INVESTMENTS An investment is any material used in dentistry to invest an object. It is also defined as the material used to enclose or surround a pattern of a dental restoration for casting or molding or to maintain the relations of metal parts during soldering. For casting procedures, the wax pattern
of an inlay or crown is
embedded or invested in a heat resistant material, which is capable of setting to a hard mass. The wax is removed from such a mould usually by burning out, before casting the molten alloy. There are in general two types of investments: casting investments and soldering investments. Requirements for an ideal investment material 1. Should be easily manipulated. It should be easy to mix and paint on to the wax pattern, and it should harden within a short time. 2. Should produce a smooth surface as well as fine detail and margins on the casting. 3. On being heated at high temperatures, it should not decompose and give off corrosive gases that may chemically injure the surface of gold alloys. 4. Should be porous enough to permit the air or other gases in the mould cavity to escape easily during the casting procedure.
5. The mould must expand to compensate for the shrinkage on cooling of the alloy. 6. Should be strong enough to withstand the force of the molten alloy entering the mould. 7. After casting, it should break away readily and should not attach itself to the surface of the metal. 8. Casting temperature should not be critical. Preferably the thermal expansion versus temperature curve should have a plateau of thermal expansion expansion over a range of casting temperatures. 9. Should be comparitively inexpensive. Types of casting investments 1. Gypsum bonded investments 2. Phosphate bonded investments 3. Silica bonded investments Gypsum bonded investments are the oldest materials and are used for casting conventional gold alloys. The phosphate bonded investments are used for metal ceramic restorations. Silica bonded investments are principally used for the casting of base metal alloy partial dentures.
Gypsum bonded investments American Dental Association specification no: 2 for casting investments used for gold alloys, lists 3 types of investments: Type I – for inlays and crowns utilizes thermal expansion of the mould. Type II – for inlays and crowns utilizes hygroscopic setting expansion. Type III – for gold partial denture utilizes thermal expansion. Composition Gypsum bonded investment is a mixture of 4 types of materials. a. Refractory materials: usually a form of silica (quartz, crystobalite or tridymite), this withstands high temperatures and provides mould expansion by thermal expansion. b. Binder : autoclaved calcium sulphate hemihydrate is used. i. To react with water and on hydration binds the silica together. ii. To impart sufficient strength to the mould and iii. To contribute to the mould expansion by its setting expansion. c. A reducing agent: such as powdered charcoal, reduces any oxide formed on the metal. d. Modifying chemicals : such as boric acid or sodium chloride to inhibit shrinkage on heating.
Gypsum The alpha hemihydrate is usually used because of its greater strength compared to the beta variety. But this binder when heated to high temperatures for complete dehydration, shrinks considerably and frequently fractures. The shrinkage on heating is due to the dehydration of the set gypsum in two stages. i. 2CaSO4 · 2H2O (CaSO4)2 H2O + 3H2O ii. (CaSO4)2 · H2O 2CaSO4 + H2O Shrinkage is due to the transformation of calcium sulphate from the hexagonal to the orthorhombic configuration.
A thermal expansion curve of gypsum shows considerable shrinkage from 200 to 400°C. A slight expansion then occurs to approximately 700°C and then a tremendous contraction is caused, probably due to decomposition, and sulphur gases like sulphur dioxide are emitted which contaminates the casting. So gypsum investments should not be heated above 700°C and these untoward effects can be minimized by ‘heat soaking’ the mould at 700°C for at least an hour to allow the reactions to be completed before casting commences.
Silica Silica (Silicon dioxide, SiO2) is the refractory material and it regulates thermal expansion. Gypsum contracts during heating and silica eliminates this contraction and changes it to an expansion. Silica exists in at least four allotropic forms: quartz, tridymite, cristoblalite and fused quartz. When quartz, tridymite or Cristobalite are heated, a change in crystalline form occurs, called “inversion”, at a transition temperature characteristic of the particular form of silica. Quartz inverts from a “low” form or alpha quartz to a “high” form called beta quartz at 575°C, Cristobalite inverts from 200 to 270°C from low or alpha cristobalite to high or beta cristobalite. Two inversions of tridymite occur, one at 117°C and the other at 163°C. Fused quartz exhibits no inversion at any temperature below its fusion point. Due to its very low coefficient of thermal expansion it is of little use in dental investments. The beta allotropic forms are stable only above the transition temperature and an inversion to the lower form occurs upon cooling. In powdered form inversion occurs over a range of temperature rather than instantaneously. 870°C Beta quartz 575°C
1475°C 1700°C Beta Beta Fused silica tridymite cristobalite
Alpha quartz Alpha
The dentistry decreases and volume increases as the alpha form changes to the beta form and a rapid increase in linear expansion occurs. This is
probably due to a straightening of chemical bonds to form a less dense crystal structure. Only quartz and cristobalite or their combination are used as investments. Depending upon the type of silica principally employed, investments are classified as quartz investments or cristobalite investments. Modifiers Modifying agents, colouring matter and reducing agents such as carbon or powdered copper are also added. Some modifiers such as boric acid and sodium chloride not only regulate the setting expansion and setting time, but prevent most of the shrinkage of gypsum when it is heated above 300째C. The usual composition of gypsum bonded investment is: Refractory
60 to 65%
30 to 35%
Modifiers & Pigments :
4 to 7%
Setting time ADA specification no.2 recommends a minimum setting time of 5 mins. and a maximum of 25 minutes. The modern investments usually have a initial setting time between 9 to 18 minutes. Normal setting expansion
A mixture of silica and gypsum investment results in a greater setting expansion than a gypsum product used alone. The silica particles interfere with the intermeshing and interlocking of the crystals as they form, resulting in a outward thrust and so an expansion. Setting expansion of modern investments is about 0.4%. Setting expansion of the mould compensates partially for the casting shrinkage of gold. The ‘normal setting expansion’ as determined in the laboratory is different from the ‘effective setting expansion’ occuring in practical usage. Factors determining the effective setting expansion a. Greater the gypsum content of the investment, greater the exothermic heat transmitted to the wax pattern and greater the mould expansion. b. Lower the W/P ratio for the investment, greater the exothermic heat and greater the setting expansion. c. Thinner the walls of the wax pattern, greater the setting expasion of the investment. d. Softer the wax, greater the setting expansion. If a wax softer than Type B inlay wax is used, the setting expansion may cause a serious distortion of the pattern. Hygroscopic setting expansion
This occurs when the gypsum is allowed to set under or in contact with water. Hygroscopic setting expansion of an investment may be more than six times the normal setting expansion. The ADA specification no.2 for Type II investments requires a minimal hygroscopic setting expansion of 1.2% and a maximal expansion of 2.2%. In one, method known as the ‘water immersion’ technique, the investment mould is placed into water. Another method is the ‘water added’ technique. Here a measured volume of water is placed on the upper surface of the investment material within the casting ring. This produces a more readily controlled expansion. Theory of hygroscopic setting expansion Hygroscopic setting expansion is a continuation of normal setting expansion and occurs as the immersion water replaces the water of hydration used up by the crystals. This prevents the confinement of the growing crystals by the surface tension of water. (Mahler D.B. and Ady A.B. ‘An explanation for the hygroscopic setting expansion of dental gypsum products’ J. Dent. Res. 39 : 578, 1960). Factors affecting hygroscopic setting expansion a. Composition Increase in silica content increases the hygroscopic setting expansion. Finer the silica particles greater the expansion. Alpha hemihydrate produces more expansion in the presence of silica, than beta hemihydrate. The hygroscopic setting expansion of stone or plaster alone is very slight.
The investment should have at least 15% binder to provide strength after hygroscopic setting expansion, and to prevent drying shrinkage.
b. W/P ratio Higher the W/P ratio of the original investment â€“ water mixture, less the hygroscopic setting expansion. c. Spatulation Shorter the mixing time, less the hygroscopic expansion. This factor is important in controlling the effective setting expansion as well. d. Shelf life of investment The older the investment, less is its hygroscopic setting expansion. The material should be stored in air tight containers and should not be exposed to humidity. It is better to purchase small amounts of the investment at a time. e. Time of immersion The greatest amount of hygroscopic setting expansion takes place if the immersion in water takes place before the initial set. The longer the time of immersion is delayed after the initial set, lesser the expansion. Also the longer the time for which it is kept immersed, greater the hygroscopic setting expansion. f. Confinement Both the normal and hygroscopic setting expansions are confined by opposing forces, such as the walls of the casting ring or the walls of the wax pattern. This confinement can be avoided largely by placing damp asbestos as a
liner on the inner wall of the ring. The water in the asbestos also is utilized for hygroscopic expansion. g. Temperature of the water bath Higher the temperature of the water bath used for immersion, greater the hygroscopic expansion. The softening of the surface of the wax results in increased effective setting expansion. h. Amount of added water An increase in the amount of water added, increases the hygroscopic setting expansion upto a certain point, after which further addition of water does not create any expansion. This degree of maximum expansion is called the â€œcritical pointâ€?. This critical point can be raised or lowered easily by changing the manipulative conditions like W/P ratio, time of spatulation, age of investment etc. i. Particle size of silica Finer particles of silica produce greater hygroscopic expansion. The hemihydrate particles have little effect on this expansion. j. Silica/binder ratio If this ratio increases, greater will be the hygroscopic expansion and lesser the strength. This is because the added water can easily diffuse through the silica particles.
Thermal Expansion Thermal expansion is directly related to the amount and type of silica employed. The contraction of gypsum is entirely balanced when the quartz content is about 75%. Cristobalite produces a much greater thermal expansion during its inversion and so the normal shrinkage on heating gypsum is easily avoided. Further, the expansion occurs at a lower temperature because of the lower inversion temperature of cristobalite as compared to quartz. Some of the modern investment contain both quartz and cristobalite. The ADA specification no. 2 requires the thermal expansions of type II investments to be between 0.0 to 0.6% and of Type I investments which rely principally on thermal expansion for compensation, to be between 1.0 to 2.0%. It is desirable that the maximal thermal expansion be attained at a temperature not greater than 700째C. Gold alloys are apt to be contaminated above this temperature. Factors affecting thermal expansion a. W/P ratio The magnitude of thermal expansion is related to the amount of solids present. So more the water used in mixing, less is the thermal expansion.
b. Chemical modifiers When the silica content of an investment is high enough to prevent any contraction during heating, the investment becomes weak. The addition of small amounts of chlorides of sodium, potassium or lithium to the investment eliminates the contraction and increases expansion without adding excessive amounts of silica. (Earnshaw R. ‘The effects of additives on the thermal behaviour of gypsum bonded casting investments’. Part I. Aust. Dent. J. 20 : 27, 1975). Silicas do not prevent shrinkage of gypsum, but counterbalance it, whereas chlorides actually reduce gypsum shrinkage below temperatures of 700°C. The total expansion of the mould from normal setting expansion, hygroscopic setting expansion or thermal expansion is generally sufficient to compensate for the shrinkage on cooling of gold alloys, which is about 1.5% by volume. PROPERTIES Thermal Contraction When an investment is allowed to cool from 700°C, its contraction curve follows the expansion curve, during inversion of the beta quartz to its stable form at room temperature. It shrinks to less than its original dimension because of the shrinkage of the gypsum when first heated.
Strength The investment must be strong enough to prevent fracture or chipping of the mould during heating and casting. But the compressive strength should not be unduly high. Sometimes a distortion of the casting may result from a directional restraint by the strong investment to the thermal contraction of the casting, as the alloy cools to room temperature. (Teteruck W.R. Mumford G. â€˜The fit of certain dental casting alloys using different investing materials and techniquesâ€™. J. Prosthet. Dent. 16 : 910, 1966). This rarely occurs in gypsum bonded investments, but may occur with other types of investments. The distortion can even lead to fracture of the casting if the hot strength of the alloy is low. The compressive strength of the investment depends on the type and amount of gypsum binder used. Modifiers aid in increasing the strength as more of the binder can be used without much reduction in thermal expansion. ADA specification no. 2 requires a minimum compressive strength of 2.5 Mpa, 2 hours after setting of the investment. However, higher strength is required for a Type III partial denture investment. More the W/P ratio employed, lower the compressive strength. The greatest reduction in strength upon heating is found in investments containing sodium chloride. The strength decreases on cooling to room temperature, because of the fine cracks that develop.
Fineness Fineness of the investment affects the setting time and surface roughness of the casting. Fine silica particles increase the hygroscopic setting expansion and gives smoothness to the casting. Porosity When the molten metal enters the mould, air must be forced out ahead of it or else back pressure builds up and prevents the alloy from completely filling the mould, causing back pressure porosity. Pores in the investment offer adequate venting. Less the hemihydrate content and more the gauging water used, more porous is the investment. The more uniform the particle size, greater is its porosity. A mixture of coarse and fine particles exhibits less porosity. Manipulation Before investing the wax pattern, it is washed with a non-foam detergent to remove any oil or grease and to facilitate the wetting of the pattern by the investment mix. The casting ring is lined with wet asbestos strip. Investing the pattern may be done: i)
under vacuum, to prevent trapping air on the surface of the pattern, or
painting investment material on to the pattern with a brush before gently forcing it into the filled casting ring.
If the hygroscopic technique is employed, (Type II) the casting ring, with the crucible former end down, is immediately immersed in a water bath at a temperature of 37 to 38°C. For the thermal expansion technique (Type I and III) the investment is allowed to harden in the ring placed on the bench. Divestment: It is a die stone-investment combination in which the die material and investing medium have a comparable composition. Divestment is a commerical gypsum-bonded material, it is mixed with colloidal silica liquid. The die is made from this mix and the wax pattern constructed on this. Then the entire assembly (die and pattern) is invested in the Divestment, thereby eliminating the possibility of distortion of the pattern upon removal from the die or during the setting of the investment. The setting expansion of the material is 0.9% and the thermal expansion is 0.6% when heated to 677°C. Since divestment is a gypsum-bonded material, it is not recommended for high-fusing alloys, as used with metal ceramic restorations. However, it is a highly accurate technique for use with conventional gold alloys, especially for extra-coronal preparation. Wax elimination is started only after atleast one hour. Limitations: Above 1200°C, a reaction occurs between calcium sulphate and silica: CaSO4 + SiO2 CaSiO3 + SO3. This sulphur trioxide gas evolved causes porosity in the casting and contributes to the corrosion of the casting. For this reason gypsum-bonded investments are not used for higher fusing alloys like cobalt-chromium. In such cases, phosphate or silica bonded materials are chosen.
Phosphate bonded investments The popularity of metal-ceramic restorations, which use higher melting gold alloys and the trend toward the use of inexpensive alloys have resulted in the greater use of phosphate bonded investments. Composition The filler is silica, in the form of cristobalite or quartz or a mixture of these two, and forms approximately 80%. It provides, refractoriness and a high thermal expansion. The binder is magnesium oxide (basic) and a phosphate that is (acidic). Originally phosphoric acid was used, but at present mono-ammonium phosphate is used as it can be incorporated into the powdered investment. The powder is mixed with an aqueous colloidal silica suspension and invested. As alloys used in metal ceramic restorations have higher melting points, their contraction during solidification is greater and the aqueous colloidal silica suspensions compensate for this. Sometimes these silica solutions freeze in cold weather and so, some phosphate investments have been produced, which utilize water as the gauging liquid. Carbon is often added to the powder to produce clean castings and to facilitate the â€˜dig-outâ€™ of the casting. It is suggested that carbon embrittles alloys like silver-palladium and base metal alloys. Recent evidence shows that
palladium does not react with carbon at temperatures below 1504°C. (J.F. Jelenko Co: Thermotrol Technician, 34 : No.1, Winter, 1980). Thus if the casting temperature of a high palladium alloy exceeds this critical point phosphate investment without carbon is used. A carbon crucible should not be employed for melting the alloy; and even gold alloys used with porcelain should not be premelted or fluxed on charcoal blocks, because the trace elements that provide high strength may be removed. Setting reactions The chemical reaction is as follows: NH4H2PO4 + MgO + 5H2O NH4MgPO4 6H2O The Magnesium ammonium phosphate formed is polymeric. (Neiman R and Sarma A.C. ‘setting and thermal reactions of phosphate cements’. J. Dent. Res. 9: 1478, 1980). The MgO is never fully reacted. Thus a predominantly colloidal multimolecular (NH4MgPO4 6H)2n is formed, coagulating around excess MgO and fillers. Upon heating, the binder of the set investment undergoes thermal reactions, as suggested by J. F. Jelenko Co., in the following way:
MgO + NH4H2PO4 + H2O Room Temperature (NH4MgPO4 6H2O)n MgO NH4H2PO4
Colloidal type particles
H2 O Prolonged setting at room temperature or dehydration at 50°C (NH4MgPO4 6H2O)n Dehydrated at 160°C (NH4MgPO4 · H2O)n
Ammonia which is liberated gives off its characteristic smell.
Heated at 300-650°C (Mg2P2O7)n
Non-crystalline polymeric phase
Heated above 690°C Mg2P2O7 +MgO Heated above 1040°C Mg3(PO4)2 The final products are crystalline Mg 2P2O7 and some excess MgO, along with unchanged quartz and/or cristobalite. Some Mg 3(PO4)2 may be formed if the investment is grossly overheated or when the molten metal contacts the mould cavity surfaces.
Setting and thermal expansion During the setting reaction a slight expansion occurs and this is more in the colloidal silica-mixed materials than for water-mixed materials. The setting expansion, as in the case of gypsum, results from the outward thrust of the growing crystals. Thermal expansion is also greater for colloidal silica-mixed materials. The combined setting and thermal expansion for phosphate investments is around 2% if the special silica liquid is used
Similar to gypsum bonded investments, the phosphate investments mixed with water show a shrinkage from 200째 to 400째C. This contraction is practically absent in the colloidal silica mixed materials. Both the setting expansion and the thermal expansion increase as the concentration of the special liquid increases. A decrease in expansion can be achieved by increasing the liquid/powder ratio rather than by decreasing the concentration of the liquid.
According to Engler et al the setting and thermal expansions do not compensate totally for the shrinkage of the metal as was formerly thought, and research need to be done to find other factors which contribute to compensate for the metal shrinkage. The magnitude of shrinkage is of the order of 1.4% for most gold alloys, 2.0% for Ni/Cr alloys and 2.3% for Co/Cr alloys. Thermal shrinkage This occurs due to decomposition of the binder, magnesium ammonium phosphate, and is accompanied by the evolution of ammonia, which is readily apparent by its odour. 300掳C 2 MgNH4PO4 路 6H2O
Mg2P2O7 + 2NH3 + 13H2O
However, some of the shrinkage is masked because of the expansion of the filler, especially cristobalite. At higher temperatures, some of the remaining phosphate reacts with silica forming complex silicophosphates. These impart significant strength to the material at the casting temperature. Working and Setting time Unlike gypsum investments, phosphate investments are markedly affected by temperature. Warmer the mix, faster the set. The reaction gives off heat, which further accelerates the setting. Increased mixing time and mixing efficiency result in a faster set; these two factors give smoothness and accuracy to the casting. Mechanical mixing under vacuum is preferred.
An increase in liquid/powder ratio increases the working time. Other properties The cohesive strength of phosphate investments is so high that they do not have to be contained in a metal casting ring. The material is allowed to set inside a plastic ring which is removed before heating. The porosity of the material is sufficient enough to prevent the build-up of ‘back pressure’. Even though it is possible to use hygroscopic setting expansion like for gypsum bonded materials, it is rarely used in practice. Technical consideration Separate mixing bowls must be used for gypsum bonded and phosphate bonded investments to prevent contamination which causes problems to the setting of gypsum. When a number of wax patterns are invested together, there should be at least 3-4 mm of investment around each pattern and each of them must be in different planes to prevent excess pressure developing and leading to fracture of the investment. The rapid expansion of cristobalite at approximately 300°C requires slow heating and holding at about 200-300°C for at least 30 minutes, so as to prevent cracking of the investment. Thus a two-stage burnout is employed.
Recovery and cleansing of the casting are more difficult when phosphate investment is used. So ultrasonic cleansing may be necessary. Neither phosphate binder nor silica is soluble in hydrochloric or sulphuric acid. Cold hydrofluoric acid will dissolve silica refractory very well, without damage to a gold-palladium-silver alloy. Base metal alloys require light grit-blasting usually with fine aluminium oxide. However, chromium based alloys are usually sand-blasted to remove the investment. Acid should never be used for cleaning base metal alloy. (Allen F.C and Asgar K. ‘Reaction of cobalt chromium casting alloy with investments’. J. Dent. Res. 45 : 1516, 1966). Silica bonded investments These are used for casting base metal alloys at high temperatures. The binder is a silica gel which reverts to silica (cristobalite) on heating. Several methods are used to produce silica or silicic acid gel binders. a. When a pH of sodium silicate is lowered by the addition of an acid or an acid salt such as monoammonium phosphate, a bonding silicic acid gel forms. The addition of Magnesium oxide will strengthen the gel. (Dootz E.R., Craig R.C. and Peyton F.A.: ‘Simplification of Chrome-cobalt partial denture casting procedure’ J. Prosthet. Dent. 17 : 464, 1967). b. An aqueous solution of colloidal silica can be made to gel by the addition of an accelerator, such as ammonium chloride.
c. A colloidal silicic acid is first formed by hydrolyzing ethyl silicate in the presence of hydrochloric acid, ethyl alcohol and water, as follows: Si(OC2H5) + 4H2O Si (OH)4 + 4C2H5OH Because a polymerized form of ethyl silicate is actually used, a colloidal sol of polysilicic acids is formed instead of the simpler silicic acid shown in the above reaction. The formation of poly silicic acid constitutes the 1 st stage of the setting reaction, called “hydrolysis”. Stage 2 is called “gelation”. Here the sol is mixed with quartz or cristobalite to which is added a small amount of finely powdered MgO to render the mixture alkaline. A coherent gel of polysilicic acid then forms accompanied by a slight ‘setting shrinkage’. Stage 3 is called “drying”. Here the soft gel is dried to a temperature below 168°C. During drying, the gel loses alcohol and water to form a hard, concentrated gel of silica particles tightly packed together. A considerable volumetric contraction accompanies the drying. Which reduces the size of the mould. This contraction is known as “green shrinkage” and it occurs in addition to the setting shrinkage. The gelation process is slow and time consuming. A faster method to obtain silica gel is by the addition of amines such as piperidine to the solution of ethyl silicate. Here hydrolysis and gelation occurs simultaneously. But an unacceptable shrinkage may occur, mainly in the stage of hydrolysis.
Stock solutions of hydrolysed ethyl silicate binder may be prepared and stored in dark bottles. The solution gels slowly on standing and its viscosity may increase noticeably after 3-4 weeks when it has to be discarded. The thermal expansion of silica investments is considerable as both the binder and the refractory are forms of silica which can invert during heating. Another shrinkage called ‘firing shrinkage’ occurs above 675°C, when the polysilicic acid gel changes to silica. As with other investments, the thermal expansion can be controlled by the amount, particle size and type of silica filler employed. Silica-bonded investments being more refractory than phosphate-bonded investments, can tolerate higher burn-out or mould-casting temperatures. Temperatures between 1090 and 1190°C are employed when the higher fusing chromium containing alloys are cast. The common burn-out temperatures generally employed. Gold alloys :
Slow burn-out 450°C
Rapid burn-out 700°C Ni/Cr alloys :
Co/Cr alloys :
(Mabie CP: ‘Petrographic study of the refractory performance of highfusing dental alloy investments: II silica – bonded investments. J. Dent. Res. 52 : 758, 1973).
After divesting and cleaning, the casting is commonly electrolytically polished in an acid bath prior to a final mechanical polishing. Porosity The particles of the set material are packed so closely together that the porosity is negligible. Air spaces or vents must be left in the investment to permit escape of gases. Soldering investment In the process of assembling the parts of a restoration by soldering, such as clasps on a removable partial denture, it is necessary first to surround the parts with a suitable ceramic or investment material before the heating operation. The assembled parts are temporarily held with sticky wax until they are surrounded with the investment after which the wax is softened and removed. The portion to be soldered is left exposed to permit removal of wax and effective heating prior to being joined with a solder. These investments contain quartz and a calcium sulphate hemihydrate binder. They have lower setting and thermal expansions than casting investments. So the assembled parts do not shift position during setting and heating of the investment. Soldering investments do not have a fine particle size as smoothness of the mass is less important.
Application of various types of investment materials Investment
Dental plaster or stone
Mould for acrylic dentures.
Mould for gold casting alloys.
Mould for base metal and gold casting alloys, mould for cast ceramics and glasses; mould for soldering.
Mould for base metal casting alloys.
Discussion Of the three main types of casting investment materials, the phosphate bonded products are becoming most widely used. Silica-bonded materials are rarely used nowadays due to the fact that they are less convenient to use than the other products and that the ethanol produced in the liquid can spontaneously ignite or explode at elevated temperatures. As phosphate bonded investments can be used even for gold castings it can be considered a universal investment material. References 1. Mahler P.B. and Ady A.B. ‘An explanation for the hygroscopic setting expansion of dental gypsum products’. J. Dent. Res. 39: pp 578, 1960. 2. Earnshaw R. ‘The effects of additives on the thermal behaviour of gypsum bonded casting investments part I. Aus. Dent. J. 20 : pp27, 1975.
3. Teteruck W.R. and Mumford G. ‘The fit of certain dental casting alloys using different investing materials and techniques’. J. Prosthet. Dent. 16 : pp910, 1966. 4. J. F. Jelenko Co: Thermotrol Technician, 34 : No.1, Winter, 1980. 5. Neiman R. and Sarma A.C. : ‘Setting and thermal reactions of phosphate cements’. J. Dent. Res. 9 : pp1478, 1980. 6. Allen F.C. and Asgar K : ‘Reaction of cobalt chromium casting alloy with investments’. J. Dent. Res. 45 : pp1516, 1966. 7. Dootz E.R., Craig R.C. and Peyton F.A. : ‘Simplification of chromecobalt partial denture casting procedure’. J. Prosthet. Dent. 17 : pp464, 1967. 8. Mabie C.P. : ‘Petrographic study of the refractory performance of highfusing dental alloy investments: II silica-bonded investments. J. Dent. Res. 52 : pp758, 1973.