Gypsum Products PDF

Gypsum Products Lecturer: Dr. Sagon-Kendall Introduction Gypsum is the common name for calcium sulfate dihydrate. Gypsum products are more frequently used in laboratory procedures than any other group of compounds. Controlled variations in the...

Gypsum Products Lecturer: Dr. Sagon-Kendall Introduction Gypsum is the common name for calcium sulfate dihydrate. Gypsum products are more frequently used in laboratory procedures than any other group of compounds. Controlled variations in the manufacture of gypsum products yield a group of dental materials that include plaster, dental stone, die stone, casting investment, and soldering investment.  Each substance is a carefully formulated powder that has the particular combination of physical properties to do a specific job. When the prepared powder is mixed with the proper amount of water, the blend initially forms a fluid paste that gradually hardens into a solid. In the fluid paste state, the mixture can be poured into molds or otherwise shaped. As gypsum sets, dense masses of crystals form and heat is liberated. This liberation of heat, called an exothermic reaction, happens while all gypsum products are setting. Uses of Gypsum 1. Impression plaster. 2. Mounting the casts to the articulation. 3. Form casts and dies. 4. Used as a binder for silica. 5. Used as a mold for processing dental polymers. 6. Used for bite registration (record centric jaw relation). Classification of Gypsum Type 1: Impression Plaster Type 2: Dental Plaster/ Model plaster Type 3: Dental Stone Type 4: High strength/low expansion dental stone. Type 5: High-strength/high-expansion die stone. Impression Plaster This is a plaster that has been specially compounded for making impressions of the mouth, as follows: Impression plaster must behave differently than model plaster. It must be able to set much faster to reduce the time it is held in the patient’s mouth. Because a plaster impression cannot spring around an undercut as it is withdrawn from the mouth, it must be broken into pieces and reassembled outside the mouth.  For this reason, it must be weak and brittle. Impression plasters are rarely used in dentistry today due to the availability of hydrocollids and elastomers. Impression plaster must have a very low setting expansion of 0.13 percent because an impression that changes size significantly is inaccurate. Various accelerators and retarders are added to control the setting time of plaster, and coloring agents are often added to distinguish one gypsum product from another. Today, impression plaster is mainly used to obtain bite registrations for dentures or orienting a fixed partial denture in the mouth for a solder index. Modeling plaster Manufacturing Process: Gypsum is converted into model plaster by grinding it into small particles and then heating it slowly in open vats to drive off the water of hydration. Under a microscope, the plaster is seen to be made up of rough irregular crystals. Each crystal contains a definite proportion of water. This is called water of crystallization or water of hydration. The amount of water eliminated by heating has a bearing on the behavior of the plaster when it is again mixed with water in the laboratory. The hemihydrate produced is called β-calcium sulfate hemihydrate. Such a powder is known to have a somewhat irregular shape and is porous in nature. A special process is used to ensure plaster made for dental use has suitable working properties. These properties must always be uniform throughout a batch of material and from one batch to another. Model Plaster One of the most important requirements of plaster is that it must set or harden within definite time limits. The amount of setting expansion must also be from 0.2 to 0.3 percent. A setting expansion of 0.3 percent is the maximum amount allowed by the American National Standards Institute (ANSI) of the ADA’s Specification Number 25 for model plaster.  Model Plaster’s Uses. 1. Constructing a matrix 2. Flasking a denture 3. Attaching casts to an articulator 4. As an ingredient in some investments. The initial setting time for most dental plasters is from 4 to 12 minutes. The final setting time is approximately 20 to 45 minutes Dental Stone Dental stone is medium strength plaster that is stronger and more resistant to abrasion. It is used primarily for casts (such as diagnostic casts), opposing arch casts, and complete and partial denture working casts. Dental stone is made by autoclaving the gypsum under pressure and then grinding it into a hemihydrate powder (hydrocal). Calcium sulfate hemihydrate produced in this manner is designated as α-calcium sulfate hemihydrate. The particles are more prismatic and regular in shape.  For this reason, dental stone requires less water in mixing and sets more slowly. When set, it is harder, much more dense, and has a higher crushing strength than model or impression plaster. The average setting expansion is approximately 0.12 percent. The manufacturer colors dental stone to make it easy to distinguish from plaster. The initial setting time of a typical stone product is from 8 to 15 minutes. The final set takes approximately 45 minutes. Die Stone (Improved stone) Improved stones are specially processed forms of gypsum products used to make crown, onlay, and inlay dies. They are harder, more dense than dental stone, and have a 0.08 to 0.18 percent setting expansion. They are also colored to distinguish them from plaster. Because the amount of setting expansion is critical, it is important to use the water-to powder ratio the manufacturer recommends. These high strength plasters are made by first boiling the gypsum in a 30-percent calcium chloride solution after which the chloride is washed away with hot water (100°C), (the product is called densite) before autoclaving and then grinding the stone into very fine particles. Some manufacturers use a 1 percent solution of sodium succinate, or they add resin particles to increase the hardness of the stone Properties of Ideal Gypsum material Dimensional stability, no expansion or contraction during or after setting. High compressive strength to withstand the force applied on it. Hardness, soft material can be easily scratched. Reproduce the fine details. Produce smooth surface. Reasonable setting time. Compatible with the impression material. Can be disinfected without damaging the surface. Physical Properties Crushing Strength. Crushing strength or compressive strength is the measure of the greatest amount of compressive force that can be applied to a substance without causing it to fracture. The strength of a gypsum product increases rapidly as it hardens. Because the relative amount of water left in the set material has a distinct effect on strength, the following kinds of gypsum product strengths (wet and dry) are recognized: a. Wet Strength. This is the strength of the material with excess water still present in the set up mass. b. Dry Strength. This is the strength of a dried gypsum specimen. Twenty-four hours after setting, the compressive strength of a gypsum specimen left to dry will double. Physical Properties Setting Time. The setting time is the time required for the material to set or harden. It is divided into the following stages: 1. Initial Set. The time starts when the powder is mixed with water and ends when the material becomes solid enough to remove from the tray and trim without distortion. 2. Final Set. This is the time required for full crystallization to occur. All exothermic heat dissipates and the and reaches about half its potential crushing strength 3. Setting expansion A gypsum product enlarges in volume as it sets. This is called setting expansion and usually amounts to about 1%. A gypsum material sets up in air or in contact with water. The setting expansion varies, depending on the conditions the material is exposed to. Normal Setting Expansion. A gypsum product expands predictably when it is allowed to solidify unconfined in a normal room temperature environment. A setting expansion that takes place under these conditions is called normal setting expansion. Hygroscopic Setting Expansion. Hygroscopic setting expansion occurs when a gypsum material is allowed to solidify under water. A hygroscopic expansion can be expected to more than double a normal setting expansion. In some dental procedures, a gypsum product solidifies in limited contact with water. For example, an investment is sometimes made to set against a wet ring liner. This expansion is greater than the normal setting expansion, but it is not as great as a hygroscopic expansion. A setting expansion that occurs as a result of limited contact with water is called semihygroscopic expansion. Thermal Expansion. This kind of expansion occurs because of a gypsum product being heated. The amount of thermal expansion is proportional to the temperature. Effect of Selected Variables on Crushing Strength. The strength of set gypsum products can be directly affected by several variables under the control of the technician: Water-Powder Ratio. The crushing strength lowers as more water is used in the mix. Gypsum products are porous, and the greater amount of water increases porosity because there will be fewer crystals formed per unit of volume of the material. Mechanical Mixing. Longer and more rapid mixing, up to a maximum of 1 minute, results in greater strength. However, overmixing breaks down the forming crystals and reduces the crushing strength of the end product. Chemical Modifiers. In general, chemical modifiers reduce crushing strength. However, borax can act to increase the surface hardness of the material. Effect of Selected Variables on Setting Time. The setting time of a gypsum product can be affected directly by certain variables the dental technician can control. These variables must be applied with extreme care. In gaining a more desirable setting time, other physical properties, such as strength, may be adversely affected as follows: Water-Powder Ratio. A longer setting time is required when more water is used in the mix. Conversely, the setting time is reduced when less water is used in the mix. Water Temperature. As the temperature of the water used in the mix is raised f rom 32 to 85 o F, the setting time is shortened. When the water is between 85 and 120 o F, the setting time is lengthened. If boiling water is used and the mixture is maintained at about 212 o F, the material will not set at all. Mixing. The setting time is shortened as the mixture is stirred (spatulated) either for a longer time or at a faster rate. Effect of spatulation The mixing process, called spatulation, has a definite effect on the setting time and setting expansion of the material. Within practical limits an increase in the amount of spatulation (either speed of spatulation or time or both) shortens the setting time. Obviously when the powder is placed in water, the chemical reaction starts, and some calcium sulfate dihydrate is formed. During spatulation the newly formed calcium sulfate dihydrate breaks down to smaller crystals and starts new centers of nucleation, around which the calcium sulfate dihydrate can be precipitated. Because an increased amount of spatulation causes more nuclei centers to be formed, the conversion of calcium sulfate hemihydrate to dihydrate requires somewhat less time. Effect of Humidity Plaster can easily absorb water vapor from a humid atmosphere to form calcium sulfate dihydrate. The presence of small amounts of calcium sulfate dihydrate on the surface of the hemihydrate powder provides additional nuclei for crystallization. Increased contamination by moisture produces sufficient dihydrate on the hemihydrate powder to retard the solution of the hemihydrate. Experience has shown that the common overall effect of contamination of gypsum products with moisture from the air during storage is a lengthening of the setting time. Accelerators and Retarders: An accelerator is a substance that, when added to a gypsum product, decreases the setting time. Conversely, a retarder increases the setting time. The manufacturer uses these substances to standardize the setting behavior of a product. At times, accelerators or retarders may be used to alter the usual setting behaviour of a product. Potassium sulfate and common table salt are accelerators; vinegar, potassium citrate, and borax are retarders. Unfortunately, accelerators and retarders also change properties other than setting time, and they tend to reduce both setting expansion and crushing strength. For this reason, chemical accelerators or retarders should never be used with casting or soldering investments because a predictable setting expansion is important in these materials. Sodium citrate is a dependable retarder. A mixture of calcium oxide (0.1%) and gum arabic (1%) reduces the amount of water necessary to mix gypsum products, resulting in improved properties. Manipulating the water temperature, mixing time, and mixing rate are safer ways of controlling setting time than using chemicals.  There are a few laboratory procedures where using a specific accelerator is acceptable. One outstanding example is when slurry water is used to accelerate plaster or dental stone mixes in cast mounting procedures. Slurry water is a concentrated suspension of gypsum particles in water made by catching the runoff from a cast trimming machine. The suspended gypsum particles are allowed to settle, and about two-thirds of the water is siphoned away. The object is to develop a more highly concentrated suspension when the sedimentary calcium sulfate dehydrate particles are reagitated. Each of these calcium sulfate dehydrate particles acts as a center of crystalline formation. Depending on the concentration of the suspension, you can expect much shorter setting times when you use slurry water than when you use plain water.  Effect of Water-Powder Ratio and Mixing Time on Setting Expansion. The manufacturer strictly controls the setting expansion of a gypsum product by using a carefully measured amount of chemical modifiers. The manufacturer recommends standard proportioning and mixing procedures that make physical properties, including setting expansion, predictable. In the case of investments, setting expansion is such a sensitive factor that deviating from the manufacturer’s directions is a questionable practice. Always be aware that a number of gypsum’s properties are interdependent. For example, steps taken to change setting time can also alter setting expansion. If there is good reason to change a gypsum material’s normal setting expansion, follow these guidelines: Thick mixes (less water) tend to result in increased setting expansion and vice versa. Long mixing times tend to increase setting expansion and vice versa. Chemical Properties Most gypsum products are obtained from natural gypsum rock. Because gypsum is the dihydrate form of calcium sulfate (CaSO4. 2H2O), on heating, it loses 1.5 g mol of its 2 g mol of H2O and is converted to calcium sulfate hemihydrate (CaSO4. 0.5H2O). When calcium sulfate hemihydrate is mixed with water, the reverse reaction takes place, and the calcium sulfate hemihydrate is converted back to calcium sulfate dihydrate. Gypsum undergoes an exothermic reaction when mixed with water. Reproduction of Detail ANSI/ADA Specification No. 25 requires that types I and II reproduce a groove 75 μm in width, whereas types III, IV, and V reproduce a groove 50 μm in width. Air bubbles are often formed at the interface of the impression and gypsum cast because freshly mixed gypsum does not wet some rubber impression materials (e.g., some silicone types). The use of vibration during the pouring of a cast reduces the presence of air bubbles. Contamination of the impression with saliva or blood can also affect the detail reproduction. Mix the stone Techniques for mixing stone are critical for an acceptable outcome. Always follow manufacturer’s directions and use the measuring instruments that come with the product. Follow these steps to ensure a reliable mix. 1. Measure the water: Add the water to a clean rubber mixing bowl. Cool water decreases set time for the stone while warmer water speeds it up. Too much water prolongs set time and reduces strength while too little water makes the mix too thick and decreases the flowability. 2. Measure the powder: Slowly add it to the water, allowing all the particles to become wet and avoid air trapping. 3. Stir and spatulate: Begin stirring with a wide lab spatula. Spatulate thoroughly ( at least 1 minute), pressing the material against the sides of the bowl until all the powder is absorbed. The consistency should be like thick cake batter or creamy peanut butter. The mix should hold its weight on the spatula and not be runny. Avoid vigorous whipping as this can introduce unwanted air bubbles.  Spatulation Mechanical spatulation: mixing is done by a machine under vacuum Manual spatulation: mixing is done via hand. A vibrating table is used to remove air bubbles during mixing. Vibrate the mix: Place the rubber bowl on a vibrator set to medium. Allow air bubbles to rise to the top and burst. This will take around 30 seconds. Methods when pouring a cast 1. Two step inverted method 2. One step upright 3. Boxing method- Home work Pour the dental model-Two step inverted method After the stone is thoroughly mixed and air bubbles removed, you are ready to begin filling the impression. 1. Fill the tooth depressions: Rest the impression tray against the vibrator’s edge. Place a small amount of stone on one of the distal ends of the arch. Allow the vibration to carry the material slowly across the impression, watching closely to ensure that the stone fills each depression with no air bubbles. 2. Fill the impression body: When the tooth depressions have filled, add stone in larger amounts to a level slightly above the impression walls. Continue to vibrate to ensure complete coverage. 3. Build the base: Place the remaining stone onto a tile or glass slab, forming a mound slightly wider than the impression. If needed, make a second mix of gypsum and water for the base. Base should be 1 inch in thickness 4. Invert the tray: Place it on top of the base, smoothing the sides up onto the stone in the impression. Take care not to lock the edge of the tray into the stone. 5. Let it set: Stone mixtures take from 45 minutes to one hour to completely set up and obtain maximum strength. 6. Separate the model: After the stone is set and is cool to the touch, carefully separate the impression tray from the stone. Surface damage may occur if the cast remains attached to the impression material as it dehydrates. Pour the dental model-One step upright Requires skill to control the thickness of the base as well as the overall size, contour, and appearance of the cast 1. Gypsum is filled in the impression. It is then laid on the bench top with face-up. More material is added until the base and the art portion of the cast have been built up to the desired form  Removing Impression. Again, timing is critical. You need to allow the stone to allow approximately 1 hour for maximum strength of stone. If you try to remove it too soon, you are likely to break off teeth. However, if you allow the impression material to remain in contact with the stone for more than a few hours, it becomes chalky, and as the alginate dries out, it becomes stiff and will tend to break off teeth as well. Bottom line: The casts should be removed, ideally, between 1-3 hours after pour- up. Trim the dental model The dental model is now ready for trimming. Factors that influence criteria for proper trimming include the following: Size of patient’s arches Position of the teeth Preferences of the dentist or lab technician Purpose of the models Keep the proportion of the base approximately half the thickness of the anatomic portion of the model. Be sure the base and occlusal plane are parallel to each other. The cast should extend slightly past the retromolar pad of the mandibular and the hamular notch of the maxillary. Investment Material Investment Material A dental investment is a refractory material that is used to surround the wax pattern during the procedure of fabricating the metallic permanent restoration. It forms the mold into which the alloy is cast after the wax has been eliminated. An investment material to be used for a casting mold should expand on setting and heating to compensate for the shrinkage of molten metal as it solidifies. Metal casting alloys have different melting ranges ¾ only pure metals and alloys of eutectic composition have a melting point. The melting range of gold casting alloys (approx. 900° C) is lower than that of Co- Cr alloys (approx. 1350° C). Therefore, investment materials used for gold casting alloys are sometimes different from those used for Co-Cr alloys.  The investment material should be of a suitable consistency for adaptation to the wax model and have a reasonable setting time. To withstand the temperatures required for the casting process there should be no distortion, no decomposition; the investment should not fragment or disintegrate under the impact of the molten metal; the material should be porous to allow the escape of air and gases and the investment should be easily removed from the casting after cooling The refractory material for these investments is either quartz or cristobalite. This material provides the thermal expansion for the investment. Note: The expansion of the investment provides a larger mold to compensate for the subsequent contraction of the alloy. Classification of Investment material According to the binder used 1. Gypsum-bonded investments: binder is gypsum (calcium sulfate hemihydrate). Used when casting conventional gold alloys containing 65% to 75% gold at temperatures near 1,100°C. 2. Phosphate-bonded investments: binder is a metallic oxide and a phosphate. 3. Silica-bonded investments: binder is ethyl silicate. Not used much today. Classification of Investment material According to the types of silica used: 1. Quartz investments 2. Cristobalite investments Classification of Investment material According to the use and melting range alloy: 1. Gypsum Bonded Investment (G.B.) They are used for casting gold alloys. They can withstand temp up to 700 C֯. G.P. divided into 3 types Type I: For casting inlays\ crowns. Mode of expansion: Thermal Type II: For casting inlays\onlays\crowns. Mode of expansion: Hygroscopic. Type III: For partial dentures with gold alloys Classification of Investment material Phosphate Bonded Investment (P.B) For alloys used to produce copings or frameworks for metal-ceramic prosthesis, pressable ceramic. Divided into 2 types Type I: For inlay, crowns, and other fixed restorations. Type II: For partial dentures and other casts, removable restorations.  Silica Bonded Investment (S.B.) Use principally in casting of partial dentures in the base metal alloy. Brazing investment or soldering investment. Used for brazing parts of restorations such as clasps on partial dentures. Divided into 2 parts Type I: gypsum-bound dental brazing investment. Type II: phosphate–bounded brazing investment. (P.B.) have certain advantages than (G.B.): 1-They are more stable at high temperatures of 650° C (1200° F), and thus are the casting material of choice. 2- They expand rapidly at high temperatures Types of Expansion of Investment Material 1. Normal setting expansion, this will occur with investment during the change from the fluid state to the solid state. The percentage of this type is (0.034- 0,4%) it occurs when the investment becomes hard as a result of crystallization. 2. Hygroscopic setting expansion. In this case, the investment set after is put in a water bath of 35C and the amount of this type of expansion is 0.35% occurs only in (G.B.) investment. 3. Thermal expansion, occurs during the burn-out procedure, due to heating the investment in the oven and it’s about 1.45%.  Expansion varies according to the: 1. Investment formula. 2. Water/ powder ratio. 3. Increase spatulation both in rate and time. 4. Aging & storage. Composition of Investment materials 1. Refractory material: Silica is the refractory material of choice, it is available in three crystalline forms quartz, cristobalite and tridymite. It adequately withstands the temperatures used during casting. It is responsible for producing much of the expansion which is necessary to compensate for the casting shrinkage of the alloy. 2. Binder material: which binds the refractory particles, and may provide additional expansion to compensate for the casting shrinkage of the alloy. The nature of the binder characterizes the material: Gypsum-bonded Investment material Silica-bonded Investment material Phosphate-bonded Investment material Other chemicals and modifiers Modifiers are used to improve the properties of the material. It regulates the setting and thermal expansion and prevents shrinkage of gypsum when heated above 300 ºC. These chemicals are sodium chloride, boric acid, potassium sulfate, graphite, copper powder or magnesium oxide Selection of Investment Material The main factors involved in the selection of investment material are: The casting temperature to be used. The type of alloy to be cast. The investment which is best able to retain its integrity at the casting temperature and able to provide the necessary compensation for casting shrinkage is chosen. Gypsum-bonded Investment Material Composition These materials are supplied as powders which are mixed with water and are composed of a mixture of silica (SiO2) and calcium sulphate hemihydrate (gypsum product) with minor components including powdered graphite or powdered copper and various modifiers to control setting time. The calcium sulphate hemihydrate reacts with water to form calcium sulphate dihydrate (gypsum) which effectively binds together the refractory silica. Gypsum alone is not satisfactory as an investment for alloy casting since it contracts on heating as water is lost and fractures before reaching the casting temperature. Gypsum-bonded Investment Material Composition The magnitude of the contraction, which occurs rapidly above 320°C, is significantly reduced in investment materials by the incorporation of sodium chloride and boric acid. The setting expansion of the calcium sulphate dihydrate, when mixed with water partially compensate for the shrinkage of the alloy which occurs on casting. Further compensation can be achieved by employing the hygroscopic setting expansion.  Phosphate-bonded Investment Material Composition These materials consist of a powder containing silica, magnesium oxide and ammonium phosphate. On mixing with water or a colloidal silica solution, a reaction between the phosphate and oxide occurs to form magnesium ammonium phosphate. This binds the silica together to form the set investment mould. The formation of the magnesium ammonium phosphate involves a hydration reaction followed by crystallization. A small setting expansion results from the outward thrust of growing crystals. Phosphate-bonded Investment Material Composition The material is also able to undergo hygroscopic expansion if placed in contact with moisture during setting. Moisture adversely affects the unmixed material and the container should always be kept closed when not in use. On heating the investment prior to casting, mould enlargement occurs by both thermal expansion and inversion of the silica. At a higher temperature some of the remaining phosphate reacts with silica forming complex silicophosphates. Silica-bonded Investment Material Composition These materials consist of powdered quartz or cristobalite which is bonded together with silica gel. On heating, the silica gel turns into silica so that the completed mould is a tightly packed mass of silica particles. The binder solution is generally prepared by mixing ethyl silicate or its oligomers with a mixture of dilute hydrochloric acid and industrial spirit (improves the mixing of ethyl silicate and water). A slow hydrolysis of ethyl silicate occurs producing a sol of silicic acid with the liberation of ethyl alcohol as a byproduct. Silica-bonded Investment Material Composition Stock solutions of the silicic acid binder are normally made and stored in dark bottles. The solution gels slowly on standing and its viscosity may increase noticeably after three or four weeks, when this happens it is necessary to make up a fresh solution. (C2H5O)4Si + 4H2O → Si(OH)4+ 4COH2H5 The silicic acid sol forms silica gel on mixing with quartz or cristobalite powder under alkaline conditions achieved by the presence of magnesium oxide in the powder. It is necessary to incorporate as much powder as possible into the binder solution to have sufficient strength at the casting temperature. This process is aided by a gradation of particle sizes such that small grains fill in the spaces between the larger grains. A very thick, almost dry mix of investment is used and it is vibrated in order to encourage close packing and produce as strong an investment as possible. Inlay Investment Inlay investments are usually gypsum bound. Inlay investments are commonly used for investing many different kinds of fixed restorations cast in conventional golds. When molten gold alloy is cast into a mold, it cools and solidifies. As it cools, it shrinks. The amount of shrinkage is approximately 1.4 (± 0.2) percent. If nothing is done to compensate for this shrinkage, the casting will be too small. The mold space must be enlarged so the molten metal is cast into a space that is 1.4 percent oversize. As the molten metal solidifies and shrinks, the casting attains the correct size.  Techniques have been devised to use setting and thermal expansion characteristics of investments to compensate for cast metal shrinkage. In one technique, high heat (1290 o F) is used to produce the majority of the required expansion. In another technique, the hygroscopic expansion of the investment is responsible for most of the compensation. Inlay investments tend to fall into two broad categories depending on how they are used-- high heat technique investments (above 1300 o F) and low heat technique investments (1300 o F or less). One type of low heat technique is used with a high water content called a hygroscopic technique. This technique creates additional expansion at a lower temperature burnout. Soldering Investment: A soldering investment is similar in composition to a casting investment  Soldering Investment A soldering investment is similar in composition to a casting investment with a quartz refractory. An investment with a quartz refractory expands less than one having cristobalite as the heat resistant component. Minimal normal setting expansion is a desirable soldering investment characteristic. A soldering investment does not expand nearly enough to compensate for the shrinkage of molten gold and should not be used for casting purposes. Like casting investments, soldering investments are made with gypsum or high heat binders. The heat resistance of the binder is matched to the anticipated soldering temperature. As a rule of thumb, a soldering procedure that takes place above 1950 o F requires an investment with a high heat binder. Investment for Chrome alloy High-Heat, Chrome-Alloy Investment. A high-heat, chrome-alloy investment is made to withstand a much higher heat than the 1300 o F normally used in eliminating wax for casting gold. Such an investment consists of a quartz powder mixed with an ethyl silicate liquid and is used with the high melting range of chrome alloys (2700 to 2800 o F). Low-Heat, Chrome-Alloy Investment: A low-heat, chrome-alloy investment is gypsum bound and has a silica refractory component. It is similar to the investment used for casting gold. A low-heat, chrome- alloy investment is used as part of the system for producing Ticonium chrome alloy castings. Ticonium metal is used throughout the Air Force Dental Service for RPD frameworks. The burnout temperature of ticonium investment molds is 1350 o F, and the casting temperature of ticonium metal is 2500 to 2600 o F. There is a sulfur dioxide liberation problem associated with gypsum bound investments at high burnout or casting temperatures. One way to com bat this problem is to increase the percentage of refractory material relative to the gypsum binder in an investment formula. Ticonium metal shrinks 1.7 percent as it solidifies. The investment and burnout techniques are balanced to furnish that amount of expansion in the mold. Investment for Ceramic Gold Alloy Gypsum-bonded investments are not adequate for casting ceramic golds. The expansion is not high enough, and the gypsum decomposes under the high temperatures. Instead, investments containing magnesium oxide and soluble phosphate should be used. The dissolved phosphate reacts with magnesium oxide to form a matrix of magnesium phosphate which binds silica particles together much the same as gypsum binds low heat investments. Phosphate-bound investments are coarse in particle size, heat resistant, strong, and sometimes difficult to remove from castings. The investment is sluggish and sets rather rapidly with a working time of 3 to 4 minutes. All-purpose investments have a smaller particle size; therefore, a smoother casting can be made. Rules for Handling Gypsum Products Use Clean Equipment. Always use a clean mixing bowl and spatula. Hardened particles left in the bowl from a previous mix alter the setting time and weaken the material. As little as 0.1 percent of the hardened particles in a mix of casting investment reduces the setting time and alters the thermal or hygroscopic expansion. The best time to clean a bowl and spatula is while the plaster is still soft and easy to remove. Tumble the Contents. Tumbling helps ensure an even distribution of the investment constituents.  Add the Powder to the Water. The powder is always added to the water; the water is never added to the powder. Place the required amount of water into the bowl and then sift the powder into the water until the powder forms an island. The powder gradually absorbs the water; consequently, the mixture is free of lumps and air. Because tap water contains contaminants, use only distilled water. Measure the Water and Weigh the Powder. To ensure the properties of any gypsum product are maintained, an accurate water-to- powder ratio must be obtained. Weigh the powder and measure the volume of water before mixing the gypsum material. Mix Well. Ensure all powder is spatulated into the water. As mixing proceeds, the water and powder form a mixture of cream y consistency. (To avoid excessive incorporation of air into the mix, do not whip the mix.)  Vacuum-Mix the Materials. Phosphate-bound investments release ammonia gas when mixing. Vacuum-mixing removes gas and air from the mix. Avoid gas entrapment by holding the mix under vacuum for 30 seconds. (Gas entrapment in the mold results in nodules on the casting.) Never Add to a Mix. Adding to a mix interferes with the setting mechanism and results in a weak and distorted product. It is better to begin a new mix. Use Good Equipment. A scarred or cracked plaster bowl allows minute particles of material to lodge in the cracks. These particles could contaminate and spoil the mix. Do Not Contaminate the Material. Never allow water or other contaminants to fall into a bin containing gypsum material. One drop of water can adversely affect the entire batch. Know the Material. An aged investment can ruin a piece of work. Be aware that investments have batch numbers and expiration dates stamped on them. Contact the manufacturer if any problems are suspected with your investments. Another good practice is to keep investments rotated, with the oldest packs being used first. Storage Improper Storage: When gypsum material is exposed to air, it absorbs water. The water may alter its working qualities and make it unfit for use. When plaster or stone is exposed to air for a short period of time, it sets faster than usual. If it is exposed for a longer period, it may set very slowly and be weak when it’s set. A prolonged period of storage in an unsealed container may alter the physical properties of casting investments, greatly changing the setting time, setting expansion, and reducing the crushing strength.  The setting time of casting and soldering investments is listed on the container along with the physical properties expected when the recommended powder to water ratios are used. This data is based on fresh material as it leaves the factory. It does not apply to aged batches of material that have been improperly stored. If an investment takes an unusually long time to reach an initial set (more than 20 minutes), the entire batch must be discarded. A prolonged setting time is a warning that some or all of the desirable physical properties may have been lost or so altered as to render the investment unfit for use.  Proper Storage: Gypsum material must be properly stored. The storage problem is more acute in a humid climate than in a dry one. All gypsum products must be stored in a sealed container in a dry room. A systematic plan for withdrawing older stock from the supply room should be used. To minimize prolonged periods of storage, large quantities must not be stockpiled due to the danger of deterioration. Some authorities also recommend that still another factor be taken into account when casting investments are stored. The heavier constituents (for example, quartz) settle to the bottom of the container, thereby altering the working properties of the investment. Therefore, investments should be tumbled before use, either mechanically or by hand, to make sure the powder is evenly mixed throughout. Proper Handling of Plaster and Dental Stone Cast Erosion of Casts: A well-poured cast can be ruined by contact with water because hardened stone is soluble in water in a ratio equal to or less than 1 part stone to 500 parts of water. When a stone cast is immersed in water, an erosion process begins immediately on the surface of the stone. The erosion is noticeable in as short a period as 10 minutes. This can be shown in the laboratory by suspending a stone cast in water so part of the cast is submerged, while part of it remains out of the water. In 10 minutes, the erosion of the submerged part will be evident because of its pitted appearance. The time necessary to produce a noticeable effect depends on the mineral content of the water, temperature of the water, and density of the stone. A poured impression should never be submerged in tap water because of the harmful effect it has on stone. Saturated Calcium Sulfate Dihydrate Solution (SDS) Preparation: SDS is a clear, true solution of water and a maximum amount of dissolved dihydrate (set) gypsum product. Cast surfaces exposed to SDS do not erode nearly as much as cast surfaces bathed in tap water. If a cast must be soaked for more than 1 or 2 minutes, SDS should be used. SDS is made by immersing fragments of gypsum casts in water for about 5 days. A saturated solution consists of about 0.2 grams of dehydrate in 100 cc of water. If a slurry water suspension is left to settle out for 3 to 4 days, the clear fluid above the sediment is SDS. For use, siphon off the SD S into another container without agitating the sediment layer. SDS can be made from plaster, dental stone, or gypsum bound investment, whichever is best suited for the kind of cast you expect to wet. Wetting Casts: Occasionally, casts require quick superficial wetting (for example, cleansing cast surfaces). SDS must be used instead of tap water for this purpose. When a cast is shaped on a cast trimmer, gypsum slurry splashes onto its surface. If this slush layer is allowed to dry, it is hard to remove and cast damage could occur. As the slurry buildup accumulates, rinse the cast in a suitable container of SDS to remove the slurry. The SDS must be changed often or it will also turn into concentrated gypsum slurry. When outright cast soaking must be done in conjunction with a laboratory procedure, the cast must not be completely submerged in SDS. Total immersion slows down the soaking process because air trapped in the cast cannot readily escape. Instead, the fluid level should be maintained below the tissue surface of the cast. A cast can be moistened in this manner in 20 to 30 minutes. The wetting process can be seen gradually working up from the base of the cast to the tips of the teeth, much the same as oil dam pens the wick in a lamp. If relief wax has been placed on the cast, there is danger of the escaping air from the cast lifting the wax from the stone. Instead of setting the cast on its base, set it on its end in the SDS.  Technique Material Setting time Heat resistance Normal Setting Hydroscopic Thermal Expansion expansion Expansion Plaster Initial: 7-13 minutes NA As low as NA NA Final: 45minutes possible Stone Initial: 8-15 minutes (Hydrocal) Final: 45 minutes Die Stone Initial: 15 minutes Final: 25-30 minutes Soldering Initial 8-12 minutes Matched to the melting Matched to the expansion of the metals being soldered Investments Final: 18-22 minutes temperature of the metals being soldered Gold- Casting Initial: about 12 Matched to the burnout Thermal expansion technique: Investments minutes and casting temperature Semihygroscopic and thermal expansion must Final: 35-45 minutes of the metal being cast compensate for gold shrinkage ( about 1.4%) Hygroscopic Expansion Technique: Hygroscopic expansion, pattern wax expansion and thermal expansion must total about 1.4% Chrome-nickel Initial: 8-12 minutes Special gypsum bound Combined semihygroscopic and thermal expansion system Final: about 20 investment for the must compensate for shrinkage of chrome nickel about investment minutes ticonium system 1.7%