much. Use more than one ball clay and talc if you can (to insulate changes). If crazing is a perennial problem, then use more talc and less ball clay (some people use a 60:40 talc:ball clay mix for casting. However if you are using the modeling version with added bentonite, think carefully before cutting the ball clay to 40%, it is needed for plasticity. Consider adding some whiting to prevent moisture expansion and a small amount of barium carbonate (0.3%) to precipitate soluble salts. If you are adventurous, consider swapping some of the ball clay for kaolin, it is a much whiter, more water permeable but less plastic clay.
Things to Watch Out For If you want to fire higher for more strength beware that the body will be darker in color (there is still lots of iron in even the cleanest talcs and ball clays and higher temperatures form glasses that reveal its presence). Ware is normally very resistant to warping when fired at the typical cone 06-04. But do not get crazy, drastically overhung pieces can warp. Of course there is a big disadvantage to stability in the kiln: at these low temperatures ware is very weak and porous, thin ware can often be torn apart with your bare hands. You can burn to cone 03-02 or maybe even as high as 1 (but no higher) and get huge increases in strength but you will not be able to use commercial glazes, you will have to formulate your own (the frit companies can help you with a recipe, get them to give you a low and high expansion frit so you can trade them off for each other to adjust thermal expansion if needed). Don't even think about firing to cone 6, talc:clay mixes can melt suddenly and completely ruin your kiln. In addition, they can produce a very brittle matrix. In addition, from 1100 C on talc decomposes and releases H20 that cause lots of bubbles in glazes. Slips, engobes: At low temperatures these may not stick to the body very well at all. If the thermal expansion of your glaze and clay body are not matched well, the glaze will simply part ways with the body at the slip or engobe buffer layer. Underglazes should be OK as long as they melt well, are not too thick and they have a matching thermal expansion.
What Will You Do? Talc bodies are fine as long as you use them within the universe for which they were designed. All those flashy commercial glazes are pretty compelling. If you do not have any compunction to understand how glazes or clay bodies work then don't worry, be happy. But if you working outside the 'hobby ceramics box' then beware. You need to know the limitations of what you are using. If you run into problems it is likely not the fault of the body manufacturer. He is assuming that you know the talc body and the world it is in the center of. If you are at the edges of that world then you are on your own. On the other hand you might be wanting to dig deeper with the objective of making stronger ware.
Think about firing to at least cone 02 (where stoneware strength can be achieved using terra cotta bodies). Learn to make your own glaze that you can adjust and control. Lots of prepared low fire bodies will fire to cone 02 easily. If you mix your own clay body then you are in an ideal position to change to a different recipe that will produce stronger and more functional ware.
Firing: What Happens to Ceramic Ware in a Firing Kiln Section: Firing, Subsection: General Description By understanding what sorts of change art taking place in the ware at each stage of a firing you can tune the curve and atmosphere to produce the best possible ware.
Article A kiln is not a microwave oven and firing one is not like heating Chinese noodles! It is not a rote timed process where you can just set the oven and go shopping. Firing a kiln is much more like baking an angel food cake. It requires awareness of kiln contents, the process, and the objective. Inflexible schedules are out; flexibility and sensitivity are in. As a potter or industry, you are basically making rocks in your kiln; metamorphic rocks. You are changing the form of matter just the way a metamorphic rock has been changed from another by the forces of heat and pressure. As I have highlighted elsewhere, there are physical and chemical things that happen in the kiln. The physical changes give us the headaches, but the magic of the chemistry makes it all worthwhile. Let's review the simple stages in a firing.
The Stages of a Firing Final Drying The ware has to dry in preparation for bisque or single fire. If you don't have a dedicated drier, then you are using your kiln as a drier. If your drier does not exceed the boiling point of water, then you are using your kiln as a drier. If your ware sits in the studio or plant after drying, then its hydroscopic nature results in the absorption of water from the air and once again, your kiln is the final drier. Whatever the case, all water has to come out and even though a piece looks and feels like it is dry, there can still be plenty of water present. If just 2 or 3 percent needs removal, a typical industrial periodic kiln setting could have hundreds of pounds of water that must escape, all of which expands a thousand times when it turns to steam. Needless to say, this makes for a damp atmosphere during this stage and proper ventilation is a must. Many modern driers (an kilns) have fans that impose a virtual hurricane of draft on the ware in the kiln to remove this water efficiently. In a fine grained clay (one containing bentonite or ball clay), it requires time to vent the moisture out, especially if ware is thick. If you fire too fast at this early stage, the water within boils, generates steam, and just blows the piece apart. If you heat just a little slower, a few chunks will be blown off at sites of thicker cross section. A little slower yet and maybe just a few cracks. Still slower and only micro cracks that will weaken the ware and encourage failure in later stages or during use. Slower yet and you have it right. Slower yet and you have some margin for getting it right on a continual basis. Another matter, which must not escape your notice, is the drying of glazed biscuit ware. The absorbent biscuit can pick up considerable water during glazing and this must not be driven out too quickly. A rapid warm up will loosen the glaze from the biscuit, causing it to fall off or crawl during the firing. A moisture-laden atmosphere during early stages can even re-wet fully dried ware, and the subsequent sudden drying and associated steam and pressures associated with rapidly increasing temperature will likewise compromise the fragile glaze-body bond. How do you tell what firing schedule is right? Experiment. There are a number of variables that make it very difficult to establish rules. The most important are the weight of the ware, the density of the setting, the water content of the clay and its ability to vent this water, and the air flow within the kiln to remove the vapor as it is generated.
Large hand-built sculptural pieces weighing hundreds of pounds can require weeks or even months of protected air drying. These must be fired over two or three days, most of this time at the boiling point stage. Lighter industrial ware like mugs can be humidity force-dried in special chambers in minutes and fired in hours. In general, for dry ware and good airflow in the kiln, most ware, including large porcelain items, can be brought through this stage in several hours. Ware that is not dry may require much more time. In a worst case scenario, namely a densely packed electric kiln having no airflow and large pieces that have not been boil dried, this stage could take 24 hours or more. Whatever the case, as long as you understand the importance of thoroughly dry ware, air flow in the kiln, clay venting ability, and density of pack, you will be able to adjust matters to encourage success. In this stage, it is not so much a matter of even firing but speed and atmosphere. It is all just common sense. Before continuing, I would like to mention the matter of clay bodies. Certain ill-conceived clays are much more vulnerable to failure during this stage of firing. Clays, which lack particle size diversity, made only from ball clays, kaolins, and very fine ground feldspar and silica, do not perform well even if grog is included. On the other hand, natural native materials with limited processing will sometimes tolerate very fast firing. I have seen clays that can survive to 2300 °F in less than an hour at thicknesses of 1 cm. To make your bodies tolerant, use large particle size kaolins and ball clays, minimize bentonite, and keep your eyes open for quality special-purpose clays and fireclays, which are known to open up the body.
The Ceramic Change Crystal bound water has to escape during bisque or single fire. At earlier stages, mechanically bound pore water, that is water between clay and mineral particles, is expelled. However, H2O is bound right into the clay crystal itself, as well as into other minerals that may be in the clay body. For example, kaolin loses 10%+ of its weight on firing due to this crystal water. This “water smoking” phase occurs over a wide range of temperatures that can extend past the red heat stage. Since large quantities of water can be generated, there is ample reason not to push the kiln too fast up to red heat. For the clay body, there is no going back after the changes that occur during this phase, thus the term “the ceramic change”. How much of a concern is this? Well, it turns out that it is not nearly as critical as the expulsion of mechanical water which occurs earlier. By the time this stage is in full swing, pores within the body matrix provide a good network of channels through which the steam can be vented. Although shrinkage is not occurring during this period, the ware is very fragile; as it lacks the particle bonding mechanisms it had in the green stage. For this reason, there is one matter of concern. Proper airflow in the kiln for single fire ware should vent all escaping steam to prevent any upset in existing glazebody bonds.
Quartz Inversions and Conversions Crystalline solids are rather temperamental and quartz is no different. Quartz is a crystalline form of silica in that it has a three dimensional regular pattern of molecular units. These form naturally in nature because lengthy cooling times allow arrangement. Quartz is made of a network of triangular pyramid (tetrahedron) shaped molecules of silicon combined with four oxygens. Unfortunately, the quartz delights in changing the orientation of the tetrahedron shaped molecules with respect to each other, thus loosening or tightening the whole mass (and thus changing its total size). It exhibits twenty or more personalities called “phases” and these show a remarkable range of physical properties. A change to another phase is called a “silica conversion”. The most significant phases are quartz, tridymite, crystobalite, and glass. The material does not even melt to change phase (except to produce silica glass of course). Only an elevated temperature to increase molecular mobility along with the required time is needed. What is more, each of the above crystal phases has two or more forms (alpha and beta, beta one, etc.). Changes which occur between these are reversible, that is, the change which occurs during heat-up is inverted during cool down. These changes are thus called “quartz inversions”. These inversions, unfortunately, often have associated, rather sudden, volume changes. That means that quartz conversions are something to consider when optimizing the fired properties; quartz inversions are something to consider when firing to prevent cracking losses. There are two important inversions you need to know about because of their sudden occurrence
during temperature increase and decrease. The first is simply called ‘quartz inversion’ and it occurs quite quickly in the 570°C range (1060°F). In this case, the crystal lattice straightens itself out slightly, thus expanding 1% or so. The second is crystobalite inversion at 226°C. This is a little more nasty because it generates a sudden change of 2.5% in volume and it occurs at a temperature within the range of a normal oven. This material has many more forms than quartz, so it is a complex animal to say the least. However, while all bodies will have some quartz, you won’t have a problem with crystobalite inversion unless there is crystobalite in your body. Crystobalite forms naturally and slowly during cooling from above cone 3. It forms much better if pure crystobalite is added to the body to seed the crystals or in the presence of catalysts (e.g. talc in earthenware bodies). You can ignore these phases. But you will never be able to fully optimize fired properties of your ware and will never fully address “inversion” related firing problems without at least a partial knowledge of silica phases. We could just melt quartz, cool it quickly, and the resulting glass (irregular arrangements of molecules) could be ground into a powder having very stable firing behavior. This would really make things much simpler. Unfortunately, silica melts at a very high temperature, so this is impossible. So we have to live with the stuff and learn to cooperate with it during the firing process. As noted already, individual particles of quartz in the body change from alpha to beta form of the quartz phase and back during heat up and cool down. It is important to realize that it is not the whole piece of ware, or even the silica within it, that undergoes the associated volume change. It is the small and even microscopic particles of the quartz that do. This behavior is, of course, dampened by the structure in which they exist. During heat up, these particles are in a non-glass bound matrix surrounded by other particles and pore space, so there is much tolerance for the volume change associated with the inversion. However, during cool down or subsequent heat ups, where the clay matrix is a solid mass of glass melted around each particle of quartz, sudden volume changes in the quartz particles are much more likely to cause micro cracks radiating around each. Since the quartz can form the skeleton of the entire structure, waves of change occur through a piece which tend to extend the micro cracks into major cracks. What does this all mean? It means there is not too much to worry about with quartz inversion in first fire ware on the way up, or about cool-down for bisque ware. In both cases, the open body is quite tolerant. However, take it easy on second-fire earthenware, very easy on second-fire stoneware, and super easy on second-fire porcelain. Watch for excessive amounts of quartz powder in dense bodies that do not fire to full vitrification. In these, the quartz has not been dissolved by the corrosive action of the fluxes, but remains part of a non-homogenous fired matrix. If possible, use the finest quartz powder available and this will make dunting (cooling cracks) during firing less of a problem.
Burnout Almost all bodies contain some organic matter that must decompose and then burn at some point to produce carbon gases (the dark color of ball clays, for example, is due to their coal content). As expected, this burning occurs at red heat and beyond. It is of interest in the firing process because proper oxidation and sufficient time are needed to prevent black coring of the body and associated expansion and strength problems. This means you should provide adequate time for this part of the process, namely, at least a few hours with some draft for thicker ware. In addition, glazes or fine slips should not flux and seal the surface too early as this could result in bloating when the last remaining gases encounter blocked escape routes.
Sintering When all the water has been removed from a clay, there is really little left to hold individual particles together other than intimate contact. At some point during heat-up, chemical bonds begin to develop between particles. These processes do not involve melting yet, but a rearrangement of the molecular structure does seem to occur as a result of the increased mobility afforded by the rising temperature. This is the sintering point of a clay. You can demonstrate this by putting a sample of powdered clay into a kiln, and firing to a temperature necessary to bond the pile of powder into a cohesive and solid lump. The sintering point is normally around red heat. When a body has reached this point, it becomes impervious to water, thus resisting slaking (particle disassociation in water). If heated a little higher, ware demonstrates considerable ability to withstand thermal shock, a property that is lost to some degree as the glassy phase develops at higher temperatures.
Decomposition At some point in the firing, fluxes begin to react strongly in bodies and glazes, and chemical changes begin to occur. “Decomposition” refers to the first stage of oxide rebuilding undertaken by the Kiln God. Although materials like feldspar and kaolin are put into the kiln, the kiln fires gradually “deconstruct” these materials into their basic oxide building blocks. In some cases (e.g. single fire glaze ware), this deconstruction yields gases like sulphur and carbon dioxide which must escape, typically by bubbling up through or out of the molten glaze (e.g. whiting, dolomite lose up to 40% of their weight during firing). Reconstruction of the glaze and body matrix occurs using the pool of oxides available after decomposition. As you might expect, when the temperature begins to drop, either after shut-off or soaking, decomposition stops and recomposition to a glass begins. Thus, the most interesting part of the show really begins when the kiln starts cooling, and up until that point the stage was just being set. The glass that forms during normal cooling is a random arrangement of oxide molecules, unlike a crystalline solid which has a regular repeating structure that requires extended cooling time to form.
Reduction Many potters and a few industries use reduction firing to achieve rich iron brown and earthtone colors and special effects like copper red glazes. Reduction firing is somewhat of a ‘black art’ and is difficult to describe on paper. The basic idea is to supply only enough oxygen in the kiln to burn the fuel. But rules tend to break down in actual practice, because many people do what they call a “heavy reduction” by supplying even less oxygen and thus introduce unburned carbon from the gas into the developing body and glaze chemistry. The reduction process denies the iron compounds in the body of the oxygen molecules that they would like to have. This forces them into the reduced form, thus producing the desired colors. Many potters begin a body reduction around 1000°C, holding this for a time. Then they move to a neutral atmosphere to bring the kiln up to glaze melting temperature; when they again apply reduction to shape the final iron chemistry of the glaze. Others begin a light reduction at 1000°C and fire this atmosphere right to final temperature. Some practitioners close the firing with a short soaking period in oxidation to clear any carbon residue, others do not. Reduction is difficult to maintain consistency within, and thus potters, who tend to love the mystery and surprise of each firing, have embraced the technique. In recent years, oxygen monitoring devices (i.e. Australian Oxytrol Systems atwww.cof.com.au/useoxy.htm) have become available which enable users to exercise tight control over the kiln atmosphere. Manufacturers provide detailed instructions on what oxygen level to maintain for each glaze type. This and the general appeal of high temperature reduction have made those using electric kilns feel they are ‘second class citizens’. However, this is not necessarily true for you.
Vitrification Earthenware and low-temperature whitewares are not fired to maturity, thus they never vitrify completely. This is not to say they lack strength. A body of considerable porosity and pore space can be remarkably strong by virtue of the glass weld between its particles. Think of vitrification as a process that develops in a clay body during firing. We take it far enough to produce the desired strength and color, but not so far that ware begins to warp excessively. Thus each person arbitrarily decides what ‘vitrified’ is for himself and his own circumstances. Some bodies vitrify over a wide range of temperatures, others do so over a very narrow range and thus require close firing control. Knowledge of this process helps us to see the importance of testing a body at temperatures below and above the actual working temperature, and testing at slower and faster rates of rise. This helps you to see it in the context of the vitrification process and alerts you if your firing is miss-targeted. Soaking the firing takes on much more meaning when you understand that vitrification is a process. The body is composed of quartz mineral and clay crystal particles (and possibly grog or alumina) which form a physical skeleton around which the flux-containing materials flow. As firing proceeds, the silica hungry fluxes become more active and begin to dissolve the quartz particles and remove silica from the metakaolin (originally the hydrated clay crystal). As this happens, the melt forms silicates and thus stiffens, needing yet higher temperatures to continue the process. Given these higher temperatures (above 1000°C), the formation of long mullite crystals from the decomposing metakaolin occurs. This rearrangement happens without melting of the crystal, and the higher Al2O3 form melts much later, further stabilizing the vitrified mass. On a chemical level, the alumina oxide present acts as somewhat of a chemical skeleton as silica comes into solution, further stabilizing the clay mass. This helps us
understand part of the magic of why the piece does not end up lying on the kiln shelf in a heap. Remember then, the firing is not just the melting of a glass cement to glue together a bunch of microscopic rocks, it is a matter of silica conversions and inversions, mullite development, chemistry development of the melt, silica dissolution, and a multitude of other things. These make it possible to produce fired products having a great variety of physical properties from the same piece of clay by adjusting only the firing schedule. One excellent reference on this complex subject is the Potter’s Dictionary under ‘silica’, ‘mullite’, and ‘crystobalite’.
Glaze Set As the final stages of firing arrive, the glaze reaches its full viscosity and mobility. Unlike the body, it melts fully and often all oxides move about freely in anticipation of arranging themselves in some semblance of order at freezing. During this period, interaction between body and glaze accelerates and an interfacial layer is formed. The chemistry of the glaze and body, and the time available determine how transitionary this layer becomes. Likewise, the fluidity and surface tension of the glaze determine its ability to wet the surface to heal minor bare spots, and its ability to pass gaseous bubbles percolating up through the melt from the ever shrinking and vitrifying body. During a soak, the glaze has further opportunity to even itself out and develop an optimum interface with the body.
Glaze Cool and Freeze Cooling is an integral part of the firing process, since this is where the actual glass-building occurs. For the body, on the other hand, the building occurs during heat-up and the beginning of cooling cements this new form of matter. In the simplest possible case, the ware cools, the glaze solidifies as a glass and it is done. However, an element of crystal formation often accompanies cooling. This is especially the case for slower cooling or where the chemistry of the melt encourages the formation of nuclei for crystal growth. The growth actually occurs right around the freezing temperature, which can be much lower than you might think. Some stoneware glazes may take many hundreds of degrees to set and crystallization continues to occur until all molecule mobility is stopped. This means you should be aware of this process and if undesired crystalization (devitrification) happens, adjust the chemistry of the glaze (i.e. raise alumina) or speed the rate of drop during the critical range. So the firing process is slightly more complicated than most people think. Admittedly, you can fire a kiln without knowing any of this and you can bake a cake by just setting the oven timer and going shopping. But realistically, how likely is it that you are going to achieve optimal results? Or even passable results? •
Low Budget Testing of the Raw and Fired Properties of a Glaze Section: Glazes, Subsection: General Description There is more to glazes than their visual character, they have other physical properties like hardness, thermal expansion, leachability, chemistry and they exhibit many defects. Here are some simple tests.
Article What if you could greatly increase the quality of your fired ware by only small changes? Would you do it? Testing it thoroughly is a key ingredient. But mention the idea of testing a glaze and 99% of us think in terms of dipping a test tile and firing it to see how it looks. Many have learned that there is often little correlation between how a glaze looks on a little test tile compared with how it looks when used on ware, thus they tend to put little effort into testing. However if you already have a glaze in production, or have made an adjustment to an existing one, you are likely willing to expend much more effort to evaluate it fully. Raw and fired glazes exhibit many properties that both production and end users are knowingly or unknowingly concerned with. These include slurry properties, dry hardness, behavior of the melt, freezing characteristics, hardness and scratch resistance of the fired glass, compatibility of host glaze and added colorants, leach resistance, glaze fit with the body, clarity of the fired glass, etc. Certain factors such susceptibility to material change and varying firing conditions are more difficult to measure. Most people and companies do not have fancy glaze testing equipment to evaluate these properties. It is to these that this article is directed, there is much you can do to greatly increase your confidence in the true 'quality' of your product. Testing your glaze thoroughly is doubly important if you are using one or two base glazes because, in a sense, you have all your 'quality eggs in one basket'. We 'hoot' a lot about the advantages of focusing your efforts on one base glossy and matte recipe and altering these to make whatever you want. Thus the following tests all assume and depend on the fact that you are testing the transparent base without any colorants, opacifiers, or other additives. Remember that improving the base improves every recipe that is based on it. Here are some tests you should consider:
Slurry Properties Mix a small pail of the glaze and bring it to the proper consistency (I am assuming you are glazing bisque ware). Most glazes work best when there is enough water to make them fluid and not so much that they do not gel. Typically you should be able to stir a glaze with a stick, pull the stick out, and the slurry should remain in motion only for 2-4 seconds. Measure its specific gravity and record this amount. If in future the glaze gels, settles, or runs excessively check its specific gravity. If it is correct then you need to investigate changes in your water supply or dry materials that could flocculate or deflocculate the slurry. Let the container of glaze sit overnight and check it the next day for any tendency to settle or gel excessively. If it settles incorporate some bentonite in the recipe. If it gels to the point that remixing does not thin it out then replace materials known to be soluble (i.e. Gerstley Borate, Nepheline Syenite) or use distilled water.
Application Properties Dip your dry finger into the slurry and pull it out. Only one or two drips should fall and a good layer of glaze should hold itself in place. For glazes intended for application to bisque ware, dip a thick test tile (2 cm thick) all the way in for two seconds, pull it out and turn it over and watch for any tendency to run and drip. Note how quick it dries. When dry dip it again half way and hold it in for several seconds. Set it out to dry and note any tendency of the glaze cover to crack during drying (this is an indication of too much clay or too plastic clay in the recipe). When the specimen is
completely dry note its tendency to 'powder off' in your hands. Will it accept overglaze painted decoration without turning to mud or will wax resist paint on and hang on well (if not the clay content is either too low or not plastic enough). Dip a thin test tile also (5mm thick or less) and apply thick and thin layers as above (the tile should be thin enough so that the clay becomes waterlogged and the glaze thus dries slowly). Note any tendency of the glaze to bubble as it dries, this indicates that the specific gravity of the glaze may be too low.
Crystallization, Clarity, Crawling Fire the thick and thin test specimens made above. The thick one will cool slower, note any tendency for it to devitrify (develop crystals) compared to the thin one. Note any tendency toward crawling on the thickly glazed sections of the specimens, especially near edges. Note any tendency of the glaze to cloud with bubbles in the thicker areas.
Stain Compatibility Using a small brush paint stain ID numbers and color swatches on a flat tile of the target clay body. Glaze the whole tile over with a transparent version of the base glaze you are testing and fire. Note colors that are off and research the required chemistry to explain why.
Leaching Add a colorant like cobalt to a transparent version of the glaze being test, make and fire a bowl, fill it with vinegar and leave it in a warm place for twenty four hours. Note any fading of color, reformulate and test again till no leaching occurs.
Hardness Take a sharp concrete nail and attempt to scratch the glaze. Compare its performance against a benchmark glaze that has proven to be durable. Or use a hardened steel file to attempt to remove glaze on the lip or corner of a piece.
Cutlery Marking Using the edge of a stainless spoon or fork attempt to mark the surface of the glaze. If it marks attempt to rub or clean the mark off. If you cannot consider reformulating the glaze to develop a smoother surface or fewer crystals, flux it to melt more, or fire it higher.
Crazing, Shivering Cycle a thin glazed shard of your body-glaze combination between three-minute immersions in boiling water and ice water. Do it three times. Use a black marker to color an area of the surface. Clean the ink off using methyl hydrate. Note any craze lines highlighted by the ink. Note any areas near edges that may be flaking off (shivering). Reformulate the glaze to reduce its thermal expansion (increase it for shivering), fire, and test again.
Glaze Over Solubles Prepare a 12cm diameter disk 5mm thick from your clay and dry it under a fan with the center 6 cm covered. Any soluble salts in your clay body will be amplified on the outer 3cm ring. Break the disk in half, glaze half, and support and fire it resting on the flat. Note any reaction between the soluble salts in your clay and the glaze (i.e. blistering, orange peeling).
Flow Check the flow of the melt using a flow tester.
Fired Volatility Many glazes work well at one temperature but if fired slightly higher or lower they can have a much different surface. This type of behavior is normally only tolerated in very special purpose glazes. Thus you should prepare sample tiles for firing one cone higher and lower than you working temperature. This will help you spot possible trouble in future if kiln firings are not totally consistent.
Microscope Using a $20 30X Lightscope (available at hardware stores) look carefully at the glaze surface for any signs of poor melting (especially in matte glazes) or surface pits that could harbor bacteria. Identify any tiny specks as either impurities or the first stages of crystals. Break a shard and check for a good interface between glaze and clay. Examine any scratches you were able to make with the concrete nail.
Make Functional Ware Mix a couple of gallons of the base glaze and make a variety of functional ceramic shapes, glaze, and fire them. Note any problems like pinholing, crawling, bubbling, devitrification, and blistering.
Understanding Glaze Calculation: An Aid to Potters Section: Glazes, Subsection: Chemistry Description Bob Kavanagh explains ceramic glaze calculation as a potter for other potters. He deals with theory and show practical examples.
Preamble This article is written by a potter with potters in mind. Many of us have a fairly straightforward aversion to the numerical and chemical aspects of glaze calculation and many potters have a problem with crazing. What I do in this article, is to open a couple of doors into understanding the underlying issues involved in glaze calculation, so that potters can become more familiar with this method of addressing a glaze problem. I do this in the context of discussing crazing and using computer programs which help with glaze calculation. This article is written for the potter who is looking for explanations about glaze and clay chemistry in fairly ordinary language, although of course some language will be technical because we are going to talk about molecules and chemical formulas and several related matters. Relax, the last time I studied chemistry in school was 36 years ago, and even I can understand some of this stuff; and now that we have some of these new computer programs available to help (e.g., Insight, Hyperglaze), it has even become interesting. The beauty of these glaze programs is that they handle a great deal of the actual calculations for us, and they contain an incredible amount of information in them so that we do not have to check our chemistry books every fifteen minutes to know what we're doing. Glaze calculation in a slightly chemical form is not as customary for us as direct practice and experience in the studio. On top of that, many of the well known leaders in glaze development (those who write books and articles on glazes for potters) present their material in a format which we know and recognize. They offer recipes or instructions which give rise to recipes, which we can just go and try in the studio. This recipe format does not encompass glaze calculation at the molecular level. Crazing is a glaze fit problem. What is glaze fit? Why should we consider it? How do molecules of a glaze relate to glaze fit? Glaze fit describes the match, a physical relationship, between a fired glaze and clay. Good glaze fit is a fundamentally important consideration for all utilitarian ware. Good glaze fit occurs when the glaze on a pot is slightly compressed and when it holds the clay in a state of slight compression. When we say a glaze fits a body we usually mean that the two most obvious problems crazing and shivering - are absent. Good glazes add a safe finish to functional ware and increases the strength of the clay. To achieve good glaze fit, we must establish an appropriate balance of ingredients in our recipes to harmonize our glaze-to-clay bond. This article will help throw light on one approach to establishing this balance by using glaze calculation with the unity formula. This article consists of four major parts: the first addresses expansion, contraction and the general nature of glazes; the second talks about glazes, recipes, and the chemical makeup of the ingredients in our glazes; the third outlines ideas on molecules and the unity formula and how these relate to expansion and contraction; the fourth, approaches the matter of glaze fit, expansion and contraction, glaze calculation and how all of these relate to the glaze in the bucket in the studio. Theoretical glaze calculation does not replace "doing" in the studio. One of the wonderful things about making pots is that only making and firing them, tells us what actually works. These computer programs are another tool which is available to us.
Part I 1. Expansion and Contraction In general, ceramic materials expand when heated and contract when cooled and in theory the amount they expand when heated, is the amount they contract when cooled. Glaze-to-clay fit becomes an issue when the clay and the glaze differ significantly in the amounts they expand and contract. If the glaze contracts in cooling much more than the clay, then the glaze will stretch; at its limit, it will develop cracks as it relieves the tension: this is crazing. This is just like jeans which are too small; you bend over, they split. On the other hand, if the clay shrinks a lot more than the glaze, the glaze bunches up and it may "pop" off the pot as the glaze relieves the extreme compression: shivering. We know that not all things expand and contract the same amount. We know for example, that we warm up the metal lid on a glass jar so that we can get it loose; we do that because the metal expands more than the glass and the lid is therefore more easily removed. Ceramic scientists have done considerable research in this area to try and outline how much specific materials expand and contract; and there is an astounding amount of research done on expansion and contraction in physics, building engineering, the space industry, etc. As a result of this scientific research, there are numbers which indicate how much things expand and contract, when heated and cooled. This number is called the coefficient of thermal expansion (although of course potters are most interested in what happens when glazes contract). The coefficient is a number which tells us how much something expands for a given unit of temperature change (e.g., for every degree increased, the unit length/size of the thing increases by X amount). These numbers are extremely small by any standard of daily life and what I am saying is a simplification. We can make use of these numbers to see how much a glaze expands and contracts according to what's in it.
2. Glaze as Glass For our purposes, we can think of glazes as akin to glass and clays as akin to composite materials (concrete is a good example of a composite material). Glass is created by melting inorganic earth materials and cooling the melt fairly quickly to form a non-crystalline, congealed, substance; it may also be called a "super-cooled liquid". A composite material is an aggregate of particles held together in a medium. In general, all ceramic products are stronger when slightly compressed than when slightly stretched. This means that they break much more easily if stretched or bent than they do if squeezed or crushed. The normal strength of a clay can actually increase under conditions of slight compression, a feature which makes glazes even more valuable for functional ware. Like glass, clear glazes are non-crystalline solutions with extremely high viscosity when they are cooled; this means that they do not flow. They do not have a predictable chemical structure. When we shut down our kilns at top cone, the glaze is in a fluid state (we know what happens for example when we fire too high or wait too long at the high end of our firings; the glaze runs all over the place - it is fluid). If we cool our kilns relatively quickly, the fluid glaze freezes in place as a clear glass. If the cooling at the highest temperatures takes a slightly longer time, crystals will likely form in the glaze and an internal structure will develop. The point here is that as the clay and glaze cool, they contract just as they expanded when heated. If you put them into your freezer after they come out of the kiln, they keep on contracting. If there are tensions between the clay and glaze, these tensions become increasingly important as the clay and glaze become rigid, because only a rigid glaze can craze. The tension in the glaze increases until a moment when the internal strength or tolerance of the glaze can no longer accommodate the stresses being developed by the ongoing contraction of the glaze, and the glaze will crack. Glazes at high temperature never craze; they are fluid.
3. Overview of a Glaze by Internal Function We can picture glazes as functionally composed of glass-forming agents, melting agents and stabilizing agents. The main source of the glass forming agent is silica. Silica is silicon dioxide - Si02 . Silicon (Si) is very widespread, as is oxygen (O); they naturally combine. Quartz is a very pure crystalline rock which
supplies us with silica. There are other sources of silica which vary in degrees of purity and chemical composition, but for now let's just stick with quartz since most of us have some familiarity with quartz crystal rocks. When silica is in a crystal form like quartz, it has a definite structure and order, and certain properties go along with this crystal state. When silica is melted, it has different properties than in the crystal state. We are all familiar with this sort of thing; for example, ice, water and steam have very different properties even though they are all H2O. For our purposes now, the most important difference in silica is that crystalline silica has a higher expansion/contraction than melted silica. Melted silica (also called fused silica glass) is extremely strong and stable. It expands and contracts very little and seems in many respects to be the ideal glaze. The trouble is that it doesn't melt until about 1710ยบC, or around 3115ยบF. Since potters do not fire at this high a temperature, our glazes have other materials in them - fluxes which aid in the melting of silica at lower temperatures (say, cone 6-cone 10). The main fluxes are materials like feldspar, talc, whiting, dolomite, zinc oxide, etc. Unlike melted silica, however, when these agents are heated and cooled, they expand and contract a fair amount, significantly more than fused silica. It is this expansion and contraction which is the problem in fitting the glaze to the clay. As a result, we find ourselves in a peculiar situation: we need fluxes to make the glaze melt but they make the glaze expand and contract a fair amount. What we seek, therefore, are fluxes which melt silica at our conventional temperatures but which do not create a glaze fit problem for the clay body we are using. We need the "appropriate balance" between the expansion/contraction of the fluxes, the silica and the clay body we are using. Stabilizing agents, of which alumina is the prime example, help stabilize glazes and inhibit excessive flow. Alumina does not expand or contract very much, and so it helps the glaze fit by countering some of the high expansion/contraction of the fluxes. Looking at these three internal functions gives us an idea of the internal operations of a glaze. How do we get to a glaze?
Part II 4. Weight: Recipes, Raw Materials and Molecules As potters, we start with glaze recipes which outline the percentage composition of raw materials; for example, what I call "base 100" for cone 6-8 oxidation:
"base 100" nepheline syenite
38 (units) 38%
13 (units) 13%
10 (units) 13%
magnesium carbonate 3 (units)
13 (units) 13%
quartz (flint, silica)
23 (units) 23%
The total of any recipe is always 100%, although it may actually weigh any amount at all. These are percentages by weight of the raw materials which I add to my glaze bucket. Our raw materials are often fairly complex, but they can be analyzed by industrial or ceramic chemists into a chemical formula. For example, we have kaolin in "base 100". When we look at kaolin from a technical point of view (not as a potter throwing a canister set), it is what we call a hydrated aluminosilicate. This tells us that it is a silicate (has SiO2 chemically bound in it); that it has alumina (Al2O3) in it; and that there is water in it (H2O). To express this as chemical formula we say Al2O3.2SiO2 .2 H2O. This tells us not only that this is a hydrated alumino-silicate, but also what the numerical relation of the
molecules is in one molecule of clay, namely: that for every molecule of alumina in the kaolin, there are two molecules of silica and two molecules of water. A molecule is a configuration of atoms held together by internal chemical forces and our ingredients can be seen as a composite of these molecules, which themselves are held together by internal chemical forces. In this case, if water, alumina and silica were not chemically bound together in this way, we would not have clay, but something else. This is a chemical picture and does not tell us anything about the structure of clay platelets, particle size, the relation of physical properties with which we are familiar (such as plasticity), firing characteristics, etc. It tells us about the chemical structure and make-up of kaolin, the internal relation of molecules in naturally occurring clay. Eventually this information will help us understand the chemical makeup of the fired glaze. Our raw materials can also be presented in the form of a typical analysis. This analysis shows the percentage by weight of each oxide in the raw materials after they have been fired. We could get an analytical chemist to do this for us, but it's too expensive to do the analysis for all our materials ourselves, so we use ones sent by the mines to our suppliers. By understanding the typical analysis of our raw materials we know which raw material provide which oxides in what relative amounts in our fired glazes. Thus, by looking at a typical analysis for all our raw materials, we can begin to see the relationship between the recipe and the oxide composition of the finished glaze. As you may recall from high school chemistry classes, and from common sense too I guess, molecules weigh something and can be weighed - just not by you and me in our studios. When they are weighed, they are not compared to our normal systems of weight such as ounces or grams. The weights of atoms (called, appropriately enough, "atomic weights") are compared to the hydrogen atom and their weights are assigned according to how many times heavier than hydrogen they are (hydrogen is called "1"). You can go to the local high school or college science teacher, or library, and see a copy of what's called a "periodic table", which amongst the many other things it will tell you, will tell you the atomic weights of the various elements we use in making glazes. Let's return to the formula for kaolin: Al2O3.2SiO2.2 H2O. We know by checking our periodic table, that: aluminum has atomic weight of 27 (rounded off from 26.981539); silicon has an atomic weight of 28 (rounded off from 28.0855); hydrogen has a weight of 1 and oxygen has a weight of 16 (rounded off from 15.9994). We can simply add them together to determine the weight of molecules since molecules are simply atoms held together. So an alumina molecule - 2 atoms of aluminum and 3 of oxygen - (Al2O3) has an atomic weight of 102 (actually 101.963); a silica molecule is one atom of silicon and two of oxygen (SiO2) with an atomic weight of 60. A water molecule (H2O) weighs 18: 2 times the weight of hydrogen, 1, and the weight of oxygen, 16. We can see, therefore, that one molecule of clay has an atomic weight of 180. As you can imagine, the water, and many other things, like carbon, etc., burn off as the kiln heats up and so the fired molecule weights less than the unfired one. Now, it so happens that scientists have done a great deal of research using atomic weights and they have devised many useful tools to help in analyses related to them. For our purposes, the most helpful tool is called the "molecular equivalent weight". The molecular equivalent weight of a material is a weight which is numerically the same as the atomic weight of a molecule of that material, but expressed in conventional weights. For example, the molecular equivalent weight of silica (molecular weight, 60), is 60. This could be 60 grams, 60 ounces, 60 kilograms, etc. The molecular equivalent weight of silica is 60 units of weight.
5. Fired Glazes and Oxides What goes into the glaze which I apply to my pot is not identical with what comes out of the kiln as the fired glaze on the pot. Some material gets burned out in the firing and what is left, melts. We all know things get burned off from the various odours and fumes which we smell when we do the firing and all of the stains and discolouration on the metal jacket on electric kilns and on bricks of all of our kilns. What is burned off is designated as loss on ignition - L.O.I. What goes into the glaze is kaolin, quartz, feldspar (nepheline syenite), limestone, magnesium carbonate, etc. What comes out of the kiln is a glass composed of "oxides" (e.g., silicon dioxide, calcium oxide, magnesium oxide, aluminum oxide, etc.). The kaolin and nepheline syenite, for example, can be analyzed and presented in a typical analysis,
which shows us the percentages of oxides (and the loss on ignition) which result from calcining (firing) the raw materials. EPK (outlined in the table below) breaks down in the following way: silica; alumina; iron oxide; potash; soda; calcium oxide; magnesium oxide; titanium dioxide; phosphorus pentoxide; loss on ignition.
46.08% 37.46% 0.69% 0.40% 0.04% 0.13% 0.12% 0.03% 0.12% 14.66% Nepheline syenite (outlined in the table below) breaks down into the following: silica; alumina; iron oxide; potash: soda; calcium oxide; magnesium oxide; loss on ignition.
Nepheline Syenite SiO2
TiO2 P2O5 L.O.I.
60.70% 23.30% 0.07% 4.60% 9.80% 0.70% 0.10%
If we use this type of analysis and look at "base 100" as a whole, we see that there are various sources for different oxides: nepheline syenite gives us silica, alumina and potash/soda, calcium oxide, magnesium oxide and traces of iron oxide; limestone gives us calcium oxide (everything else burns off); zinc oxide gives us zinc oxide and magnesium carbonate gives us magnesium oxide (everything else burns off); EPK gives us silica and alumina, trace minerals and a fairly important LOI; quartz gives silica. We have three sources of silica, two of alumina and one for potash/soda, etc. In the fired glaze we no longer have kaolin, feldspar, etc. In the fired glaze "base 100", we have glass composed of randomly arranged oxides. When we do a similar analysis of the fired glaze "base 100" as a whole, and look at a percentage breakdown of all oxides by weight from all sources (outlined in the table below), Insight gives us the following result: silica; alumina; iron oxide; potash; soda; calcium oxide; magnesium oxide; titanium dioxide; phosphorus pentoxide and zinc oxide (leaving out loss on ignition).
Base 100 Glaze SiO2
Fe2O3 K2O Na2O CaO
57.44% 15.31% 0.07
2.0% 4.13% 8.34% 1.64% 0.05% 0.03% 10.99%
The programs Hyperglaze and Insight do this calculation in the blink of the eye. The first several times I went through this without the computer programs to help me, it took a significant amount of time, a dramatic amount of referencing to chemistry books and a fairly disorienting headache. If you had a way of measuring the weight of your fired glaze on a pot, you would be able to know the actual weight of each oxide in that glass because we know the percentages of the oxides in the fired glaze. In any case, we know the relative weights because we know their percentages of the whole, even if we do not know their exact weight in grams or milligrams. You can now imagine that since we know the relative percentages by weight of oxides in the glaze and the atomic weights of the oxides at the molecular level, we can begin to calculate the relative numbers of molecules of each oxide in the glaze. Thankfully, we never actually calculate the number of molecules in a glaze. We are able, however, to figure out the ratios of flux molecules to alumina and silica molecules. because we know the relative weights. The reason we want to determine a ratio of molecules is that this ratio will help us calculate the expansion and contraction of the glaze which is made up of these very molecules. This of course takes us back to the issue of glaze fit. Figuring out ratios of oxide molecules seems like fairly esoteric for work in a potter's studio, given that what we really want is a nice glaze which works well. I agree. How do we do this?
6. Molecules and Ratios of Molecules Permit me a couple of extreme examples to make my point about ratios of molecules and then I will move on to a more reasonable example. Choose a glaze recipe with only silica and alumina in it, in equal weights: 50% each. Look at a typical analysis of the fired glaze (below). Because silica and alumina are each quite pure, the typical analysis will give us the same result as the recipe. We should remember that if we use flint or quartz to supply us with the silica, there will be a slight variation because flint and quartz are not pure silica, but for this example we will use silica itself.
Two Part Glaze SiO2
Half the weight of the fired glaze is provided by silica (with an atomic weight of 60) and half by alumina (which has an atomic weight of 102). That is, 50 units of the glaze weight are alumina and 50 units are silica, regardless what the units are as long as they are a standard weight (grams, pounds, etc.). Remember what I said about molecular equivalent weights above (page 6). We know that to get the same weight of alumina and silica, we need to have more silica molecules than alumina molecules because alumina is so much heavier than silica. We know, for example, that 1.7 molecules of silica equal the weight of 1 molecule of alumina (i.e., 1.7 X 60= 102): i.e., the ratio of silica to alumina molecules is 1.7 to 1 in this fired glaze. So, let's remember that we have a fired glaze in which 50 units of the weight is silica and 50 units is alumina. If we express the ratio of molecules as a function of the 100 units of weight (100%, 100 atomic weight, 100 grams, etc.), we can see that it would take the weight of 0.83 molecules of silica (50 units of weight divided by the molecular equivalent weight - 50/60) to equal the weight to .49 molecules of alumina (50/102). Remember it is the weights of these molecules (and the molecular equivalent weights) which add up to 100 units, not the numbers of molecules themselves. Next imagine a glaze recipe which has by weight 50% silica, 25% alumina, 25% potash feldspar. Unlike the two part glaze recipe, however, this recipe uses a feldspar which itself ideally has three ingredients in it (potash, silica, alumina). Note the typical analysis of nepheline syenite above to remember how complex our actual raw materials are (page 7). The composition of the feldspar alters how much silica and alumina there are in the fired glaze, over and above the silica and alumina which are added by themselves. The typical analysis therefore looks like this:
Three Part Glaze SiO2
66.2% 29.56% 4.2% So we can see that 66.2% of the weight of the fired glaze comes from silica (atomic weight - 60), 29.56% from alumina (atomic weight 102) and 4.2% from potash (atomic weight 94). We can assume that the weight of the fired glaze is 100 units of weight, whatever the units happen to be, and we can therefore establish the ratio of the molecules to one another. At this point matters do get a little more elaborate. For example, if the weight of the glaze were 100 "atomic weight" units, it would take 1.10 molecules of silica (66.2/60), 0.29 molecules of alumina (29.56/102) and 0.04 molecules of potash to give that weight. If the weight of the glaze were 100 grams, the ratio of molecules would stay exactly the same, and if it were 100 pounds, the ratio of molecules would stay the same - even though the numbers of molecules would change absolutely dramatically. When we look at these kinds of numbers and do further analysis, the traditional way we classify the oxides is according to the three internal functions mentioned above: fluxes, stabilizers, glass formers. The three part glaze appears in the following table in terms of ratios of molecules and put into these functional categories.
Three Part Glaze Flux
K2O 0.04 Al2O3 0.29 SiO2
The ratio of molecules will eventually help us calculate the expansion and contraction of the glaze and that is the key to calculating a solution to the glaze fit problem in the studio. So yes, ratios of molecules do have something to do with getting a glaze to fit our clay when we use glaze calculation as our main tool. Now you can perform the same operation for the glaze "base 100" as we did for the two part and the three part glazes, but with more variables. This appears in the table below. In order to undertake this analysis we need to refer to typical analysis of "base 100" above for details (page 8). Once again, I extol the virtues of Insight and Hyperglaze for this purpose.
Base 100 glaze Fluxes
CaO 0.14 Al2O3 0.14 SiO2 0.87 MgO 0.04 K2O
Na2O 0.06 ZnO
There are no fractions of molecules anywhere; these are ratios of molecules in "base 100"; we are only seeing them as ratios according the three categories. All of this is leading us to address the question of getting a glaze to fit our pots in the best way. To get to that issue we need to take one more side trip into a more refined analysis. The normal way clay chemists and technically oriented potters study the set of random relations in the glaze, is with a "unity formula". So, take a deep breath and let's go.
7. The Unity or Molecular Formula and Expansion The tradition of unity formulas, begun by Hermann Seger about 1886, assumes that the fluxes are the focal point for glaze calculation, and says that the glass formers and the stabilizers should be compared to the fluxes. As a result, the formula assumes that all of the fluxes will be viewed as a single function and designated with the number "1", thereby, unity formula. A "unity formula" looks at the fired glaze from the point of view of ratios of numbers of molecules in the glaze (which is one reason it is also called the molecular formula), rather than chemical relations between oxides, or percentage by weight of oxides. This means that if there are several fluxes, then they will be seen as fractions of this unity. For example, the calcium oxide, potash, soda, zinc oxide and magnesium oxide are fluxes in the overall melting action of the glaze "base 100". If they were all equal by weight in the fired glaze, then each one would provide one fifth of the flux by weight to melt the glaze. Even if there were all equal by weight in the fired glaze, however, we would know that there would not be the same number of molecules of each oxide, because the atomic weights of the molecules are so different. The ones which have a heavier atomic weight (e.g., potash and zinc) would have less molecules than the others because their molecules weigh so much more, and it would take less of them to add up to the same weight as the others. So where are we now? The results of these molecular ratio calculations for "base 100" look like this as a unity formula, when I
Base 100 Glaze with Flux Unity Fluxes
Intermediates Glass Formers
MgO .10 K2O
Na2O .16 ZnO
Coefficient of expansion: 7.56 (times 10-6). Silica to alumina ration: 6.37 What does this tell us? Overall it says that for the equivalent of one molecule of flux (calcium oxide, magnesium oxide, potash, soda, zinc oxide all taken together), there are 0.36 molecules of alumina and 2.32 molecules of silica. Now we are able to calculate the overall expansion and contraction of the glaze by cross reference to expansion charts. How do we do this? Firstly, we need to know how much our oxides expand and contract so that we can alter the overall expansion in our glaze if we have a glaze fit problem (and almost everyone does). If a glaze consisted of only one molecule type then once we knew the coefficient of that molecule, we would know the expansion of the glaze, but we do not have such a glaze. Now it's a handy feature of clear glazes that "the thermal expansion of a multioxide glass can be estimated by assuming that the coefficient of expansion is an additive property." We can calculate the total coefficient of thermal expansion by simply calculating the addition of the constituent parts. "For example, if a glaze consists of 50% oxide A which has an expansion of 5, and 50% oxide B which has a expansion of 10, then the expansion is: (.50 X 5) + (.50 X 10) = 7.5."
Part IV 8. Glaze Fit and Glaze Calculation: Aiming for Successful Glaze Fit Now let's suppose that you have a glaze which crazes (and who doesn't?). For now we are addressing only what can be managed through changing the glaze and not yet what can be managed through changing the clay. What should you do? To get a fairly clear picture of what might be the cause of the specific problem, we would be well advised to find out how much silica, alumina, potash, soda, magnesia, etc., there is in our glaze and then check their coefficients of expansion. Once we know how much of each oxide there is and their expansions, we can see which ones most affect the expansion of the glaze and we can plan from there. Since we do not know exactly how many molecules there are in "base 100" or what the exact weight of the glaze on the pot is, we work from ratios of molecules for this. We act as if there were 1 molecule of flux (and then we are able to factor in each of their coefficients of expansion) and .36 molecules of alumina (and factor in the coefficient of alumina based on .36) and 2.32 molecules of silica (and factor in its coefficient) Once we get this idea and we have some idea about how much a given ingredient increases or decreases the expansion/contraction of the glaze, we can begin to influence this expansion/contraction to resolve our problem. Insight suggests that the expansion/contraction of "base 100" is approximately 7.56 (times 10-6) and that the ratio of silica molecules to alumina molecules is 6.37 to 1. It just so happens that glaze "base 100" crazes slightly on my stoneware body (A) and significantly
more on stoneware body (B). Now, the first question is, how do I modify the expansion of this glaze so that it fits my clay (A), on which it now crazes slightly? To modify the glaze so that it does not craze, I must lower the expansion/contraction of the glaze. To be able to change my glaze in the most helpful way, I want to alter the glaze at the "recipe - raw materials" level and yet have it show up at in the unity formula level, along with the changes in the thermal expansion. If we can see what changes take place at the molecular level and with thermal expansion at the same time that we are altering our raw materials, we would be able to see the whole picture of glaze calculation at the same time. Lo and behold! This is exactly what these new computer glaze calculation programs do best. The only way to know anything for certain when testing glaze fit, is to test the glaze on your clay. The point of the glaze calculation software is to facilitate the work of arriving at a glaze recipe which is the most likely to succeed. With the computer doing the actual calculations for me, I looked at several options (101-125) for lowering the coefficient of expansion while trying to keep other features of the glaze as much like the original as possible. If one has a glaze which crazes slightly ("base 100" on stoneware clay A), the first thing one does traditionally is to add a little quartz because the silica provided by it will certainly lower the coefficient and thereby lower the tension in the crazed glaze. Sometimes, doing this is enough the solve the problem. Using Hyperglaze or Insight, I can add small amounts of quartz to my recipe and see instantaneously the impact of this change on the ratio of molecules in the unity formula and on the coefficient of expansion. It so happens that by adding 4 units of silica to the glaze (thereby adding up to 104 units), the coefficient drops to 7.39, a healthier number and a thermal expansion which does in fact fit clay A. It does not fit clay B however. You will note that the ratio of silica to alumina has changed slightly: from 6.37 to 1, to 6.86 to 1 suggesting to us that the glaze will have a higher gloss. This is important to note because the silica to alumina ratio is fairly good indicator of the finish of the glaze (whether it is gloss, satin, etc.) and this may slightly alter the visual character of the glaze. In order to know whether it is affected or not, you must fire the piece, look and see.
8.1 "base 101" nepheline syenite
38 (units) 36.54%
magnesium carbonate 3
Base 101 Glaze (Unity Formula)
Intermediates Glass Formers
MgO .10 K2O
Na2O .16 ZnO
Coefficient of expansion: 7.39 Silica to alumina ratio: 6.86 to 1 8.2 "base 102" Another test which keeps the silica/alumina ratio the same (6.86 to 1) and has a coefficient of 7.39, follows as "base 102". The purpose of this example is to indicate that one can have exactly the same silica/alumina ratio as "base 101" by altering the other raw materials in the glaze and not only the silica. This example is very interesting because the quartz in the recipe is actually less than in the original and yet the coefficient has been lowered significantly and the silica/alumina ratio kept constant. This is possible because other materials add silica and low expansion fluxes. By using these computer programs we can see this in a flash because the program does all the calculations instantaneously and they have in them the data on a large number of raw materials.
"base 102" nepheline Syenite
magnesium Carbonate 2.00
102.65 (units) 100.00%
As you can see, all the changes are slight, but the accumulated effect of them is a lower expansion/contraction of the glaze. Does the glaze fit without crazing? Only testing it will tell us for certain. In this particular case, it fit A but did not fit B.
Glaze 102 Base (Unity Formula) Fluxes
Intermediates Glass Formers
CaO .35 Al2O3
MgO .13 K2O
Na2O .15 ZnO
Silica to alumina ratio - 6.86 to 1 Coefficient of expansion - 7.39 8.3 "base 107" To address the matter of glaze fit and clay B, it is fairly clear that we cannot simply add more silica ("base 101") because there would be a very significant change in the amount of silica and thereby in the finish of the glaze. The other changes ("base 102") were not enough (remember that the coefficient of expansion on 101 and 102 are the same even though the recipes are different). The materials databases which are in these programs proves to be very helpful at this point, because we can play with types of kaolins, types of feldspars and different ingredients to vary the coefficient of expansion, keep the silica/alumina ratio and maintain a constant watch on our other ingredients while we're at it.
A wonderful feature of Hyperglaze for example, is that we can pull up an information sheet on each material while we are actually in the midst of doing a complex calculation and see it at the same time. Once we see whether the material might be helpful or not, we can immediately insert it into our recipe and see the changes at the level of molecules and thermal expansion. While we are analyzing this glaze, I would like to go back to the original silica to alumina ratio of 6.37 to 1 because this offers the most likely similarity to the original glaze.
"base 107" nepheline syenite 43.51
112.70 (units) 100.00%
What becomes clear in this example is that there is no magnesium carbonate. The magnesium oxide originally provided by it is now provided by the talc, which also adds silica and has virtually no loss on ignition; the nepheline syenite and kaolin are up slightly and the whiting and zinc are down slightly. It's the overall mix which provides the final result and this appears as a unity formula:
Glaze Base 107 Fluxes
Intermediates Glass Formers
CaO .30 Al2O3
MgO .17 K2O
Na2O .18 ZnO
We have a ratio of silica to alumina of 6.37 to 1 and a projected expansion of 7.16. In testing "base 107" on clay B, I found that it fit B and it fit A. I then subjected the test results to some severe stresses on both clays to ensure that my initial successes had some foundation. Both of these glazes survived all the stress tests.
9. Recipes Revisited in the Studio Now what does all of this tell us? Fundamentally we have arrived at a secure, safe glaze for a wide array of functional ware. We have arrived at it through a fairly analytic approach basing as much of our calculations as we could, on scientific information, using glaze chemistry calculation programs to help us zero in on the most likely solutions to a known problem. Now we take the recipe, mix it, glaze the pot and fire. Is the glaze as beautiful as it is solid and safe? Ah, well, that is of course an important question. Beauty in a glaze transports aesthetic value into daily use, but, it is the subject for another article.
ENDNOTES . Insight is a very fast glaze calculation program with a programmable materials database holding an immense wealth of technical information on a wide variety of common ceramics materials (copies are
available for both macs and pc's and now in Windows: by Tony Hansen in Medicine Hat, Alberta, Canada. Hyperglaze is a package of functions including glaze calculation, materials database, glaze and clay recipe database storage, a hypercard support system. It runs on macs: by Richard Burkett at San Diego State University, firstname.lastname@example.org). It is fully integrated and easy to use. Note the highly informative articles by Rick Malgrem in Ceramics Monthly, January 1992 and March 1994. . "When a glaze is subject to tensile stresses in excess of its ultimate tensile strength, it develops fractures, or crazing. The reverse case is that in which the glaze [expansion] is low and develops excessive compression, resulting in. . .shivering." Cullen W. Parmelee (revised by C.G. Harman), Ceramic Glazes (CBI Publishing: Boston, 1973), p. 251. Note also, W.G. Lawrence, Ceramic Science for the Potter (Chilton Book Company: Radnor, Pennsylvania, 1972), chapter 11. Charles Lynch, Practical Handbook of Materials Science (CRC Press: Boca Raton, Florida, 1990). For a couple of quite technical references on this type of issue, look at: Y.S. Touloukian, Themophysical Properties of Matter (IFI/Plenum: New York, 1977), vol. 12-13; also, on expansion note: Touloukian, Ibid., pp. 14a 16a. . We also know that expansion is not uniform and that linear expansion is somewhat different than expansion in volume. In addition, we know that materials may show slightly different expansions depending on the precise context of the experiments. It is comforting to know, however, that the numbers are close enough and consistent enough to be able to undertake high level calculations. . For example, Lawrence says that silica's coefficient is 0.37 times 10-7 (i.e., 0.000000037) and that alumina's is 0.61 times 10-7, Lawrence, Ibid., pp 142-47. . One handy item in Hyperglaze is something called "Potter's Friend" which does all kinds of small calculations for the potter, such as converting centigrade and fahrenheit degrees. You can also call up definitions at any time; if for example, you want to be reminded what the unity formula is or the definition of, say,"mole", you call up their explanations as you are working. . W.E. Worral, Clay and Ceramic Raw Materials (New York: Halstead Press, 1975), note Chapter 6: The Effect of Heat on Clays; Prudence M. Rice, Pottery Analysis: A Sourcebook (Chicago: University of Chicago Press, 1987), chapter 12, "Properties of Clays II: Firing Behavior"; Tony Hansen, Magic of Fire (Digitalfire Corp.: Medicine Hat, Alberta, 1995). In addition, the material databases of Hyperglaze and Insight outline loss on ignition. . When you calculate these figures using Insight or Hyperglaze, the exact results may vary slightly from program to program. This divergence should remind us that these programs are, after all, only tools to assist us in formulating better glazes. Practice with one of these programs gives rise to better results when we are consistent in our approach and do not just switch back and forth between them without thinking. We should remember as well, that because all of these numbers are established by experimental investigation, there will be slight variations in test procedures and exact materials studied and this will give rise to divergence of results. . Read David Green, A Handbook of Pottery Glazes (Watson-Guptill Publications: New York, 1979), chapter 1, for a good review of this matter. . These numbers are not exactly the same when I use Hyperglaze and I take this opportunity to remind potters that these programs are tools which require constant updating and scouting around with various companies to ensure current data. For example, Ron Roy out of Toronto, uses Insight but has established a unique database which he has collected himself from companies with which he has a wide familiarity. The authors of these programs update them on a regular basis. On top of all of this, there is no complete agreement in the scientific community about coefficients of expansion for example. We should not be surprised; the world is not as consistent and clear-headed as we might want it to be. This lack of agreement only means we must ourselves be careful and be consistent in our use of the data and information available to us. . There are also 0.13 molecules of boric oxide, 0.09 molecules of phosphoric oxide, and trace amounts of iron oxide or titanium oxide, which I did not mark down, for the sake of simplicity. . Touloukian, Ibid., page 10a with an example on 11a in Table 1. This feature is not true of clay because of the unpredictable development of crystals, each of which is unknown without empirical investigation. As a result, we cannot calculate the expansion and contraction of a clay body. They can
be measured with the proper tools however. . Tony Hansen, Ibid., p. 82. . For example, Parmelee (ibid.) suggests simple coefficients for ingredients on pages 242 and Lawrence (ibid.) on pages 144-5. Other books address this matter as well (e.g., Rhodes)
By Bob Kavanagh
Glossy Glaze 'Gloss' refers to how shiny and light-reflective a glaze is. Glazes high in glass former (SiO2, B2O3) are glossy. Those high in Al2O3 tend to be matte. Fluid glazes can crystallize to a matte surface if cooled slowly or a glossy surface if cooled quickly. The SiO2:Al2O3 ratio is taken as a general indicator of glaze gloss, ratios of more than 8:1 are likely to be glossy. In some industries, gloss is a more of a product of firing than chemistry. For example, a glaze may normally fire matte (by having a chemistry that crystallizes heavily on cooling, for example), but when super-cooled it will fire glossy.
Matte Glaze A glaze that is not glossy. Of course, unmelted glazes will not be glossy, but to be a true matte a glaze must be melted and still not glossy. To be a functional matte it must also resist cultery marking, clean well and not leach into food and drink. Thus it is not easy to make a good matte glaze. It is common to see poor quality matte surfaces on name-brand table ware sold in major stores. The vast majority of random material mixes that melt well want to be glossy. Matteness can be a product of the physical or mineral form of a material used, the chemistry and selection of materials to source that chemistry and often the firing schedule. While some types of mattes are stable, with others it can be difficult to maintain the same fired texture through material and firing variations. The best mattes are those whose mechanism is understood and have an adjuster (a firing change or a material whose percentage can be raised or lowered to fine tune the degree or character of the matteness). The visual character of mattes, even those within the same mechanism, varies widely and is often difficult to characterize. Matteness is often part of a larger visual character that involves color and variegation. Mechanisms that produce matte glazes produce surfaces that scatter light: -Micro crystalline surfaces. High CaO glazes, for example, form minute calcium silicate crystals when cooling (at normal cooling rates). Wollastonite especially can do this, but also other sources of CaO. Another oxide that crystallizes well if oversupplied is ZnO, the size of the crystal being determined by the rate of cooling. -Micro-wavy (non flat) surfaces can be produced multiple ways. High Al2O3 (if supplied in a form that can decompose to enable Al2O3 to enter the melt), for example, stiffens the melt preventing level-out during cooling. Glaze melts that contain multiple melt phases solidify in a non-homogeneous way to produce a glass that both scatters light from within and from its surface. High temperature talc and dolomite glazes create this effect (although different in appearance) because the MgO creates multiple phases in the melt that have different fluidity and refractive indexes. These are sometimes called 'silky mattes' and are pleasant to the touch. -Crowbar method! Materials whose individual particles are so refractory that they simply do not dissolve in the melt, if added judiciously to the right base, can produce a workable matte. Magnesium carbonate is an example. Even calcium carbonate, if supplied in raw form, does not melt at lower temperatures and can thus matte a glaze. But the best example is calcined alumina, if used in sufficiently fine particle size, can matte a glaze even with a small addition. However, alumina hydrate, by contrast requires a much greater addition. Why? It enters the chemistry of the melt and imparts a true alumina matte, the latter just increases the melting temperature because it is so refractory. Employing combinations of these mechanisms is normally not practical because they can conflict. For example, a crystal matte is based on a highly fluid, well melting glaze, whereas an alumina matte is the opposite. Functional matte glazes are more difficult to formulate (especially at middle and low temperatures) because they have a narrow window of chemistries or have recipes containing matting agents that are highly active (resulting large changes in the degree matteness for small variations in the recipe or process). For crystal mattes, specific firing methods are also needed (e.g. slower cooling). They may stiffen the glaze melt and prevent it from leveling completely during cooling.
A Low Cost Tester of Glaze Melt Fluidity Section: Glazes, Subsection: Adjustment, Adaptation Description This device to measure glaze melt fluidity helps you better understand your glazes and materials and solve all sorts of problems.
Article There are many complex and expensive instruments designed to observe and measure the goings-on in firing kilns. Generally this type of equipment is expensive and measures absolute physical properties that can be quantified easily. However glaze melt flow is like clay plasticity, it is more subjective and not so easy to quantify. It is best measured comparatively, that is, one specimen directly compared with another. Fortunately such tests can be done using inexpensive methods and devices. I would like to submit a general purpose testing method for many glaze melt properties that is both inexpensive and easy to use. So many factors related to the melting, solidification and physical properties and defects of fired glaze surfaces are related to melt viscosity. Thus a test that provides information about this has the potential of being very valuable. Before going on, I will give credit where credit is due. This is not an original idea. I have seen this device described in industry literature to compare melt properties of nepheline syenite and feldspar. Also, I was sent a very nice dual-flow mold by Hugh Nile at Sterling China (it had the initials IMC embossed on it). I am aware that other industries also use similar devices. However I want to take it to the next level by clearing documenting its advantages and a procedure to use it. I have made a rubber master mold of the one described herein and can making working molds for others. If you would like one please see the bottom of this article.
Testers that do not work well Small or steep angle testers: Although I have messed with smaller sizes in the past I have now seen the light. They just do not work as well. You need a large enough reservoir, and long enough flow ramp at a shallow enough angle to get repeatable and sensitive tests. Inclined tile testers: Some companies prepare a lump of the glaze to be tested and glue it to one end of a tile using a slurry made from the same material. While this will often work it is problematic with compounds that shrink a lot or those lacking dry hardness. The former could crack off and the latter may crumble off. I'll leave it to your imagination what might happen if pieces or the whole sample rolls into contact with a kiln element.
The Dual-Flow Large Tester This is shown in the picture. It is 13.5cm high while standing (5.5 inches). The long runway is at less than a 45 degree angle for extra sensitivity (there are actually two orientations for two different angles). One of the big advantages of the dual tester is that it can be employed for side-by-side testing of two specimens (e.g. one alongside a benchmark). It is amazing how close you can match the melt fluidity of two materials using this method. This device is cast in a plaster mold using a mix refractory enough to resist warping if walls are cast thin (in Large and small glaze fluidity flow testers production situations flow testers should be made from the same clay that ware is made from but if such is too vitreous you can reduce the feldspar content somewhat. See below for more information on the slip
recipe. I usually bisque fire these testers for extra strength. The reservoir accepts a 10-12 gram ball of material that you can just drop right in. These balls are easy to make by dewatering the glaze or material slurry on a plaster surface to the right working consistency and then rolling the ball in your hands, drying it and shaving material off to achieve the right weight. (thus the glaze does need to have enough plastic ingredients to enable this workability or you need to add some bentonite to impart it). I have defined a procedure for this test in the testing area of this site. As noted in the procedure there, for repeatable results it is important that your testers be the same thickness, made from the same clay, fired at the same rate of rise and to the same temperature, and the ball sample must be the same dry weight each time. In case you are not yet clear on how this tester is used: Two glazes are compared by dropping dried balls of each into the reservoirs at the top and the whole thing is fired to the desired temperature (with a tile below to catch any glaze that runs right off the end of the runway). During the firing, the glazes flow down the runway according to melt development, melt surface tension (and other factors like bubble development). What this tester can show you about glaze s: If glaze ingredients shift in particle size or chemistry and thus change the melt, it will be immediately evident either by the flow reaching further down the runway or by a change in the character of the flow. This information is valuable in quality situations since it is so hard to guage glaze fluidity by simple observation of a normal thin layer on glazed ware or a test tile. Information from a flow tester is valuable when adjusting the recipe of a glaze for other things like thermal expansion, color, material substitution for no-longer-available materials, etc. while trying to maintain the same fluidity. This test helps to rationalize discrepancies between between what a glazes chemistry indicates should happen and what actually does happen in the melt. Often glazes of very similar chemistry will have different flow properties due to factors related to mineralogy and physical properties of materials. For example, the tester shown here is two glazes with the same chemistry, one sources CaO from calcium carbonate, the other from wollastonite. A flow test can be helpful in evaluating basic mechanisms in glazes. For example, is a stony matte glaze matte because of lack of melting activity or is it due to surface crystal development in a fluid melt? Are bubbles not breaking at the surface because of surface tension or lack of fluidity? Is crawling due to loosening of dry glaze from the ware or interface problems with the melt? Ball milling time: By extracting samples from your mill at regular intervals, firing, and comparing the degree of flow you will be able to assess the mill's effect on glaze maturity and melt development. Because the glaze is so thick in a flow tester, bubbles resulting from products of decomposition within the glaze will be evident by the character of the thick flow and in the broken cross section (bubbles can even disrupt the melt flow). The glazes ability to wet the surface of the clay is evident by the angle at which the leading edge and sides of the flow meet the runway surface. These testers are great educational tools. This one, for example, shows the impact that a simple addition of opacifier has on glaze flow. Changes in properties like opacity or tendency to crawl, blister, pinhole, crystallize, craze or shiver, develop entrained bubbles or boron-blue clouding are often amplified by this test. The flow provides for an opportunity to see a very thick layer of your glaze and this can reveal differences not noted in thinner layers. Glazes which do not necessarily run on ware may run very badly on a flow tester, this indicates a lack of SiO2 and Al2O3 and is a warning for susceptibility to cutlery marking and leaching. Since so many glaze defects are either related to melt viscosity or revealed by a thick flow, monitoring this property is important. This device can even be used to help determine the optimal firing temperature, by experience you will know the fluidity of glazes that perform well. In addition firing a glaze in a flow tester at a range of temperatures may reveal that fluidity begins to increase more quickly through a narrow temperature range. After all, what is more significant to determine the freeze-point than flow of the
glaze melt? Raw M aterials Testing Most companies can readily test clay materials for use in bodies and glazes using physical testing methods that require a minimum of equipment. But it is not so obvious how to compare and test fluxing materials like feldspar for consistency. One can just trust the particle size and chemistry information provided by the manufacturer for each shipment and compare numbers. But what is the actual relationship between these numbers and the consistency of product on a production line? Can you trust the numbers anyway? The tester is an elegant simple alternative. It accurately shows melting power, color and impurities, you need to see two feldspars side-by-side to see how sensitive it is (see pictures at bottom for an example). Product Development Many ceramic products are tuned to melt to a certain extent to achieve their function. For example, an engobe needs to have a stiffer melt than a glaze, but much more maturity than the underlying body. Likewise, a ceramic printing ink must have a specific degree of melt fluidity, enough to adhere or melt to a smooth hard surface, but not so much as to bleed into the covering or underlying glaze. Melts used for bonding purposes likewise need to develop enough glass to bond, but not so much that fired geometry cannot be maintained. A standard and a test can be evaluated sideby-side using this tester. If the melt is not fluid enough, then it can be fired higher, or a percentage of frit can be added. Taking Photos Since these fired testers are quite large, storing them for future reference can be a problem. Taking a picture of them and scanning it onto the computer for archival purposes makes more sense. Make them at least twice as large as the ones shown here and they should still take less than 100kb of memory. You may find that making the testers from an off-white, grey or even tan body might be better to prevent washed-out results when taking photos. Also, have plenty of side lighting so that gloss is highlighted.
Slip Recipe A good starting recipe is #L2540, it is 50% ball clay, 25% feldspar and 25% silica. This does not cast quickly but the pieces have good green strength and the clay will vitrify around cone 10-11. For a more refractory mix replace some of the feldspar with kyanite, calcined alumina or some other non-plastic high temperature material. You will need to know how to mix and deflocculate a clay slip, search in this library for the word "deflocculation" for an excellent article on understanding the casting slip mixing process. Getting a Tester If you would like a mold of the flow tester shown here, use the email form on this page and ask for a quote. Plainsman Clays has the master rubber mold and can make plaster working molds from it for $100 each plus shipping. If you need alot of these we recommend you buy one plaster mold and use it to make a master rubber block mold so that you can make as many plaster working molds as you need. We can contact them on your behalf to arrange it and get a quote. Delivery time will likely be a month.
Understanding Thermal Expansion in Ceramic Glazes Section: Glazes, Subsection: Thermal Expansion Description The way to deal with crazing or shivering is to understand what thermal expansion is, how it relates to the chemistry to the glaze and practical ways to calculate it. There is a rich mans and poor mans way to fit glazes.
Article Almost any fired ceramic object experiences expansion as it is heated and contraction as it is cooled. A typical piece of functional ware is a two-part system in that body and glaze possess independent expansion characteristics. However the glaze is fixed to the underlying body and is therefore obliged to conform to the body's thermally induced size changes. Stresses are thus part of what we could call a 'glaze-body marriage'. To succeed a marriage needs two important things: Compatibility: In a body:glaze 'marriage' they must have compatible expansion curves (we will see what that means in a minute). A key fact to remember is that fired ceramic is strong under compression but very weak under tension. If a glaze is stretched onto the body at any time (even as a result of contraction due to quick surface cooling) it will likely form a network of cracks to relieve the stress. This is why having a glaze under slight compression is good. Firmly bonded: In most cases a significant reaction layer or interface does bond them together. This buffer forms as the liquid glaze melt attacks the body, penetrating into it and forming intermediate compositions layered against the body. The temperature, soaking, cooling rate, glaze and body chemistry, and material particle sizes all affect the development of the intermediate zones. On one extreme earthenware will have a poor interface while on the other high temperature slow fired porcelain will have a highly developed one. Fritted glazes will typically be more reactive and produce a better interface. Crazing (the fired glaze forms a network of crack lines) and shivering (the fired glaze flakes off at rims and edges) are among the most common problems glaze technicians have to deal with. Many manufacturers and individual potters suffer serious loss of product strength and compromised hygienic properties because of these. Unfortunately some do not even realize it. Glazes that have a higher expansion than the body by implication also contract more on cooling. This puts the glaze under tension, stretching it, sort of a "size 6 mug in a size 5 glaze" situation. If you would like to demonstrate some dramatic crazing, mix nepheline syenite and water and apply a thick layer to a test piece made from a typical cone 10 clay body and fire. Tension can also occur where a normally compatible glaze is subjected to stretching by a moisture absorbing and expanding body (e.g. water reacts with alkaline or alkaline earths remaining uncombined in under fired bodies). However I will assume that your clay body is either vitreous or contains additives to prevent this. Shivering is the opposite, a "size 6 mug in a size 7 glaze" situation where areas of the glaze unable to 'hang on' can actually flake off the fired ware. If you would like to see some pretty dramatic shivering, make a body composed of ball clay and silica 50:50 and apply a typical cone 10 low-feldspar glaze. There are plenty of common misconceptions about how to deal with crazing problems, most tend to attack the symptoms instead of the real cause, thermal expansion mismatch. It is not difficult to create a glaze:body marriage that survives the initial contraction test of cooling slowly in the kiln. The real trial is achieving a 'working fit' that ensures the glaze is under the correct amount of compression over the entire range of heat/cool cycles it will experience during many years use. You must create a two-part system that achieves a degree of compression in the glaze that is within the "interfacial layer's" ability
to hold it comfortably over a long time. Thus, compressive stress actually becomes a contributing factor to the ability of the 'marriage' to stand up under thermal attack during use.
Rich Man's Way to Fit Glazes If you can afford an instrument called a dilatometer, then you can take a broader view of thermal expansion. This device is the standard instrument used to measure thermal expansion of small test samples from room temperature to set point (for glazes) or an arbitrary temperature (for bodies). It is basically a small furnace in which a small bar made from the glaze or body is heated. The bar is positioned in a refractory tube against which a sensitive push-rod measuring probe rests (newer designs use lasers). Length changes during a fixed-rate heat-up from room temperature to the softening point of the glaze are recorded and plotted as a 'dilatometric curve'. Visualize a reversal of the dilatometric heat-up curve: On cooling, a molten glass solidifies at its "set point". From here, it is capable of accepting differential stress from the body to which it is attached. The total thermal expansion could be considered as the percentage increase in length of the bar at its 'set point'. However glazes have different set points so a more useful standard has evolved: divide the total expansion by the number of degrees taken to produce it (if the curve is fairly linear over the range this value is reliable). This produces a figure that represents the change in length per Â°C. By the time the mathematics are finished, the result for ceramics is a value in the 10-6 decimal range. Thus an expansion of '7.0' is really 7.0 X 10-6 in/in/Â°C (the length units are obviously arbitrary, it could be cm/cm/Â°C if you like). As already stated, the ideal glaze should have a slightly lower expansion than the body to put it under some compression. It is thus not difficult to imagine a technician superimposing the curves for variations of a glaze on top of the one for the body to find one whose curve tracks a little lower than the body. How much lower? That would be determined by that companies experience with the type of glaze and body they use. It is important to realize that this method of controlling the expansion relationship between body and glaze is not practical for people who just want to make one test of a body and glaze in isolation and expect the results to be a definitive indication of fit. This method is only useful if done over a period of time in parallel with production to develop an understanding the expansion relationship between body and glaze. In some ways, the poor man's way to doing this is actually better for most people.
Poor Man's Way to Fit Glazes An interesting point is that although a dilatometer provides a graphic view of the history of thermal expansion for a glaze and body, the technician still has to decide what the proper spatial relationship between body and glaze curves should be. How do they do this? Other kinds of tests. They must subject ware to extreme thermal stresses and mechanical tests to try to induce crazing. Over the years a history of these test results and a record of how ware stands up over time produces the body of knowledge that enables them say where the line for a glaze should be in relation to a specific body. From that point on new glazes can be brought on line based on dilatometric testing without the need for all the other tests. It may have already struck you that you can fit a glaze just fine without the use of a dilatometer. Industrial tests for glaze fit are not nearly as complicated as most people might think. While dilatometric curves of body and glaze are nice to have, isn't it the "acid-test" of subjecting the body-glaze 'marriage' to stress that tells the real story? Typically, multiple specimens of glazed ware are repeatedly subjected to an atmosphere of steam at high pressure, heated in air or boiling water, and then
quenched in ice water. It is important to standardize the test. Make sure that the ware is thoroughly heated and cooled throughout on each cycle and examined closely to record results. Some have been able to translate the failure point in their tests to the expected failure rate in the field. For bodies with an absorption, it is also important to realize that body expansion can occur if, and when, water is absorbed. So in the above tests, an important element is long exposure to heat and being sure the ware is thoroughly and completely cooled (some people will put ware in a freezer overnight to take it well below the temperature of ice water). Many have standardized on a 5 minute cycle ice-water boiling-water test. A second valuable test is fired strength. The idea is to glaze and fire a sample bar of the body, then break multiple specimens in a device that records the necessary force for each. Calculations and subsequent averaging yield a strength figure that can be compared with the unglazed body's strength. In this way, you can create a profile comparing strength with formulation changes designed to vary the expansion of the glaze. While achieving a high strength is good, it is important that a glaze not be under too much compression, this might produce stronger ware out of the kiln but under continued use it may eventually fail. Another interesting observation you can make is a fracture test. Make a thin-walled vessel of the clay (as close to spherical as possible) and glaze it on the inside and fire. Then drop it on a concrete floor (cover your eyes). If the glaze is under compression the piece will almost explode into dozens of pieces, often with a popping sound (glazed edges of shards will be razer-sharp). If the glaze is crazing the piece should break with a dead thud into many pieces with some grainy material produced by disintegration along break lines (because of craze-induced weakness). A vitreous piece with a fitted glaze should be strong and break into only a few pieces. Glazed edges on shards should follow the contour of the crack is if the body and glaze were one.
Using Calculation to Fit a Glaze It might seem logical that we measure the thermal expansion of each of the materials used in a glaze recipe and then calculate the expansion additively based on the percentage of each. However such an approach is completely wrong. The problem with calculating glaze expansion from the expansions of the raw materials is that those expansions are a product of the mineralogy and crystal structure of the material (or should I say: 'of the material particles') rather than its chemistry. The classic example of this is carbon. Graphite is a soft lubricating powder, diamond is the hardest material known. They have the same chemistry but different mineralogy. In ceramics the classic example is silica powder. It has a very high thermal expansion, particles of quartz are literaly the 'kings of thermal expansion'. Fused silica (melted and then quickly solidified before it can crystallize) is the lowest (or one of the lowest) expansion ceramic materials known! How is that possible? Because quartz is a crystal, fused silica is a glass. Yet they have exactly the same chemistry! This story can be repeated for almost any other raw ceramic material, they are almost all crystalline. Most materials are mixtures of minerals and even mixtures of different forms of the same mineral (and others). A fired glaze is a glass, it is not crystalline. Thus its expansion is a theoretical product of the contributions of the oxides that its chemistry enumerates. Each recipe material contributes one or more oxides, the same oxide can be contributed from a number of materials in the recipe (SiO2 for example). Thus calculating a glazes expansion additively from the percentage of each oxide is practical (although there are caveats as you will see later). INSIGHT knows the expansion of each of the oxides (expansions of ceramic oxides, especially the commons ones, are well known) and calculates the expansion of the glaze as a whole from the formula. Thus expansion is a formula level property. We must be aware of the limitations of this to employ it effectively. First, it is not possible to calculate a glaze's absolute coefficient of thermal expansion accurately. Various mathematical techniques have been employed that produce good results within certain systems, but there is no universal method that works everywhere. Remember also that dilatometers have their own absolute vs. relative interpretation problems and a glaze on a piece of ware might not exhibit the same thermal expansion as a test specimen (for a variety of reasons). The simple additive method of calculating expansion is the most common and has proven quite workable. Why? Different authorities disagree on absolute expansion but they generally agree comparatively. Also the differences between oxide expansions are so profound that calculations unquestionably give direction. So remember: Calculations produce relative, not absolute thermal
expansion values. But isn't that what we need? If a glaze is crazing then we need to take its expansion downward. Do we need an absolute number to represent the expansion of the glaze and clay? No. This is why calculated expansion is effective, oxides that increase or decrease it are well known. This is similar to driving with a broken speedometer and judging your speed according to surrounding traffic. Little old ladies are likely driving below the limit, teenage men likely above. Different roads are like different clay bodies. How much should you adjust expansion value to fix a crazing problem? If a glaze is crazing soon after a piece is fired this is an indicator of a serious, not a minor problem. Thus, if the expansion of the crazing glaze calculates to 7.5, then try to move it to 7.0. After stress testing, then move it more or less.
Establishing Calculated Expansion Targets for Your Clay Bodies Anyone who has done a lot of calculations on recipes they have tested on their own clay bodies soon learns to predict whether or not new ones will craze. However, remember that calculations are relative within 'systems', although a dolomite matte may need to calculate to 7.0 to fit your body don't expect a highly fluid zinc lithia crystalline to work at the same calculated value.
Consider an example of a silky matte cone 10 glaze. Silky Matte CUSTER FELDSPAR..... DOLOMITE............ EPK KAOLIN.......... SILICA.............. FORMULA & ANALYSIS *CaO .41 8.33% *MgO .41 5.90% *K2O .12 3.97% *Na2O .05 1.16% *Fe2O3 .00 .23% *TiO2 .00 .09% *P2O5 .00 .05% Al2O3 .44 16.25% SiO2 2.98 64.02% COST/KG .23 RATIO 6.70 EXPAN 6.45
33.0 23.0 22.0 22.0
The company using it has found it to be successful with an ironware brown body at cone 10 reduction. Not only does it not craze but it considerably strengthens the ware. This is important because such bodies are usually fairly weak (iron bodies develop their characteristic warm color as a function of stopping the firing short of vitrification). A glaze that fits poorly on such a clay can cut its strength quite dramatically whereas one that fits well can significantly increase strength. In the case of this company, they were producing ware that was tougher than others who used a stronger clay body but with a poorly fitted glaze. In fact, the strength difference in the ware was so great that simply breaking a mug of each made it obvious which was stronger. Since this glaze is known to function well, the calculated expansion of any other glaze using the same or a similar material suite can be compared and fairly reliable fit predictions can be made. Although a dilatometer-equipped factory technician correlates strength and thermal stressing glaze fit tests to actual measured expansions, you can correlate to calculated numbers. This method is not quite as precise but it is more flexible in that it allows prediction of fit without having to make fired samples and measure them. Predictions become increasingly accurate with time and experience.
How expansion is calculated?
As already noted, it has been shown that thermal expansion is considered an additive property (with the limitations noted). This means that a knowledge of the expansion values of a glaze's constituent oxides makes it possible to calculate the expansion of the glaze as a whole by simple addition. The equation for a glazes calculated expansion, G, is: G = Ea x Pa + Eb x Pb + Ec X Pc etc. Where a, b, and c are oxides, E is the oxide expansion and P is the proportion of oxide in the glaze. For example, if a glaze consists of 50% oxide A, which has an expansion of 5, and 50% oxide B, which has an expansion of 10, then the expansion is: (.50 x 5) + (.50 x 10) = 7.5 Remember, this is only an approximate way to deduce a glaze's expansion since other factors (especially interactions) besides the oxide make-up are involved. The issue is further clouded by the fact that basic expansion data from different authorities can vary, use different units, and be intended for use with molar formulas or percentage analyses. There is, thus, the challenge of either averaging the data or accepting the set one feels is more applicable or credible. In actual practice, I have found this to be less of an impact than expected since there is good agreement on the oxides that typical comprise 90% of the glaze (e.g. the huge expansion contributions of Na2O and K2O, moderate CaO, low MgO, Al2O3, SiO2, B2O3). There are, of course, irregularities not yet accounted for. One interesting exercise is to page through the frits in DOS FORESIGHT and compare calculated expansion figures with the physical expansion values shown for many of them. For most the correlation is good, but for some there is a considerable discrepancy (B and Li containing frits are examples, this is understandable since these two oxides are known to have non-linear response to proportion). Like the drug industry, we must also consider interactions between oxides, this is not well understood.
The Art Glass Industry: An interesting parallel The importance of knowing expansion is paramount to glass artists who cool their thick pieces in minutes instead of hours. Pieces of glass of different colors and chemistry are routinely joined and they must be expansion compatible to cool without cracking. The supply chain considers thermal expansion a fundamental piece of information that should never be separated from a glass or glass mix. 90-compatible, for example, refers to soda glass that has an expansion of 90 x 10-7. Built in stresses that cause failure over time are a great concern also and glass artists use hot/cold tests and special instruments to identify and deal with them. Ceramic artists can take a good lesson from this.
Thermal Expansion Numbers Provided with Frits Frit companies normally publish expansion numbers with their frits. These may be derived by measurement in a dilatometer or by calculation. Various frit companies have different ways of calculating (usually involving the standard additive process with exceptions). Ferro, likely pretty typical, calculates coefficient in degrees C using Hall factors if available and M & H factors otherwise. The numbers are the COE x 10 -6. When they measure the thermal expansion of glazes or frits in the laboratory they log the expansion change between 0-450 degrees C using a 3 degree C per minute heating rate.
Adjusting Glaze Expansion by Calculation to Solve Shivering Section: Glazes, Subsection: Thermal Expansion Description This page demonstrates how you might use INSIGHT software to do calculations that will help you increase the thermal expansion of a glaze while having minimal impact on other properties.
Article Shivering and crazing are probably the two most common glazing problems in industry and hobby ceramics. When one becomes familiar with some thermal expansion theory and how easily crazing can be dealt with, it becomes a mystery why others seem puzzled by the problem. I have watched people who are in love with one touchy glaze try a myriad of different bodies, alter firing curves in every possible way, and yet, achieve only limited success. And then, there are the "snake oil" remedies. "Just add some silica to stop crazing and take some out to stop shivering" it says in many textbooks. This sounds fine at first but as already discussed, you will likely upset the oxide balance and probably kill its surface character if you add enough to fix the problem. Each glaze has its own personality and complexities, a "one solution fits all" method will not work. Until now, calculating glaze formulas and their expansion values has been a little tedious to say the least. INSIGHT and similar software changed that, providing instant expansion results for any formula or recipe. Below is a set of source figures I have chosen to use. As you will see, using the calculation approach is an exercise in relating one glaze to another, one oxide to another. It is not necessary to know what a crazing glaze's expansion is; all we really need to know is whether and adjustment to its recipe will give it a lower expansion to make it fit better onto the body. While we could get into the units of these numbers, this is not really important, their relationship to each other is the key.
Expansion Values BaO 0.13 K2O 0.33 MnO
CaO 0.15 Na2O 0.39 TiO2
PbO 0.08 ZnO
0.09 B2O3 0.03
MgO 0.03 Fe2O3 0.13 Al2O3 0.06 SiO2
From the numbers above, it becomes obvious why the addition of silica has a lowering effect on a glaze's expansion (although not to the degree you might think). One also begins to appreciate why high-temperature glazes (which have so much more SiO2 and Al2O3) are much easier to fit to clay bodies without crazing. Yes, SiO2 has a very low expansion, so glazes containing plenty of it will tend to have a low expansion also and thus resist crazing. However there is a caveat to this: Even high silica glazes will craze if they contain significant amounts of Na2O and K2O (from feldspar, for example). Let's look at a shivering problem that occurred in one studio. Below is an INSIGHT report of the original shivering glaze. DETAIL PRINT - Matte White GlazeMATERIAL Cost/kg
Notice that the expansion is calculated based on the figures shown under each oxide column title. It is very clear which oxides need to be increased to move the expansion up. Shivering is much less common than crazing, so it is not likely that too much change is needed. It should be possible to leave the SiO2 and Al2O3 alone and thereby minimize fired property changes associated with disruptions in the overall balance. If exotic color compatibility is not at issue (it depends on the absence or presence of certain fluxes), a conservative starting point is to redistribute the fluxes, increasing one at the expense of another. As noted, sodium has the highest expansion, so it is logical to increase it at the expense lower ones like CaO and MgO. You have probably noticed that high sodium glazes (ones with plenty of nepheline syenite and soda feldspar) tend to craze. However Na2O contributes properties much more similar to K2O, therefore I will first try to remove Na2O and replace with an equal amount of K2O. A common sodium sourcing material, as mentioned, is nepheline syenite. Using INSIGHT, I made a quick substitution for potash feldspar, then juggled remaining material amounts to compensate for the differences in these two materials. Here is the result. DETAIL PRINT - Matte White GlazeMATERIAL Cost/kg
Notice that the calculated expansion is up modestly to 7.2 (from about 7.0). The relative amounts of the fluxing oxides (refer to the "UNITY FORMULA" line) have altered somewhat, but the SiO2 and Al2O3 remain unchanged as a result of the compensatory recipe adjustments I did. It is possible that this change in expansion might not be enough. There is still room for more movement. The obvious target is the flux MgO, it has the lowest expansion by far. I am going to remove some of it in favor of CaO, this should have a greater effect on overall glaze expansion than the above change (note however that it is possible that the glaze depends partly on the magnesia (MgO) for its silky appearance, substituting CaO for MgO is more likely to preserve this than Na2O for MgO).
Following is a calculation where I have eliminated the MgO sourced by talc, and added CaO. DETAIL PRINT - Matte White GlazeMATERIAL Cost/kg
As you can see, the expansion has increased much more this time; possibly it was not necessary to remove all the talc. A line blend of this adjustment with the original recipe would determine some intermediate compromise mixture. A simple three-interval line blend is done by mixing the trial 75:25, 50:50 and 25:75 with the original. An easy way to do this is described in the section about altering glaze temperature. I am going to try one more change. I will go back to the original recipe again, leave the potash feldspar alone, remove the talc, and adjust the rest of the recipe ingredients to preserve the SiO2 : Al2O3 ratio. DETAIL PRINT - Matte White GlazeMATERIAL Cost/kg
The expansion has moved from 7.0, for the original, to 7.23 with this modest change. Again, it is possible that the optimum recipe is a mixture of this and the original as determined by a line blend. Notice that throughout this calculation process, I kept the total recipe amount at 100. Also, I rounded recipe amounts on each calculation to avoid accuracy overkill. Since the Superpax and bentonite are added for non-chemical purposes of melt opacity and slurry suspension, as a final step I recalculated the recipe total to 96.5 and restored the superpax to 1.5 and the bentonite to 2.0. You probably want to know whether this really worked! Well, it performed perfectly! All three trials had a surface quality that is almost identical to the original. The first two high sodium tests made the glaze a little less opaque, requiring the addition of more Superpax. A little work with the three in production indicated the best one.
Dealing with Crazing In actual practice, it is not that common to find a glaze that shivers and requires the kind of adjustment just done (except perhaps at low temperatures using high talc bodies). Middle and high temperature glazes naturally tend toward crazing, it seems it is always a challenge to keep the expansion low enough. The process is simply the opposite of that shown above, the best strategy is to reduce Na2O and K20 in favor of MgO and CaO or to add boron which enables increasing the silica and alumina.
Barium Carbonate Formula: BaCO3 Alternate Names: Barium Carb, Witherite
Oxide Analysis Formula BaO 77.66% 1.00 CO2 22.34 Oxide Weight 153.30 Formula Weight 197.40
Enter the formula and formula weight directly into the Insight MDT dialog (since it records materials as formulas). Enter the analysis into an Insight recipe and enter the LOI using Override Calculated LOI (in the Calc menu). It will calculate the formula.
DENS - Density (Specific Gravity) 4.27 Barium carbonate powder is dense and white and is manufactured either from the mineral barite (BaSO4) or from barium chloride. Subsequently a precipitation process is used to get the carbonate form. There are several crystalline forms of BaCO3, alpha is the most stable. Barium carbonate is very stable thermally and does not readily disassociate unless at least some CO is available in the kiln atmosphere (i.e. reduction). BaCO3 is reduced to the unstable BaCO2 in the reaction: BaCO3 + CO -> BaCO2 + CO2 While BaCO2 has a high melting temperature, it will break down much readily in a glaze melt (liberating the BaO for glass building). It decomposes even more readily during glaze melting in a reduction atmospheres. The dissolution process happen most quickly if BaCO3 is present in small amounts (e.g. 5% or less). Even if present in larger amounts, the glaze matrix can solidify with both types, one participating in the glass microstructure and the other acting as a refractory filler, opacifier and matting agent (especially in low temperature glazes). Effects produced when baria is acting as a filler are sometimes mistaken for those of a true baria crystal matte. Such will likely leach toxic BaO (other oxides will opacify or produce a low fire matte i.e. CaO, MgO, Alumina, Zircon). Barium carbonate produces gases as it decomposes and these can sometimes cause many pinholes or blisters in glazes. There are many barium frits available and incorporating one of them to source the BaO instead is a classic application of ceramic chemistry calculations. The resultant glaze will be more fusible and will have better clarity and fewer defects. In art ceramics barium carbonate is popular for the production of classic barium crystal mattes, BaO readily forms crystalline phases during cooling. These are dependent on adequate kiln temperatures, cooling cycle and the chemistry of the host glaze (a slightly reducing atmosphere is also beneficial). Barium can act to initiate crystal development in other chemistries, for example metallic glazes can benefit by the addition of some barium carbonate. Barium carbonate is commonly added to clay bodies in small amounts (0.2-0.8%) to halt fired surface scumming or efflorescence It is slightly soluble in water and provides Ba++ ions to link with SO4-- ions in the water to form BaSO4 (barium sulfate). This new sulfate molecular form is much less soluble (2-3 mg/L), so it stays internal (rather than migrating to the surface during drying). However companies try minimize the use of barium (or even high clays with high soluble salts) because the barium sulphate generates sulphuric acid during firing and it corrodes kiln refractories. To get the best dissolution it is best to add the barium to the water first and mix as long as possible, then either add the water to the other dry ingredients (for plastic bodies) or add the other ingredients to the water (for slips).
Glaze Opacifier - White If available in sufficient amount, barium oxide will promote crystallization of a melt during cooling, thus imparting a measure of opacity.
Titanium Dioxide Anatase, Brookite Formula: TiO2 Alternate Names: TiO2
Oxide Analysis Formula TiO2 100.00% 1.00
Enter the formula and formula weight directly into the Insight MDT dialog (since it records materials as formulas). Enter the analysis into an Insight recipe and enter the LOI using Override Calculated LOI (in the Calc menu). It will calculate the formula.
DENS - Density (Specific Gravity) 4.26 HM OH - Hardness (Moh) 6.5 M LPT - M elting Point (MP) 1830C TiO2 occurs in many silicates in nature, accounting for over 1% of the earth's crust. Thus it is manufactured using a variety of materials and processes. Although titanium is the strongest white pigment known for many uses, in ceramics the whiteness (and opacity) it imparts to glazes is due to its tendency to crystallize during cooling. While titanium dioxide is used in glazes as an opacifier, it is not as effective and easy-to-use as tin oxide or zircon. It can be used as an additive to enliven (variegate, crystallize) the color and texture of glazes (rutile works in a similar manner). In moderate amounts it encourages strong melts, durable surfaces and rich visual textures. Titanium is available both as raw and surface treated products. Non-pigmentary grades flow more freely in the dry state. Self opacified enamels are made by adding titanium during smelting to super saturation. Upon firing the enamel, the titanium crystallizes or precipitates to produce the opacity. Titania is also used in dry process enameling on cast iron appliances for its effect on acid resistance, color and texture. In glass, non-pigmentary titanium dioxide increases refractive index, intensifies color.
Wollastonite Calcium Silicate Formula: CaSiO3 Alternate Names: Wolastonite
Oxide Analysis Formula CaO 48.28% 1.00 SiO2 51.72% 1.00
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DENS - Density (Specific Gravity) 2.8-2.9 HM OH - Hardness (Moh) 4.5-5 SLBY - Solubility Soluble in HCl Wollastonite is a naturally occurring calcium metasilicate. It is the only commercially available pure white mineral that is wholly acicular (needle-like crystals). Wollastonite is available in fine particle size powders as well as fibrous 'high aspect ratio' products (20:1). This material has a very unusual texture, it does not flow at all (a hand full can be picked up with fingers downward). Wollastonite's unique qualities were first recognized in 1822 by an English scientist, Sir William Wollaston. However as a commercially available raw material wollastonite has only been available since the 1950s. Explosive market growth took place during the 1980s and 90s and major industrial sectors have adopted the material. Deposits are mined mainly in US, China, India, Mexico, Canada, and Finland. They vary in purity; some require almost no beneficiation; others may require removal of up to 80% impurities such as garnet, diopside, limestone, and dolomite (e.g. by magnetic separation, froth flotation, optical sorting). Synthetic wollastonite is also made by combining quicklime with quartz, calcium carbonate and calcium hydrate. No commercial products have the theoretical chemistry shown here. Example of Typical Data Appearance: Brilliant White Shape: Acicular Molecular weight: 116 Specific gravity: 2.9 Refractive Index: 1.63 pH (aqueous solution): 9.9 Water solubility (gms./100cc): 0.0095 Density (lbs./solid gallons): 24.2 Bulking balue (gal./lbs): 0.0413 Moh's hardness: 4.5 Coefficient of expansion: (in/in/degree C): 6.5 x 10 -8 Melting point: 1540 Nyad 325 on 325# sieve: 1.0 The fibrous form of wollastonite can be very beneficial in bodies. In low fired ceramics wollastonite reduces drying and firing shrinkage and drying and firing warpage. It also promotes lower moisture and thermal expansion in the fired product. It fires with no LOI and its fibers help vent out gassing. These factors have made it a valuable component in tile bodies, especially for fast fire. Vitreous and semi
vitreous bodies can also show reduced shrinkage with small additions (2-5%), however wollastonite becomes a stronger flux as temperatures go above 1100C. Wollastonite exhibits a slight solubility in water, but slips containing it can become more alkaline (potentially affecting rheological properties). At higher temperatures the powder form is valuable as a source of CaO flux in glazes (and bodies). The other main raw source of CaO is whiting but it releases a high volume of gases of decomposition which produce suspended micro-bubbles that demand slow firing to clear. Also, since wollastonite sources silica as well, glaze recipes employing it do not need as much raw silica powder. Further the SiO2 and CaO react more readily to form silicates. Thus wollastonite is used as a major flux in high temperature sanitaryware and electrical insulators. In glass and fiberglass making wollastonite melts more readily (lower energy costs) and microbubble generation is lower than limestone-sand mixes. Wollastonite has the ability to seed crystals (in glaze melts of sympathetic chemistry), and can be valuable to create special effects which depend on devitrification (crystallization during cooling). Since CaO tends to devitrify in high temperature slow cooled glazes wollastonite can be employed to make faster cooled lower CaO content ones exhibit the same effect. Wollastonite is also used in stain and frit formulations to supply CaO in a more easily melted form. Mineralogy vs. Chemistry, Wollastonite vs. Calcium Carbonate Wollastonite is an excellent demonstration of the fact that we must consider ceramic chemistry is a relative science and it is one piece in the glaze puzzle. The mineralogy of materials is another important factor to consider. For example, the melting temperature of a frit or glass is predictable, but since raw minerals are most often crystalline, the bonds holding the molecular structure together are more complex. The melting temperature of minerals of similar or even identical chemistry, for example, can be vastly different. To demonstrate we took a reliable cone 6 calcium matte glaze (Wollastonite - 34.0, Ferro Frit 3134 21.0, Kaolin - 45.0) and used the above technique to calculate an equivalent recipe employing whiting to source the CaO. We fired the two glazes side-by-side on upright tiles and in a flow tester to cone 6 (picture shown below). You might expect these glazes to fire the same since they have the same chemistry. Not so. The wollastonite version runs much more on the flow tester. This is because the wollastonite melts at a lower temperature than whiting or is more easily dissolved in the melting frit glass. Also, the entrained bubble population is much higher in the whiting version (whiting has an LOI of 45%). Additionally the wollastonite version is a silky pleasant matte, the whiting version is glossy. The former more fluid melt gives the crystals much more freedom to grow during cooling. In simply looking at the glazed tiles one might easily assume that the transparent glossy whiting version is melting more than the matte wollastonite one, however the opposite is clearly the case. This is a good reminder that ceramic calculations need to be viewed in perspective. They excel in ongoing predictions of how changes to existing material amounts in a recipe will affect fired properties. They are much less reliable as absolute indicators of properties of unknown glazes. Always remember that glazes are made of materials that have a chemistry, mineralogy and physical properties and you cannot ignore any of these.
Zinc Oxide Pure Source Of Zinc Formula: ZnO Alternate Names: ZnO, Zincite
Oxide Analysis Formula ZnO 100.00% 1.00
DENS - Density (Specific Gravity) 5.6 Zinc oxide is a fluffy white to yellow white powder having a very fine physical particle size (99.9% should pass a 325 mesh screen). It is made using one of two processes that produce different densities. The French process vaporizes and oxidizes zinc metal, the American process smelts a coal/zinc sulfide mix and oxidizes the zinc fumes. Ceramic grades are calcined, they have a larger particle size and much lower surface area (e.g. 3 square meters per gram vs. less than 1; however 99.9% still passes 325 mesh). Calcined grades are said to produce less glaze surface defect problems (although many ceramists have used the raw grades for many years without serious issues). You can calcine zinc on your own in a bisque kiln, fire it at around 815C. Calcined zinc tends to rehydrate from atmospheric water (and get lumpy in the process, calcining a mix of zinc and kaolin produces a more workable powder). Alot of zinc is used in crystalline glazes (typically 25%), because these have no clay content, they bring out the best and worst of both the calcined and raw materials. The raw zinc suspends glazes much better (the calcined settles out much more). The raw zinc takes more water, but since the glaze can thin out over time it is better to add less than needed at mixing time and mix thoroughly. The raw zinc screens better (although it can be a challenge to get either slurry through an 80 mesh screen). Zinc oxide is soluble in strong alkalies and acids. It can be an active flux in smaller amounts. It generally promotes crystalline effects and matteness/softness in greater amounts. If too much is used the glaze surface can become dry and the heavily crystalline surface can present problems with cutlery marking. Other surface defects like pitting, pinholing, blistering and crawling can also occur (because its fine particle size contributes to glaze shrinkage during drying and it pulls the glaze together during fusion). Zinc oxide is thermally stable on its own to high temperatures, however in glazes it readily dissolves and acts as a flux. Zinc oxide sublimes at 1800C but it reduces to Zn metal in reduction firing and then boils at around 900C (either causing glaze defects or volatilizing into the atmosphere; note that electric kilns with poor ventilation can have local reduction). While it might seem that zinc would not be useful in reduction glazes, when zincless and zinc containing glazes are compared it is often clear that there is an effect (e.g. earlier melting). Thus some zinc has either remained or it has acted as a catalyst. The use of zinc in standard glazes is limited by its price, its hostility to the development of certain colors and its tendency to make glazes more leachable in acids (although zinc itself is not considered a hazardous substance). Zinc oxide is used in glass, frits, enamels and ferrites. Zinc oxide is also used in large quantities in the rubber and paint industries; in insulated wire, lubricants, and advanced ceramics.
Mechanisms Glaze Opacifier - White Zinc oxide will produce opacity or whiteness, especially at low temperatures, if the calcium content is low. It does not opacify as well in boron glazes. It works well in combination with tin.
Zircon Formula: ZrO2.SiO2 Alternate Names: Zirconium Silicate
Oxide Analysis Formula SiO2 32.79% 1.00 ZrO2 67.21% 1.00
DENS - Density (Specific Gravity) 4.3-4.7 HM OH - Hardness (Moh) 7.5 M LPT - M elting Point (MP) 2550C SLBY - Solubility Insoluble in water, weakly soluble in acids Zirconium silicate (or zircon) is extremely stable and will survive to very high temperatures in a glaze melt without dissolving (although small amounts do dissolve). Zircon is the generic name for zirconium silicate, the trade names are different (for example, zircopax). The refractive index of zircon is high (particularly with micronized zircon, size less than 5 microns). Zircon is normally used in glazes for opacification (converting a transparent glaze to an opaque). Up to 20% may be required to opacify some transparent glazes, amounts beyond this reach saturation where crystallization begins to occur. The exact amount needed varies between different glaze types. It is thus most effective at low temperatures. Tin oxide can be a more effective opacifier than zircon (it has various advantages and disadvantages). Opacification can be a complex subject, and there are many other mechanisms for opacifiation (see Opacity in the glossary). Individual zircon particles are angular, very hard and refractory (amazingly, they do not dissolve into the glaze melt even when ball milled to exceedingly small particle sizes). High amounts of zircon opacifier can cause cutlery marking (because of abrading angular micro-particles projecting from the glaze surface). Zircon lowers glaze thermal expansion, but not by the same mechanism as other oxides that dissolve in the melt. Its low expansion will thus tend to reduce crazing. Thus, if the presence of zircon reduces thermal expansion enough that there is a danger of shivering, the glaze formulation may need to be adjusted to accommodate the zircon addition. It is often best to exclude the chemistry of the zircon materials from participation in glaze chemistry calculations, it is better to treat them simply as an addition and take into consideration their effect on glaze properties on a physical rather than chemical level. Zircon stiffens the glaze melt and this often needs to be taken into account (since the chemistry of a zircon-containing glaze might suggest that it is more fluid that it really is). Glazes lacking melt fluidity can experience problems with crawling, blistering and pinholing since they are lacking in the ability to heal disruptions. Adjustment of the glaze chemistry may be needed (increasing the flux content while maintaining the SiO2:Al2O3, for example). Many people do not fully appreciate the relationship between glaze color and opacifier content. Transparent glazes have depth, thus any colorants present produce deep and rich color (if the glaze does not crystallize of course). But when an opacifier is added to the glaze, the color depth is affected (according to the percentage employed). Thus a rich blue can turn into a dull pastel blue in a fully opacified glaze. The degree of color in a transparent colored glaze varies according to thickness whereas opacified colors to not suffer this problem (although this phenomenon is considered a benefit by many for its decorative appeal).
Because of its high thermal stability zircon is also employed in making various hi-tech porcelain bodies and materials. It is a major source for the production of zirconium oxide ZrO2. Zircon sand (which is milled to produce zirconium silicate powders) has become so expensive in 2011 that major manufacturers are considering or are already diluting their products with other materials (like kaolin). So 'heads up' if your glaze is not as opacified as it was before! Also, use care in pretesting zircon materials, they vary in quality.
Body Cracking and Dunting During Firing At the Medalta Potteries (in Medicine Hat, Alberta, Canada) during the 1920s they made stoneware crocks up to 60 gallons. These monsters weighed more than 200 pounds and had walls between one and two inches thick. These crocks were made from the same clays that are employed in Plainsman H550 functional stoneware today and the glaze was typical of the feldspar recipes we still use. Even if you could fabricate one of these and figure out how on earth they dip-glazed a 250 pound unbisqued vessel, it would certainly crack into many pieces as it split during firing heat up and dunted during cooling. What was the firing secret? Simple. Energy was cheap, huge beehive kilns the size of a house could be fired for less than a dollar a month! Kilns were hard brick and massive and the firing cycle was one week. That's right, seven days. To me the moral of this story is that firing needs to be tuned to the ware. Consider another case that the average potter would find equally mystifying. In industry today it is common for roller kilns to fire stoneware in a 2-3 hour cold-to-cold cycle. My experience tells me that this is impossible. How do they do it? Obviously, they can only do this with ware that is lighter. But still, it is amazing, there must be much more to it. Here are a few factors: First, they use a body that employs low lignite, large particle, lower plasticity clays in the lowest possible proportion so water and gases can be vented out quickly. Second, they fire smarter. There are firing consultants in industry that do nothing but design custom firing curves for manufacturers. In the firing of any body there are periods when temperature rise can be much faster and others when it must be slower. But how much faster and where? A thermogravimetric analysis (TGA) test weighs a clay sample during firing to determine when it expels the most gases. A differential thermal analysis (DTA) test reveals the periods of firing where the body is exothermic and where it is endothermic. With this and other information one can design a firing curve that provides for the shortest possible firing time. Third, commercial kilns fire very evenly, some expose ware to less than a degree of gradient. Draft is one factor, many kilns have burners that double as blowers and create a kind of 'hurricane' within the kiln that exposes every part of ware to heat. A fourth factor is not directly related to cracking but I will mention it. Modern glazes are formulated to be fast-fire. They are low in boron and remain unmelted until just before the end of the firing. This makes allowance for easy channeling of gases of body decomposition before the glaze melts. How long should a firing be? If ware is cracking and you don't want to get into complicated analysis of your firing curve then it should just be longer, it is a relative thing. However it does not take brain surgeons to fire smarter also. Hold at boiling point as long as possible (over night candling is best) and go up (and especially down) through quartz inversion slower (1050F, 570C). In electric kilns there is no draft, this is a real problem in avoiding gradients; you have no choice but fire slower in the hopes of getting a more even firing.
In Bound Links (Project) To Do For each trouble, link to appropriate tests.
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Example of dunting, where a crack has released the stresses produced by uneven thermal contraction during cool-down in the kiln. This usually happens by cooling too quickly through quartz inversion.
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Cracking during heatup. They start inward on the concave angles. It is important to create shapes not prone to cracking and smooth, compress and round abrupt contours and areas prone to cracks (to deny them a place to start).
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This crack likely starting during bisque. It started at a sharp angled indent on the outside (that coincided with a thin wall section) and grew around the perimeter (not visible). From there it branched to the base.
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This crack began as stresses created during uneven drying (the rim was allowed to get ahead of the base). A thinner section (that happened during throwing) was exploited by the stresses and a crack appeared during heatup, likely during the bisque.
Glaze Blisters Blisters are evident on the fired glaze surface as a 'moonscape' of craters, some with sharp edges and others rounded. These craters are the remnants of bubbles that have burst during final approach to temperature or early stages of cooling. In some cases there will be some unburst bubbles with a fragile 'dome' than can be broken. Blisters can vary in size and tend to be larger where the glaze is thicker.
Is the glaze fluid enough? Often glazes appear like the melt should have plenty of mobility to heal but this can be deceptive, a melt flow testing regimen is the only way to know for sure (melt flow testers have a reservoir at the top of a steep incline and the glaze runs down a calibrated runway). Generally a more fluid glaze will heal blisters much better (see section below on blisters occurring even after refire).
Are excessive gases generated during glaze fire? Significant amounts of gases can be generated within the glaze itself due to the decomposition of some materials after melting has started (i.e. dolomite, whiting, manganese dioxide, clays, carbonate colorants, etc). Substitute these materials for others that melt cleanly. For example, use frits, supply CaO from wollastonite instead of whiting or dolomite, use cleaner clay materials, or use stains instead of metallic carbonates. If you are using organic additives be aware that some of these can generate considerable gases during decomposition; do tests without them, use an inorganic substitute or find way to disperse them better into the slurry. You might be under estimating the amount of gases that are coming out. Are you holding the top temperature long enough? Perhaps a much longer than expected soak might be necessary (on very thick tile or sculptural pieces, for example, 24 hours might be needed). Could you do a test on a small piece to confirm this? It might also work to adjust the firing schedule to soak, decrease the temperature a little (so the glaze is still pretty fluid), hold it and then cool quickly for the next few hundred degrees to solidify the glaze.
Is the glaze recipe or chemistry the problem? The approaches to dealing with glaze chemistry issues differ in fast fire (e.g. tiles) and slow fire (studio pottery). In slow fire we want glazes that are mobile and can heal imperfections over a long soaking period. In fast fire we want glazes that remain unmelted until after 950C (gases from decomposition can occur up until this temperature) and then melt quickly after this. If you are firing fast then you need to use a fast-fire glaze formulation so the glaze does not begin to melt until after body gassing is complete (the whole modern whiteware and tile industries are built on this principle). In fast fire, matte glazes automatically have this property because the formulations to make a crystalline matte and a late melting glaze are the largely same. Glossy glazes, however require extra attention. Reduce zircon or alumina in the glaze melt to give it better flow properties. Or source them from a frit rather than raw materials. Reformulate the glaze to have more fluidity to heal imperfections (i.e. more flux or a lower alumina:silica ratio). Strontium carbonate can help smooth viscous zirconium glazes, small amounts of ZnO and Li2O can do miracles for glaze flow. Adjust the glaze so that it has a lower surface tension so that bubbles break more easily at the surface. Does the recipe contain binders? When do these decompose to create gases (it might be higher than you think)? Boron can induce blistering, especially if its amount is quite high (check limit/target formulas for guidance). The reasons for this phenomenon are not because of gassing (this is demonstrated by the fact that high boron glazes often blister worse on a second firing). Boron is a glass like silica
and it wants to form its own glass structures. High boron can thus cause phase separation (areas of discontinuous glass chemistry in the fired glaze, e.g. globules of a sodium borate glass in a calcium silicate glass matrix). Considering the important function of alumina in glass structure, the lack thereof would be an agravating factor in the separation. Phase boundary phenomenon and the differences in surface tension and melt fluidity of the phases could breed blisters. This process likely continues in a second firing (this accounts for blistering getting worse). Ferro Frit 3134, for example, has no alumina, lots of boron and plenty of CaO/Na2O, glazes high in it make ideal candidates for this phase separation.
Is the system is intolerant of gases? Gas release from decomposing materials in the body can continue until 950C. Many glazes begin melting long before this. In the single fire process (i.e. tile) gases have to bubble up through the glaze if it melts too early. The most important factors in producing flawless glaze results in single fire ware are a dense properly pugged or pressed clay matrix that is not too thick, the use of fast-fire glazes specially formulated to melt as late as possible, a firing curve that recognizes the need for a slower rate-ofrise at glaze finish temperatures, and a body made from clean materials and containing a minimum or organics. Use a body of finer particle size so that gases are channelled to many more surface sites of lower volume and thus do not overwhelm the glaze if they have to bubble through it. Minimize techniques that roughen or remove fines from the leather hard or dry clay surface of bodies that contain coarser particles. If necessary apply a fine particled slip to leather hard or dry ware to filter internal body gases into finer bubbles during firing. Apply the glaze in a thinner layer to minimize its ability to contain large bubbles. Use clays not containing large gas generating particles (i.e. pyrites, sulphates) Some fluid glazes (i.e. rutile-blue) tend to be quite sensitive to thick application and fast firing and cooling and bubbling problems with them seem out-of-place. Experiment with firing curves to learn where heat-up or cool-down rates need to be slowed.
Is the glaze firing part of the problem? Fire the glaze higher or adjust its formulation so that it melts better and more readily heals surface bubbles. In a slow-firing setting, you may need to soak the kiln longer at maturing temperature to give the glaze a chance to heal itself. In a fast-fire you need to do the opposite, soak only long enough to melt the glaze but not long enough to allow bubbles to grow. Fire the kiln slower during the approach to final temperature or down through transformation temperature. It is not easy to understand why very fluid glazes sometimes do not heal blisters well. Sometimes they are not as fluid as they appear, do flow testing to find out. It may be possible that they need to be cooled slower through the transformation process at which they begin to stiffen and solidify; this can be hundreds of degrees lower than the actual firing temperature if you are not using a fast-fire type glaze. Rather than trial and error firing tests to find a schedule that is sympathetic to your body-glaze combination have your body evaluated for TGA and DTA. Thermal Gravimetric Analysis provides information on body weight loss during the whole firing curve so it tells you when gases are being generated. Differential Thermal Analysis shows where in the firing curve the body behaves endothermically and exothermically. An expert can use information from these tests and others to tune a firing schedule perfect for your situation. In the USA The Orton Ceramic Foundation can do this type of evaluation.
Is it being firing in a gas kiln? Avoid very heavy reduction followed by periods of oxidation. It is best to start reduction one or two cones higher than the bisque temperature, this period in the glaze kiln can oxidize any remaining potential 'blister producing' volatiles that the bisque did not
take care of. Avoid flame impingement on the ware. Make sure that stage one of the glaze fire is truly oxidizing to avoid buildup of internal carbon in the body. Watch the kiln to make sure there is plenty of oxygen present at all times.
Is the body the problem? Does the bare fired clay have a glassy film? Soluble salts within the body can move out to the surface during drying. If these are high in fluxing oxides they act as a very reactive intermediate layer between glaze and body. This can amplify existing pinhole contributors or produce glaze surface irregularities that are akin to pinholing. Add barium carbonate to the body mix to precipitate the solubles within the body or substitute implicated materials in the body batch. Is the ware being dried too slowly after firing? In industry great care is taken to accelerate drying to minimize dissolution of compounds by the water from the glaze (and too minimize issues with glaze adherence that extended drying times can raise). These compounds, that can thus cause blistering, are thus trapped inside the body. Use a body that generates less gases of decomposition. For example, the tile industry uses low lignite ball clays both to enable fast fire and to get better glaze surfaces. The ball clays can be surprisingly light in color, some resembling kaolins (i.e. Spinks Champion ball clay). Does the body contain barium carbonate particles? Screen a sample on a 200 mesh sieve. Does it contain non-white particles or unground barium? Contaminated barium can cause severe pinholing. Is the body too dense to enable the gases to free escape during the period before the glaze melts Blisters and pinholes can share the same causes. Check the article on pinholing for more information on body problems that can cause glaze defects.
Is the problem in the glaze mixing? Most companies ball mill their glazes and for good reason. It is not uncommon to mill glazes up to 12 hours. Blisters and surface imperfections are often caused by impurity particles in the glaze layer itself, grinding these down as small as possible will minimize the ability of individual ones to cause problems. Don't assume your ball mill is working, some configurations will not grind a glaze fine enough no matter how long they run. A variety of contaminants can find their way into glazes and certain materials are potent sources of bubbles (e.g. plaster). Silicon carbide is an example, if you are doing any cutting or grinding using an SiC abrasive particles could be finding their way into your process (you might consider using alumina based abrasives).
Is the problem glaze application? Have you changed the way glaze is applied? Have you changed employees doing the glazing? Do you have a quality control mechanism to record aspects of the glaze lay down (e.g. thickness, density, microscopy, resistance to rub-off/dusting). A thicker glaze application is, of course, much more likely to blister. Many companies target very high specific gravities in their glazes (i.e. 1.8) to achieve a dense laydown (your application method may limit this). This minimizes entrained air and thus imperfections. Certain application techniques produce a better laydown, others produce a fluffier layer (e.g. spraying). If this is your case thinking about ways to densify the dry layer. Do not put wet ware into the kiln, a variety of problems related to the nature of glaze laydown can result. It is surprising how high the temperature can get and yet steam still be present inside the kiln.
Are you bisque firing? Is it done right? All clays release gases from burning of carbon material and decomposition of other compounds. Some clays release sulphur compounds also. If the glaze is melting during release of these gases, they must bubble up through it. If the melt is stiff, the kiln is ramped up too quickly, cooled too rapidly, or the
glaze melts too early, it will not have opportunity to heal properly. Make sure the bisque fire has good ventilation, has a clean oxidizing atmosphere, is long enough, and that ware is stacked to expose maximum surface to oxidation. Tightly packed electric kilns lacking a venting system require extremely slow and thorough firing (especially through the red heat to 900C range). The superior ventilation in gas kilns makes them best for bisque firing. Bisque fire as hot as is practical (cone 04-02) and vindicate bisque temperature with standard cones. A hot bisque is necessary to burn out any sulfur that might be present. A hotter bisque means denser ware and it may be necessary to adjust glazes to be thixotropic so they will apply well to the less absorbent body. Although you may not be accustomed to glazes that will stick to less absorbent bodies, be assured that this is very feasible. One caution however: If you ware is burnished, it is not usually advisable to bisque above cone 08 or the burnish can be lost. Bisque fire as slow as is practical. Slow fire through the period where the most gases are generated from the oxidation of organics in the body (usually from 700C to 950C). 50C per hour is considered 'slow' If you have an electronic kiln controller experiment using a fast firing curve slowed down at various temperature ranges. This will help you determine the range at which it is most critical to fire slower. Make sure that reduction does not occur during any phase of bisque or reduced iron (FeO) could play havoc with latter stage of the firing). If you do not have humidity drying equipment candle periodic kilns overnight before bisque firing the next day. This will assure that ware is completely dry and that firing can proceed quickly to past red heat, leaving more time for the carbon burnout phase.
Do blisters get worse even if you fire ware again? This often happens and it is not easy to understand since one would think that there can be no source of gases if the piece has already been glost fired. Regardless of the reason if a glaze is not healing its blisters on multiple firings then it is not fluid enough. One does not fully appreciate how stiff the average glaze melt is until you work with crystalline glazes that are so fluid a bowl must be placed under the ware to catch the runoff. However the fired surfaces of these glazes are incredibly glossy and perfect. If your glaze melted more it would run more, however you can counter this by putting it on thinner. The melt fluidity of a glaze is primarily affected by the amount of flux, so you need to increase it. However if the flux you choose has a higher thermal expansion be prepared for the glaze to craze. This is actually a job forINSIGHT.
Glaze Crawling Crawling is where the molten glaze withdraws into 'islands' leaving bare clay patches. The edges of the islands are thickened and smoothly rounded. In moderate cases there are only a few bare patches of clay, in severe cases the glaze forms beads on the clay surface and drips off onto the shelf. The problem is most prevalent in once-fire ware.
Is the glaze shrinking too much during drying? If the dried glaze forms cracks (or in serious cases flakes that peel and curl up at the edges) it is a sign that the glaze is shrinking too much. These fault lines provide places for the crawling to start. There are a number of possible contributors: If very fine-particled materials are present (i.e. zinc, bone ash, light magnesium carbonate) these will contribute to higher shrinkage during drying. Try using calcined zinc, synthetic bone ash or another source of calcia, talc or dolomite to source magnesia instead of magnesium carbonate. It is normal to see 20% clays (ball clay, kaolin). If significantly more is present try using a less plastic clay (i.e. kaolin instead of ball clay, low plasticity kaolin instead of high plasticity kaolin, or a mix of calcined and raw kaolin). If you are using Gerstley borate, try a boron frit. You may need to do calculations to make these adjustments. Ultimately you need to tune the glaze's clay content to achieve a compromise of good hardness and minimal shrinkage. If a glaze has been ball milled for too long it may shrink excessively (for example, zircon opacified glazes can be ground more finely than tin ones). Highly ground glazes may produce a fluffy lay down. If a slurry has flocculated (due to changes in water, dry material additions like iron oxide, or addition of an acid, epsom salts, calcium chloride, etc) it will require more water to achieve the same flow and will therefore shrink more during drying and require a longer period to dry. Try using distilled water. Always measure the specific gravity to maintain solids content and use deflocculants/flocculants if necessary to thin/thicken the slurry (you can remove water from an existing glaze slurry by pouring some on a plaster batt, then mixing the water-reduced mass back in). Gerstley Borate is plastic and therefore contributes to glaze shrinkage, especially if the recipe already contains kaolin or ball clay. It also tends to gel glazes so they need excessive water. Use boron stains instead. It is possible to create glaze slurries that gel and flow extremely well using the right kaolin (i.e. EPK) in adequate amounts. This requires a glaze base whose other materials do not contribute too much Al2O3. We have a separate article on glaze slurry properties that deals with this (see links).
Is the glaze's dry-bond with the ware surface inadequate? The mechanism of the bond is simply one of physical contact, the roughness of the ware surface combined with the hardness of the glaze determines its ability to 'hang on'. Some surfaces can be very smooth (e.g. slip cast surfaces). To give the glaze better ability to hang on, there should be some clay in the glaze mix to both suspend the slurry and toughen the dried layer. If ware is also excessively powdery to handle this is a signal to incorporate more plastic clay, add a little bentonite, or add a hardener like gum. Add gum to glaze to bond better to bisque. If a glaze is deflocculated it may lack the necessary fluidity to run into tiny surface irregularities in the bisque and establish a firm foothold. Wetting agents are available and can be added to the slurry to improve bond.
Does application technique or handling compromise the fragile glaze-body bond? Make sure ware is clean and dust free, even oil from ones skin can affect glaze bond. If glaze is applied too thickly the forces imposed by its shrinkage will overcome its ability to maintain a bond with the ware surface (especially inside corners or at sudden discontinuities). If a glaze can be applied more thinly, you should do so. Use a fountain glazing machine to do the insides of bowls and containers to achieve a thinner layer. If glaze needs to be applied in a thick layer, you can achieve a lower water content by deflocculating the glaze (i.e. with some sodium silicate or Darvan), however it may then tend to dry very slowly or form drips that crack and peel and instigate crawling. When applying the glaze in the normal layer thickness be careful to prevent drips that form thicker sections that can crack away during drying. It is practical to 'gel' the glaze slightly (i.e. with vinegar, Epsom salts) so that it 'stays put' after dipping or pouring. If a double-layer of glaze needs to be applied be careful that the second does not shrink excessively and pull at the first, compromising its bond with the body. If possible, the upper layer should have less clay and lower shrinkage and should dry quickly. It may be necessary to bisque each layer on before applying the next. Double-layering typical raw art and pottery glazes is difficult, special consideration must be given. If you have successfully done it in the past without any special attention then you may have simply been very lucky. When doing double-layer glazing be careful that the second layer is not flocculated (with an associated high water content). This will rewet the first layer and loosen it from the body. Adding iron oxide, for example, to a glaze will often flocculate it and require the addition of much more water to restore the same fluidity. Spraying glaze on in such a way that the glaze-body bond is repeatedly dried and rewetted could produce shrinkage-expansion cycle that compromises a glaze-bisque bond that could otherwise withstand one drying-shrink cycle. Force-drying of the ware can make the glaze visibly crack when it otherwise would not (slower shrinkage associated with slower drying gives it the glaze time to ease body interface tension by micro cracking). Preheating the bisque may cause escaping steam to rupture the bond with the ware. Rough handling of ware can compromise sections of the glaze body bond. Consider pouring a thin glaze slurry into the mold of a just-drained piece (perhaps a minute or two after the mold has been drained) and immediately pouring it out again. This base layer can be fired on in the bisque. It might be enough to prevent crawling when the piece is glazed later.
Is the glaze drying too slow? If the glaze dries too slowly the most fragile stages of adhesion are extended and cracks in the dried glaze layer can appear. Bubbles in the wet glaze layer can also form during the drying, these become areas of no bond with the underlying body and therefore can instigate crawling during melting. This can occur if ware is very thin, glaze has a high water content, or if ware is already wet when glaze is applied. To speed up drying try preheating the bisque (in a kiln to 150C or more if necessary), doing separate interior and exterior glazing, make ware thicker and better able to absorb water or apply the glaze in a thinner layer.
Is the ware once-fire? Once-fired ware is much more prone to crawling because the mechanical glaze-body bond is more difficult to achieve and maintain. If glaze is applied to leather hard ware it must shrink with the body. During the early stages of firing the ware also goes through volume changes and chemical changes that generate gases, these make it difficult for the glaze to hang on. When glaze is applied to leather hard ware you must be able to tune its shrinkage by adjusting the amounts and nature of the clays in the recipe (calculations may be needed). Once-fire ware must not be fired too quickly, especially through the water-smoking period. Make sure ware is absolutely dry before firing.
In damp conditions the powdery layer may reabsorb water from the air causing slight expansion and breaking of the adhesion.
Is the problem happening during firing? If glaze is applied over stains or oxides that lack flux (e.g. chrome pinks, manganese types, greens, cobalt aluminate) they will act to prevent bonding with the underlying body. Mix underglaze stains with a flux medium so that over lying glazes can 'wet' them and form a glassy bond. If the glazed ware is put into the kiln wet and therefore dried quickly during the early stages of firing the glaze layer will tend to crack and curl and crawling can occur. If glazed ware is put into a kiln containing heavy damp ware such that early stages of firing occur in very high humidity conditions the glaze could be rewetted and forced through an expansionshrinkage cycle that could affect its bond with the body. If a glaze contains significant organic materials (i.e. gums, binders) that gas off excessively during firing the glaze-body bond may be affected. Decomposition of materials like whiting can also generate significant amounts of gas within the glaze layer (try switching to wollastonite, it supplies SiO2 also and will allow you to reduce the silica content accordingly). Raw zinc oxide is very fine and tends to pull a glaze together during firing, use calcined zinc instead. If the glaze contains significant zircon opacifier, alumina, some stains, magnesium carbonate, the melt may be much 'stiffer' and flow less. This can affect its ability to resist crawling. Watch out for glazes with slightly soluble materials like Gerstley Borate or wood ash. With the former the partly soluble and the soluble portion tends to be the borate which will be absorbed into the bisque during application and then during firing creates a highly fluid layer between the body and the less developed glaze and thereby prevents adhesion of the glaze to the body (use frit to source boron instead). In addition soluble materials tend to flocculate (thicken) the slurry and attempts to thin them result in higher water content and therefore increased shrinkage. If the bisque firing is reduced or not adequately oxidized and excessive gases are generated during certain stages of the glaze firing, these can affect the glaze-body bond. If bisque ware is dense and non-absorbent (fired too high) it may not form a good bond with the glaze. The chemistry of glaze may be such that the surface tension of the melt encourages crawling (e.g. high alumina, high tin, significant chrome/manganese colorants, lack of fluxes of low surface tension).
Is there a problem with the body? If the clay body contains soluble salts that come to the surface during drying, these can affect the fired melt's ability to form a glassy bond with the body. Precipitate these salts with a small addition of barium carbonate to the body (for information on how this works search for Barium Carbonate in the materials section). An noted above, if the body surface is too smooth, the glaze may not be able to adhere properly.
Glaze Crazing The fired glaze exhibits a network of fine cracks. These may be plainly visible after firing or may need enhancement with ink. Crazing may also appear after a period of time or after ware has been exposed to thermal shock. Fired strength(an thus functional ware quality) are directly related to crazing since ware strength is enhanced by having the glaze under slight compression whereas it is severely reduced (up to four times less) when the glaze is under tension. If the underlying clay matrix is porous and soaks up water thensafety could be a concern with crazed ware since the cracks could be wide enough to provide a friendly breeding ground for colonies of bacteria. Containers used to store food are a special concern since a small colony in a crack can become a large culture in the food. If you have any doubt whether this is an important issue ask a commercial food service inspector about the subject.
Is the crazing a result of mistreatment of ware during use? If pieces must survive considerable thermal shock during use, then both ware and glaze need to have a low and linear thermal expansion curve and they must be compatible. This is difficult to achieve in low fire ware because little mullite or other low-expansion silicate minerals develop during firing. If your low fire body contains significant talc, reduce or eliminate it (also adjust glazes to have a lower expansion so they continue to fit the body). If your high fire body develops non-linear expanding cristobalite during firing, find a way to reduce this.
Is crazing a result of inappropriate choice of manufacturing method or materials? High temperature firing is by far the best for the production of low-expansion ware. Many more minerals are available for both body and glaze mixes and higher temperatures produce lowexpansion silicates and aluminates that give tough glaze and body matrixes capable of withstanding forces that might otherwise cause crazing. If ceramic ware is porous it can soak up water that causes the ware to expand, thereby putting tension on the glaze and crazing it.
Is crazing due to a simple thermal expansion mismatch between body and glaze? Fired ceramic expands and contracts as it is heated. If the fired glaze has a significantly higher co-efficient of expansion than the body then no amount of careful firing or thin glazing will avoid the inevitable crazing. This is by far the most common cause of crazing and solution strategies are case studies of applying ceramic calculations to solve problems. If even only one piece crazes it is often a sign that all the other ware in that kiln will eventually craze. Such glazes usually need drastic changes since crazing is a visible manifestation of a fit problem that has already greatly reduced ware strength. Lower temperatures are far more sensitive in this respect in that there is a much narrower range within which a glaze and body will be compatible. To improve glaze fit adjust the clay body to give it higher expansion and thereby the greater contraction that compresses glazes to prevent crazing (i.e. increase silica for high temperature bodies, talc at low fire). You can also adjust the glaze to reduce its expansion. There are many ways to do this. For example, if the glaze is melting well and it is not a matte, try increasing the silica. Or try introducing boron at the expense of some of the flux since B2O3 contributes to both glass development and melting. You can also introduce fluxing oxides of lower expansion at the expense of those with higher expansion in such a way that the fired properties are not changed too much; for example try adding CaO, MgO, or ZnO at the expense of Na2O and K2O (crazing is most serious with sodium and potassium glazes, to demonstrate mix nepheline
syenite and water and apply as a glaze and fire at high temperature). If your glaze is opaque try using more low-expansion zirconium opacifier or use it instead of tin or titanium. Zirconium opacifiers are also useful in transparent glazes; they have a threshold amount under which they do not normally opacify. Thus it might be possible to add as much as 5% to make the glaze both more durable and reduce its expansion. Consider also the elasticity of the glaze as even relatively well fitted ones can craze if exposed to radical temperature changes. High levels of sodium, potassium and calcium can make the glaze more brittle (the former also increase thermal expansion). Boric oxide is known to improve elasticity. If the body expansion is too low (i.e. ovenware and flameware bodies) it can be very difficult to fit a glaze that has the desired visual characteristics. Lithium can dramatically reduce the thermal expansion of glazes, but its use requires a lot of testing since its contribution is not linear and it introduces other dynamics that must be considered.
Could the Coloring Oxides in the Glaze be Involved? Generally increased additions of iron and copper oxide to a glaze will reduce crazing (if they are present in adequate amounts; beyond 1 or 2 percent). Cobalt could have a moderate lowering effect, but since so little is typically used in glazes it will not be significant.
Is the crazing a result an under fired body? Underfired bodies may contain uncombined alkali or alkaline earths than can react with water and swell the body. You can test this by putting a glazed sample in a pressure cooker for several hours or put a shard into an autoclave to see if crazing appears. Calcium carbonate is added to talc bodies to minimize moisture expansion.
Is the crazing a result of sloppy manufacture? Normally a glaze/body combination with compatible expansion characteristics will withstand considerable firing and usage abuse without displaying signs of crazing. However, in some cases, a glaze that otherwise 'fits' will craze if applied very thick. Also, if the kiln is cooled very quickly or unevenly, especially if ware is thicker, the severe stresses can produce crazing. However remember that a glaze's ability to withstand normal or even quick kiln cooling is an indicator of its ability to resist crazing in normal use.
Is ware crazing days or even months after firing? If you are cooling your kiln very slowly to prevent ware from crazing it is likely the glaze does not fit. While it may be true that slower firing seems to solve the problem, time will bring out the crazing that the kiln did not. In fact if you must slow cool to prevent crazing it is a virtual certainty that your glaze needs to have its thermal expansion reduced. Special Note: Solving crazing and shivering problems while retaining the visual character of a glaze is a classic problem for the application of ceramic chemistry calculations. There is a chapter in the lesson section of theINSIGHT manual on how to deal with this problem, it is a very practical approach.
Glaze is Off-Color If your fired glaze is not the expected color here are some questions to ask.
Does the development of the color depend on the chemistry of the glaze? In ceramics, color is about chemistry and melt dynamics, colors do not normally 'burn out'. The development of many colors requires that the host glaze's chemistry be sympathetic. For example chrome-tin pinks require glazes with minimum 10% CaO (calcium oxide) and B2O3 (boric oxide) must be 1/3 or less the CaO content. Certain blues require the presence of BaO (barium oxide). The presence of ZnO (zinc oxide) is hostile to the development of many colors, as is MgO (magnesium oxide). Stain companies know all about this. Their websites and brochures have notations for many of the colors that tell you what chemistry the host needs and what conflicts to watch for. You might even consider phoning their technical staff.
Is there enough color in the glaze? Or too much? Metal oxide colorants or colorant blends darken glaze color as their proportion is increased. But the change is usually not linear and at some point maximum color is achieved and further additions will often begin to produce metallic, crystalline or matte effects (at this point the glaze can be unstable and leach metals into liquids and may even oxidize in air). The saturation point of a color may also be different in different host glazes.
Is the glaze opacity correct? The brightness of color also depends on host glaze opacity. Opaque glazes give flatter and lighter colors because you are only seeing the color on the surface, translucent and transparent bases enable you to see down into the glaze (thus the increased depth and vibrancy color).
Is the glaze developing micro-bubbles? Excessive bubble entrainment in the glaze matrix can alter color considerably. Micro-bubbled transparents become quite cloudy and colors will be subdued, especially if the glaze is transparent and lies over oxide decoration (which might be gassing to create the bubbles).
Is the glaze developing crystals? Does its color depend on the development of crystals? Crystals grow in some glazes during cooling of the kiln. Certain glaze chemistries and (mineralogies of ingredients) encourage crystal growth (i.e. low alumina, high zinc, too much flux). Cooling the kiln slowly during the period when the glaze is freezing promotes crystal growth. Many of the metal oxides freely participate in crystallization and the range of mineral crystal species they can form is amazing. A high-iron fluid glaze, for example, may fire glossy and almost black on quick cooling, but it may turn a muddy yellow on slow cooling (because the surface is covered with micro-crystals of iron).
Is it a reactive glaze? The character of a glaze can depend on additives that mottle and variegate the character of the color (i.e. titanium, rutile). Such additives may produce a melt of discontinuous fluidity (rivulets flowing around more viscous areas of the melt). These effects can combine with crystalization and variations in opacity to make stunning surfaces. Alas, such are troublesome. Materials like rutile can be variable and the effects they create are usually fragile. It is easy to predict consistency problems for such mechanisms. Potters can fiddle with reactive glazes, but industry generally stays away from them.
Do the results depend on a fragile melting mechanism? Is vigorous melting (and running) required to develop the color and character? As noted above, such glazes may not only be prone to color problems, but also running and blistering. Glossy rutile-blues are an example. Another thing to remember is that certain raw colors and stains volatilize (vaporize) above certain temperatures.
Kiln atmosphere, Ramp The mechanism of color development in a glaze may depend on kiln atmosphere (i.e. strong reduction, weak reduction, strong oxidation), or on the speed or curve of both the ramp up and down. Your kiln may have variations in the atmosphere or your electric kiln might be firing near reduction because of poor airflow combined with carbon burn-off.
Has it been put on the right side of the glaze layer? The same metal oxide will develop different colors depending or whether it is painted under or over a glaze. If it is painted under, for example, glaze thickness, bubble population, crystal development and chemical interaction between glaze and color will shape the effect.
Would a stain be better? Achieving and maintaining an exact shade of color can be quite difficult with raw coloring oxides, especially if a blend is being used. For example, many people use cobalt, iron and manganese for black. However color shifts are common with this approach and it is usually not obvious which metal oxide should be increased or reduced to stabilize the color. Stain companies have invested considerable time to develop colors that are reliable and stable (often containing zircon, alumina, silica in addition to the metal oxides). Stains are more expensive, but the stain company assumes a burden that is often difficult for most companies or potters to handle.
Is the glaze the right thickness? On the right body? Many glazes develop deep color only if they are applied thickly enough. Others develop the desired effect when they are thin and the underlying body imposes some color. Light colored clay bodies foster the development of bright colors, iron bearing bodies subdue colors (especially when the glaze is thin). Many glazes will develop color of different character on refractory porous bodies compared to vitreous ones.
Water Contaminants It is standard practice to use filtered or distilled water for all glazes in industry. There are so many possible contaminants in water that companies cannot possibly deal with the kind of variation that can occur. Water can contain compounds of iron, sulphur, manganese and a host of sulfates and salts (and even particulates like coal dust). You might conclude that the proportions of these impurities is not sufficient to stain a body or glaze, however it is important to remember that they are soluble. That means that during drying, they are all transported to the surface by evaporating water and left concentrated there in a thin layer that will vary according to the thickness of that section of the piece. This is certainly enough to create a yellowish or brownish tinge, for example. In addition, soluble impurities in the water can and probably will affect the rheological (e.g. viscosity, thixotropy) properties of the glaze slurry. This in turn can cause thinning and settling and separation of the glaze suspension, crystallization of certain materials, thickening, etc. All of these will affect the chemical and physical homogeneity of the glaze laydown and its thickness, these of course, can effect the fired results (which include color).
Conclusion Try taking a cheap microscope and have a really close look at your glaze surface. You might be surprised at now much you learn about why the glaze looks the way it does. Understanding the
mechanism of the color and surface will help you understand how to trouble-shoot problems. It does not take rocket science, anyone can note the transparency, micro-bubbles, crystalization, variegation in color and surface (phase differences), etc. And do not shy away from chemistry, in many cases you just need to know if an oxide is present or not and how much is there. Search for 'ceramic chemistry' on google, download a free trial of INSIGHT and work through the lessons section of the manual to learn how to enter a recipe and see its formula and analysis.
Glaze Marks or Scratches 'Cutlery Marking' occurs where metal instruments leave marks on glazed functional ware. This happens because the glaze is not smooth, it is abrasing microscopic particles of the metal. However if the marks left by these particles cannot be removed easily this is more than a cosmetic problem. It suggests that they are trapped in surface pores or irregularities (pores are a possible sign of under melting). This is a very different situation than if a sharp hard metal object can scratch the surface. Such a glaze is definitely soft and lacks resistance to wear (and has the potential of harboring bacteria). Even glossy glazes that appear hard can often be scratched easily. In general, the higher a glaze is fired, the better the potential to produce a hard and smooth surface. This is because high fire glazes require less flux and therefore have more silica and alumina. While a capable technician can produce a relatively hard glaze at any temperature range, a less knowledgeable or attentive person can make soft glazes in any range also. The chemistry principles of making a hard glaze are well known.
Compare the glaze to a known hard glaze using a simple scratching test. Use a concrete nail or the sharp corner of a file (these are about 6.5 hardness on the Mohs approximate scale of 1=talc, 2=gypsum, 3=calcite, 4=fluorite, 5=apatite, 6=orthoclase, 7=quartz, 8=topaz, 9=corundum (ruby or sapphire), 10=diamond). Another excellent hardness testing method is to direct a sandblast at the surface at a 45 degree angle. Microsurface optical or electron analysis can then be used to accurately rate abrasion resistance (equipment to do accurate surface plots is now quite common in many industries, search the internet or check with some labs or universities).
Is the surface smooth? Can you mark the surface with a fork or knife? If a glaze surface has angular protrusions then it will be abrasive. This is often the case in glaze that feels silky to the touch. Microscopic sharp edges will cut away minute chunks of metal, possibly holding them in surface voids. Does the glaze contain zirconium opacifier? Zirconium-silicate particles do not enter the melt and they are angular and can protrude from the glaze surface. If you can make a line even with a hard metal object this confirms that the surface obstructions are very hard. You may need to ball mill finer, use a different or less opacifier, use a transparent overglaze, or employ a different base glaze that better envelopes the zircon. Use a microscope to check this. Does the glaze contain calcined alumina? As with zircon, you may need to use a finer size or mill the glaze more. Don't assume your ball mill is doing the job without testing particle size or surface area, a badly configured mill won't grind fine enough no matter how long it runs. Surface crystallization can produce an angular irregular abrasive surface. Islands of micro crystallization may be occurring even though the surface looks and feels smooth. Use a microscope. Check the glaze's chemistry to see if it is susceptible to crystal growth during cooling. Typically glazes low in alumina will devitrify (crystallize) during cooling. Increase the alumina to stiffen the melt and reduce the problem. Try cooling the kiln faster if other factors allow. Sometimes a slightly faster cooling cycle will not only reduce the crystals, but change their character to be less problematic. Is something nucleating the crystals (i.e. illmenite, wollastonite, titanium)? If the glaze is a crystalline matte you will need to rationalize it's appearance. Changes made to reduce or eliminate crystallization will affect the visual character. Sometimes smaller changes to glaze make-up to simply reduce devitrificaion are helpful. Or changes to the firing curve can be made to grow a finer crystal mesh. Consider switching to a high alumina matte since they have smooth (although not flat) surface. Or you might consider employing a different crystalline mechanism.
Are marks difficult to remove? Is the glaze mature? If the glaze is not fired high enough it will simply not melt adequately. The incompletely developed surface will be both abrasive (from undissolved abrasive particles) and lacking in hardness. Try firing the glaze higher to see if it improves. If it does, adjust your body to work at higher temperatures, or adjust glaze chemistry to melt lower. Sometimes only small additions of Li2O or ZnO, for example, can give much better melts. Some soft glazes are volatile. If fired exactly right they are OK, but variations in the process result in problems with cutlery marking from time to time. Test your glaze at higher and lower temperatures to span variation typical in your kiln. Volatile glazes are typically unbalanced in their chemistry (one oxide will be very high or silica/alumina very low). Alumina is a key to glaze hardness, the more present the harder a glaze will be. Inadequate alumina will contribute to glaze solubility also. While it is true that matte glazes often have high alumina, glossy results are dominant and most glossy glazes can tolerate additional alumina without noticeable visual change. Higher temperature glazes or low to medium ones containing significant boron can often tolerate a higher than expected alumina increase, especially if you source it from a feldspar or frit. Thus you might even consider adding a little boron to lower firing glazes so they can accommodate more alumina. Although keep in mind that excessive alumina in a well-melted glaze can crystallize aluminates. Glazes lacking glass former SiO2 are likely to lack hardness. Check typical limits for the temperature range and type of glaze. If your glaze will tolerate more silica then put it in. If not then firing higher or adding some B2O3 will enable the use of more SiO2. Better yet, use a finer grade of quartz (i.e. 15 micron, however make sure it is does not agglomerate during application). Zircon will improve hardness so use it as the opacifier (however remember that it can contribute to cutlery marking as outlined above). Although zirconium is considered an opacifier, many transparent glazes can tolerate 3-4% of a fine grade without loss of transparency (especially borate glazes). Put as much in as your glaze will tolerate. Source it from a zircon frit if necessary). Magnesia can reduce hardness so reduce it if you can. Magnesia holds thermal expansion down (and therefore tendency to craze) so consider carefully what to replace it with (perhaps one or more of SrO, Li2O, CaO). If you are firing ware at low temperatures, consider using a fritted base or a commercially mixed powder. While durable ware can be made at lower temperatures, it is much more technically challenging. High borate glazes are often unbalanced and not only lack resistance to marking, but are leachable. Flux saturated reactive art ware or pottery glazes are often lacking in hardness. It is common to see high temperature glazes, for example, that contain 70% or more feldspar and little or no silica or kaolin. While they are visually pleasing, they lack the necessary silica and alumina to form a hard glass.
Glaze Pinholing, Pitting 'Pinholes' are small holes in the fired glaze surface penetrating down to the body below, often into a surface pore or opening. 'Pits' are smaller, they mar the surface but to not penetrate all the way down. Pinholes or pits are often no larger than the head of a pin. During firing bodies typically generate gases associated with the decomposition of organic materials and other minerals, escape of crystal water, etc. If ware is glazed these gases may need to bubble up through the glaze melt, depending on how early it begins to melt. The causes of pinholes can often be similar to those of blistering. Keep in mind also that larger pinholes may actually be crawling (see links to other articles). In the following I may confuse pinholing and pitting or may neglect to mention one or the other, I apologize for this. When pinholes or pits occur there are often more than one contributing factor. Generally a true pinhole is a problem with the body that extends up into the glaze whereas a pit could be considered a problem with the glaze or the firing. Still most strategies to eliminate these involve attack on several fronts: Reducing burn-off by higher bisque or cleaner body (less lignite for example) Distributing body out-gassing by finer grinding Giving the gases more time to escape by slower firing or using a fast-fire glaze that melts later Giving the glaze time to heal by soaking or slower cooling Providing more kiln draft to oxidize and carry away products of decomposition coming from the body or glaze Making the glaze more fluid or altering its surface tension to enable it to better heal itself Selecting glaze materials that decompose to form less gases Being careful to apply a dense even lay down of glaze. Hobby and small scale producers have the flexibility to do much longer firings and generally must do so for the lack of fast-fire equipment and materials. Industrial producers must find ways to fire quickly, often in an hour or less. Strangely, even though small scale producers fire much slower, they can have just as many problems with pitting and pinholing. Some are using prepared bodies and/or glazes and thus have less flexibility to change things. Keep this factor in mind as you read the material below, the world you are in will determine the validity of the comments being made. If a pitting or pinholing problem has started to happen and it has not occurred before do not assume that there is some new problem. If reading this article makes it clear that there are some things that you have been overlooking, then the success you have had up until now might be accidental. This may be an opportunity to make your process better and more stable.
Is the body the problem? Are large particles or gas producing materials present? Do a sieve analysis of the body to determine if large particles are present. Weigh, fire to cone 04, and re-weigh a sample of the coarse particle material to see if it loses significant weight (due to decomposition and associated gas generation). If the particles are volatile (i.e. lignite, sulfur compounds) they will generate high volumes of gases at individual sites, possibly overwhelming the glaze's ability to heal itself there. The most practical solution is to either remove the implicated material from the body batch in favor of a finer particle grade (to distribute gas generation to more sites of less volume) or use a cleaner alternative (by cleaner I mean low-lignite and low-sulphur ball clays).
Are the particles melting vigorously? Use a sieve to isolate some of the coarser particles and fire them to body temperature. Fire to see fi any of them are active melters. Examine pinholes under the microscope so see if a glassy pool exists at its base. If this is the case it is possible that a combination of vigorous melting activity and the resultant creation of a glass chemistry that resists pinhole healing could be
occurring. In this case, the offending particles in the body must be eliminated or ground more finely.
Troublesome materials in the body? If you can see 'white spots' and dimples on the glaze surface this suggests that pinholes and imperfections existed but have healed incompletely (these may also suggest that the glaze melt does not flow as well as its glossy surface might suggest, more flux or later melting might be needed). Even fine particled bodies can gas badly, especially if they contain materials like talc, dolomite, or whiting that release high volumes of gas. It is common for some talc to be used as a flux in middle fire bodies (e.g. 2-5%) and there is not really a practical alternative that is as effective and inexpensive. That means that the firing curve must take the decomposition of talc into account slowing down the firing when this occurs.
Are there soluble salts in the body? Does the bare fired clay have a glassy film? Soluble salts within the body can move out to the surface during drying. If these are high in fluxing oxides they can act as a reactive intermediate layer between glaze and body. This can amplify existing pinhole contributors or produce glaze surface irregularities that are akin to pinholing. Add barium carbonate to the body mix to precipitate the solubles within the body or substitute implicated materials in the body batch.
Is the body too open? What is the fired porosity of the body? Does it have an open porous structure resulting from many coarser particles or laminations and air pockets (e.g. from poor pugging or sand, grog, shale, unground clay in the batch)? If pores are networked in a body that produces alot of gases on firing then these gases escaping from within are channeled into the network and converge at high volume surface vents (gas volume may be too large for the glaze to heal). Use a finer particled body or perhaps a fine slip between glaze and body. Is the body lacking maturity (not vitrifying)? For example, using a body intended for cone 10 used at cone 6 can actually impede the melt of the glaze since body silica and alumina can rob the glaze of some of its fluxes and therefore impede its ability to smooth out.
Is the body bisque surface rough or irregular? If the body surface is rough (because it contains grog or sand, or the ware has been mechanically trimmed during leather hard stage opening imperfections in the surface), pinholes often occur as the glaze dries on the body. This is a poor lay-down and these raw pinholes may turn out as fired pinholes. In addition, a rough surface exposes pore networks inside the body to larger volume 'exit vents' that produce pinholes in glazes. You can prevent this by using a finer body, smoothing the body surface in the leather hard state after trimming, or by applying a fine-grained slip. You can also wash bisque ware (do not soak it) prior to glazing, this will tend to make the wet glaze application fill surface irregularities rather than compress air into the voids then have it blow back out as a raw pinhole a few seconds later.
Do you understand the gas evolution profile of the body? There are many ways to study the characteristics of your body in this regard so that you can adjust your firing to slow down during the high gas evolution phases.
Is there a problem with the glaze recipe? Do you use binders? Glaze binders have been known to produce serious pinholing and pitting problems. Some decompose at higher temperatures than you might think. Switch to another binder that decomposes at a lower temperature, eliminate it if there is adequate clay to harden the dry glaze layer, or reformulate the glaze to melt later and more quickly using a fast-fire frit. Once again I ask, do you really need a binder, or could bentonite do the same job?
Are any glaze materials contributing to the problem? Some glaze materials produce large volumes of gases as they decompose during firing (e.g. whiting, dolomite, talc, coloring carbonates like copper, cobalt). These materials can decompose as late at 1000C, if this is after the glaze has started to melt it means trouble. In serious cases the glaze may not just pit or pinhole, but it may blister, the problem can be reduced or eliminated by employing other sources of the needed oxides (i.e. wollastonite for CaO, frits for MgO, stains or coloring oxides for carbonates). Calculation will be required to make the substitution (so that the formula stays the same).
Do you need a fast-fire glaze In industry the chemistry of fast-fire glazes is well understood (e.g. they have zinc and lower boron, this produces a later melt). If you are fast firing and are not using a glaze formulated for fast fire then you will almost certainly be having glaze pitting and surface imperfections.
Is the glaze melt is too viscous? If the glaze melt is too thick it will resist flow, impede the passage of gas bubbles, tending to trap them in its matrix. Most often a glaze melt is viscous because it is not melting enough. However even well melting glazes can have a chemistry that makes them resist flow (i.e. high alumina content) or they may contain a material like Zirconium that stiffens the melt because it does not go into solution. Using melt flow testers to gauge the melt mobility of your glaze is a good idea, it is very difficult to detect melt flow changes by simple inspection of a glaze layer. You might think that the melt is fluid enough, but only a melt flow test will say for sure. Increasing flux content to produce a more fluid melt often works well to combat pinholes and pits. Sometimes very small additions of ZnO, SrO, or Li2O can have a dramatic effect on glaze flow. Sourcing fluxes from frit or using a finer particle size material will improve the melt flow also. Or, you could simply fire higher. Likewise, a decrease in the Al2O3 content will make a glaze more fluid but could add unwanted gloss if you are using a matte. As already noted, if the glaze contains a melt stiffener like zircon, check to see if trading off some of it for tin oxide helps. It is possible that the glaze may be melting too much and blisters associated with glaze boiling may contribute to surface imperfections, however this is more likely to cause blisters or be associated with soluble salts from the body boiling below the glaze. Try adding Al2O3 to the formulation and note an improvement to confirm this.
Is the glaze melt and sealing the surface too early? Ideally the body should expel its gases before the glaze melts. Modern fast fire frits are specially formulated to melt much later. The modern whiteware industry is build on this premise and glaze formulations have been completely transformed in recent times. Fusion frit 300 is an example. If you are using early melting high boron frits reformulate your glaze to take advantage of fast fire formulations even if you don't fast fire.
Is there a problem with glaze application? If a glaze layer is too thin pinholes may be a product of a simple lack of glaze to heal them. Increasing the glaze thickness may dramatically reduce the pinhole population (of course your glaze must be stable enough not to run if applied thicker and it must fit well enough not to start crazing due to increased tension between it and the body). Keep in mind that what may appear to be pinholes may actually be blistering, this is often evident when increased glaze thickness reduces the pinhole count but reveals the remnants of many healed blister craters (dough nut shaped rounded bumps on the surface when viewed at an angle in the light). It is possible that improper application could contribute to pinhole formation. Such pinholes will usually be larger and possibly not be true pinholes, and they may be accompanied by crawling. To deal with this make sure your glaze slurry does not have too much water, that it lays down
into a dense layer on the body and that it bonds well to produce a homogeneous dried surface with minimum airspace. To encourage the production of a good surface during drying make sure ware is clean and dust free and that glaze does not form pinholes during drying (try prewetting the ware slightly if the latter happens). Many companies deflocculate their glazes to get a denser lay down.
Is the glaze contaminated? If pinholes are isolated and few in number it may be possible that a contaminant is getting into the glaze. Pour a sample through a fine screen to check. Do not underestimate the value of ball milling to improve fired glaze surface qualities, many a problem with pinholing and blistering has been solved this way. Many companies ball mill up to 12 hours for best results.
Is the ware once-fire? Once-fired ware is much more prone to crawling and pinholing because the glaze-body bond is more fragile after application and much more gas is generated during firing than for a body that has already be bisquit fired. Thus, while crawling is the most frequent complaint in once-fire glazed ware, pinholes are more common because of the significant out gassing associated with first-fire. If you add fast-fire to this mix sometimes it is a wonder that it is even possible to get a nice fired surface on a glaze! Try bisque firing to see if this eliminates the problem. If it does then the gases of firing a raw body are not being passed by your glaze; reassess the whole process to reduce all contributing factors as much as possible. Use a fast-fire glaze. See the article on blisters for related information.
Is there problem with the glaze firing? If ware is fired too rapidly the glaze melt may not have a chance to smooth over. If thicker or protected sections of ware have more pinholes this is usually an indication that slower more even firing will improve the surface over the entire piece. Also, if glaze does not pit or pinhole in sections opposite an unglazed surface that it is clear that body gases are the problem and firing needs to be compensated at the right time (of the body needs to use cleaner materials). You need to consider both the needs of the glaze and body to determine where in the firing curve to fire more slowly. In most cases non-fast-fire settings fire slower toward the high end (i.e. an hour per cone at cone 6), soak if possible, and slow the initial cooling phase. If the glaze contains an early melting material (i.e. a high boron low alumina frit) you may need to slow the firing just before the frit begins to fuse to allow as much gas to vent as possible before continuing. Most frit suppliers supply melting or softening temperature information. Modern automatic kiln firing devices make it very easy to control the firing curve. Serious pinholing problems have often been completely eliminated after studying the gas evolution characteristics of body and glaze and employing a firing curve that slows down at appropriate times. Many engineers in industry specialize in the study of firing curves and the programming of automatic kilns. For an example of a TGA (thermal gravimetric analysis) curve, see Copper Carboante and Copper Oxide on this site). A very important factor to consider also is that modern industrial kilns supply a lot of airflow to the chamber and this carries away products of decomposition. If you are using a kiln without adequate ventilation then there may be not be enough oxygen available at the glaze surface to oxidize and carry away the carbon products of decomposition. Ventilation systems can be added to kilns but that does not mean they are adequate, the air may not be passing over all sections of the ware or at a great enough rate. Some industrial kilns have so much airflow that taller ware can actually blow over if it is not set correctly! If you are doing fast-fire this is critical, a fast fire kiln absolutely must have good air flow. If you are using an electric kiln without airflow, then expect glaze imperfections unless you are firing very slowly. This is especially true if you are firing heavy masses of ware in an electric kiln, that ware may simply not be heating up as fast as your firing schedule might mislead you to believe; heat it up slower. Another factor to consider is that surface pitting can occur even on cool down (e.g. high
sulphur bodies). Thus you may need to adjust the kiln firing program to cool more slowly until the glaze stiffens.
Is the problem with the bisque firing? Since most pinholes are the product of escaping gases, it is logical to bisque as high and as long as possible to eliminate the bulk of gases during that firing. The only disadvantage of bisquing higher is that ware will be less absorbent and thus may not be as easy to glaze. Find a good compromise temperature. Also, do not stack ware too tightly in the bisque and make sure there is good airflow in the kiln. It is important that the bisque fire be conducted in an oxidation atmosphere. If not Fe2O3 within the body may be reduced to FeO, a strong flux. During the glaze firing an active glass will be formed within the body and the associated decomposition processes will generate gases that may cause bloating, blistering, or pinholing.
Is the problem spit-out? If the surface of the glaze is covered with minute broken blisters then the problem is probably spit-out, a condition caused by expulsion of trapped water vapor inside porous ceramics on refire for luster decoration. It is amazing how long it can take to drive off all the water in a fast firing, it may still be coming off past red heat! Make sure the ware you put in a glaze firing kiln is dry.
Glaze Shivering Shivering is just the opposirazing glaze crazing, the glaze is under compression and flakes off, especially at edges. It is much less common simply because glazes tend to have a higher thermal expansion than bodies and because they can tolerate being under compression much better than being under tension. When the body-glaze interface is not well developed the problem will be much worse. This is serious because a few shivered tiles coming out of the kiln, for example, could mean that all of them will shiver with time! The problem is a mismatch between the thermal expansion of body and glaze, nothing will fix it except raising the COE of the glaze (or lowering the body COE). It is conceptually easy to adjust a glaze using INSIGHT software, just increase the Na2O. However in fastfire settings, Na2O can cause bubbling. Fast fire glazes have lower B2O3, higher ZnO and CaO, lower Na2O and higher SiO2. You just need to work within these guidelines. Still, there is a good chance your glaze can handle the addition of some Na2O. But you cannot just add soda feldspar, it contains Na2O, Al2O3 and SiO2 (theoretically of course), you do not want the Al2O3 and SiO2, you only want the Na2O. That is the purpose of INSIGHT software, to figure out how to adjust the glaze recipe so that the only change in the chemistry is an increase in the Na2O. In this case you would have to calculate how much to reduce the kaolin and silica in the recipe (because they contribute Al2O3 and SiO2). The lessons section in the INSIGHT software manual demonstrates this. However, before doing that, check your clay body. Has it changed? Can you measure its COE and compare with past runs. A common cause of changes in body COE is the inclusion of scrap and recycle in production material (alteration in material makeup is common in scrap).
Powdering, Cracking and Settling Glazes When glazes 'powder' onto your hands and create dust during handling it can be more than just aggravating. The causes of dusting generally contribute to other problems (slurries settle quickly and lay-down varies in thickness). By contrast, when glazes shrink excessively and crack and fall off during drying it is totally frustrating. These two glaze problems are actually closely related, that is, they have a common cause as we shall see. I've seen normally impatient people demonstrate a remarkable tolerance for these situations. After all that glaze recipe 'came from the Gods and we can't mess with it'. Right? On the contrary, this situation is one that can be dealt with logically. It might seem that the chemistry of the glaze could not possibly have anything to do with problems like this. But think again. This is exactly the kind of problem where it really shines. Why? Because many of the solutions involve altering the glaze recipe without changing its overall chemistry. There are lots of examples of doing this in the tutorial videos you can watch at digitalfire.com. First, what causes dusting? The answer is lack of particle binding (a binder is needed to 'glue' the particles together). What about glaze shrinkage and cracking? Too much particle binding and associated shrinkage. Let us consider a little background. Glaze slurries are suspensions of mineral powders (a bunch of microscopic rocks floating in water). What makes them float? The same thing that hardens the glaze powder: Clay (e.g. kaolin, ball clay, bentonite). Clay particles are thin and flat and very small. One gram of clay has an unbelievably large total surface area compared to other minerals used in ceramics. Clay particles have a curious surface chemistry that produces opposite electrical charges on the faces and edges. This results an affinity for water on the faces, this is what produces plasticity in clay bodies, the water glues together yet lubricates movement of the particle faces one against the other. In high water systems, like glaze slurries, suspended clay particles hang on to each other directly (edges against faces) and indirectly (faces against faces) using water as the glue. This is often referred to as 'a house-of-cards arrangement' and it can accommodate large amounts of other mineral particles within the matrix and still exhibit the same properties (to a lesser degree of course). Conceptually the other mineral powders are just 'dead microscopic rocks' along for the ride! The mechanism of the 'bonding' that takes place during dewatering (drying) is not commonly understood. As interparticle water is removed during drying, clay particles move closer together (and pull others with them). The packing results in shrinkage of the entire matrix. Large particle clays (like kaolins) shrink 5% or less from plastic to dry whereas really fine particled clays might shrink 25% or more (shrinkage is more complex than simply particle size, but for our purposes we will not get into that). However mere particle proximity does not in itself create a bond. The chemistry on the surface results in the migration of some chemical species across the boundary. While this creates a very weak bond, the fact that there are billions of particles bonded together in such a fashion creates a clay surface that we perceive to be a fairly hard product. The finer the clay particles the harder it will be. However clay bodies and glazes also contain all kinds of other particles in the mix that do not bond, and as noted, they reduce the number of clay-to-clay bonds (which is bad) but also reduce the drying shrinkage (which is good). Therefore a dried matrix, whether clay body or glaze layer, is a bunch of rock particles held together by billions of weakly bonded clay particles. Now, the question is: What bonds a dry glaze layer to a piece of bisque ware? Well there is no obvious dry adhesion mechanism or boundary chemical reaction. The mechanism of the bond relates to the sticky nature of the wet glaze and the microscopically rough surface of the bisque ware. During hardening the glaze layer loses its wet adhesion and simple mechanical contact is the only microscopic bond. The layer stays on because all the minute surface cracks and pores give it places to hang on to. As you can imagine, this bond is weak at best. Since all glazes shrink during drying, it is not clear how the weak bond with the bisque is able to withstand the pulling forces associated with the shrinkage. Some glazes hardly shrink at all because they lack clay content and that is, of course, why they dust off excessively. However glazes that harden properly during drying always crack, you just do not see the cracks. Micro-cracks must develop to relieve the stress. However when there is too much shrinkage they become visible cracks. With even more stress the glaze cracks to form 'islands' with curled up edges (like a dried up lake bottom). You
can see this effect clearly if you watch a slurry of pure kaolin or ball clay dry on a bisque surface. As you can see, we want a glaze to have enough clay so that it forms a hard dry layer but not so clay that it shrinks excessively and cracks off the bisque. There are a number of strategies you can employ if your glaze is powdering on one extreme or shrinking and cracking off on the other. It follows that a powdering glaze needs either more clay or a finer more plastic clay whereas a glaze that is shrinking and cracking needs less clay or less plastic clay. Typically pottery glazes need a minimum of 15% kaolin to harden adequately. Ball clays and bentonites can be used, as well as other clay materials, but kaolin works the best to gel the slurry. It might seem that because ball clay is much more plastic than kaolin, you could use a lot less, but in practice, 15% ball is also needed. The same can be said for bentonite, while 5% bentonite might plasticize a body as much as 15% kaolin, this alone is not enough to harden a glaze well, the bulk is needed. -If your cracking and shrinking glaze employs a relatively plastic kaolin (like #6 Tile or Sapphire), try switching to a less plastic one like EPK or Pioneer. This will not affect glaze chemistry much. A similar switch of one ball clay for another is not as likely to work since pretty well all common ball clays are very plastic. If your glaze is powdering then switch from the less plastic material to a more plastic one. However I must say that if your glaze has a problem, this one change is not likely going to solve it, more will be needed. -Add some bentonite for powdering glazes (e.g. 3%), remove it from cracking glazes if it is there. Bentonite is super fine and super plastic and therefore dries very hard and shrinks alot. The small amount of bentonite does not affect the glaze chemistry too much. Remember you can't add bentonite to an existing slurry, it agglomerates into balls that even a propeller mixer won't break up; you need to shake it up with the powder in a new batch to separate the particles). -Add CMC gum to powdering glazes. Like bentonite, it needs to be added during dry mixing. Gum is very sticky and it hardens, using it is a way of 'gluing' a glaze on the ware. Strangely gum also helps suspend, but I have no idea why. Gum burns away so it has no effect on glaze chemistry (although the decomposition can produce glaze faults like blistering and pinholing). One problem: gummed glazes dry slower and drip-drip-drip after glaze dipping pull-out. Experiment with the amount, try 0.5% to start. Add it as a gum solution, not as a powder. Do not use gum unless you need it. -Use kaolin instead of ball clay for cracking glazes (and vice versa for powdering ones). Since kaolin has less silica you will need to use ceramic chemistry to figure out how to compensate for the change in alumina and silica. No big deal? Think again, the amount of SiO2 and Al2O3 is the primary determining factor for many fired glaze properties and kaolin is the number one source of Al2O3. Matte glazes are more likely to over-shrink because they have more clay to supply the Al2O3, but they also have a more critical chemistry balance, the substitution needs to be done correctly. -Check the specific gravity of your glaze (its weight per cc). If it is too low (below 1.4) then it is gelling and there is too much water in the slurry. Perhaps your water supply contains electrolytes that are flocculating the mix, that is, thickening it. Try using distilled water. Also, look out for slightly soluble materials in your glaze, they might be the source of electrolytes. High nepheline syenite glazes can do this. If you have a big container of flocculated glaze there is not much you can do with it except throw it out. You might try adding a small amount of deflocculant like Darvan or Sodium Silicate (e.g. 0.1%) to thin it but then you still have to figure out how to get all the water out and it might be thick again next week! One last note about powdering glazes: Because they generally lack clay they settle out also. Often a layer of water forms at the surface only a minute or two after stirring (generally not easily seen). Although an adequately thick layer may still build up on the piece during dip, on pull-out the water layer may wash glaze off on the last-to-leave sections (usually the rim). The principles mentioned above apply, if you don't want to stir it every minute then the glaze needs reformulation so the chemistry stays the same but more plastic materials are used to source alumina. A classic source of this problem is too much feldspar: Some glazes have 60%, this is way too much. Using ceramic chemistry you can reduce the feldspar drastically and source the lost Al2O3 from kaolin, the SiO2 from silica and the alkalies from a frit (e.g. Ferro Frit 3110). Another note about glaze bonding: If you fire your bisque too high it might not be absorbent enough to build up a good layer of glaze on dipping and still dry out quickly. If a glaze needs to dry over a long period on water logged ware, then it will usually crack. Likewise, if your ware has very thin walls then
there simply will not be enough porosity in the matrix to pull the water out of the glaze quickly enough (normally a glaze should lose its wet sheen within 30 seconds, many do in less than 10 seconds) to form the mechanical bond with the ware. One solution is to heat the bisque and dip it hot into the glaze using dipping tongs (of course that is not an option if ware is delicate). Alternatively, you could heat and then spray the glaze onto it. Also, remember that smooth porcelain surfaces do not provide as many imperfections for the glaze to hang on to so you need even more control of the drying shrinkage. The moral of the story is that you need to understand the purpose of each material in the glaze (and even the water) to fix problems. And glaze chemistry is pretty hard to avoid, here again it is one of the most valuable tools to solve the problem. You can download Digitalfire Insight for a free two month trial or sign up for Insight-live.com and do your work online. It is alot better to control what your glaze is doing that to switch to another that just has a different set of problems.