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
Oxide
Percentage
SiO2
61.5
Al2O3
9
B2O3
1
BaO CaO MgO ZnO K2O
4 14 2 5 2.5
Na2O
1
The percentage of zinc oxide is kept as low as possible for cost reasons. The calcium and magnesium oxides contribute to formation of crystals