retarders. Manufacturers usually include both accelerators and retarders in dental gypsum products to control setting time and expansion. Therefore, extreme caution must be exercised in adding any modifier to a gypsum product in the dental laboratory.

c. Crushing Strength. Crushing strength, or compressive strength, is the measure of the greatest amount of compressive force which can be applied to a substance without causing it to fracture. The strength of a gypsum product increases rapidly as it hardens. Two strengths of gypsum products are recognized, since the relative amount of water in the set material has a distinct effect upon its strength.

(1) Wet strength. Wet strength is the strength of the material with all the excess water present in the mass.

(2) Dry strength. Dry strength is the strength of a thoroughly dried gypsum specimen. This is usually two or more times greater than the wet strength. A gypsum specimen reaches its maximum dry strength after it has bench-set about 24 hours.

d. Effect of Variables on Crushing Strength. The strength of set gypsum products can be directly affected by several variables which are under the control of the dental prosthetic specialist as he mixes the material.

(1) Water-powder ratio. The more water used, the lower will be the crushing strength. Gypsum products are porous and the greater amount of water will increase the porosity, since there will be fewer crystals per unit of volume of the material.

(2) Mixing. Longer and more rapid mixing, up to 1 minute, will yield greater strength. Overmixing, however, will break down the forming crystals and reduce the crushing strength of the end product.

(3) Modifiers. In general, crushing strength is reduced by modifiers (accelerators and retarders). However, borax can act to increase the surface hardness of the material.

e. Expansion.

(1) Setting expansion. This is the amount that a material will expand or enlarge as it sets or hardens. It is usually expressed in terms of percent and must be rigidly controlled to insure accuracy in a finished restoration. This control can be exercised by the manufacturer or by the dental prosthetic specialist. The addition of accelerators or retarders by the manufacturer will result in a small amount of setting expansion of gypsum products. If the dental prosthetic specialist makes a thin mix, the setting expansion

will be less whereas if the mixing time is longer, the setting expansion will be greater.

(2) Hygroscopic expansion. This is the amount that a material will expand when it contacts water while it is setting. This physical property is important in the use of cast investment materials.

(3) Thermal expansion. This is the amount that a material will expand when it is heat-soaked at 800° to 1290° F. This physical property is also important when cast investment materials are used.

3-5. Storage of Gypsum Products

a. Gypsum material exposed to air will absorb moisture which will alter its working qualities, making the product unfit for use. This alteration may appear in several ways. When plaster or artificial stone has been exposed to air for a short time, it may set faster than usual. If exposed for longer periods, it may set very slowly and be noticeably weakened when set. Prolonged periods of storage in an unsealed container may alter the physical properties of casting investment materials so much as to drastically change the setting time and the setting, thermal, and hygroscopic expansion as well as to reduce the crushing strength. The setting time of casting and soldering investment materials is listed on the container along with physical properties which may be expected when the recommended powder-water ratios are used. These data are based on fresh material as it leaves the factory and they will not apply to material which is old or has been stored improperly. If an investment material takes longer than 20 minutes to set, it is a warning that the desirable properties have been lost or altered and the rest of the material should be discarded.

b. The storage problem is more acute in a humid climate than in a dry climate. Air-conditioning systems may complicate the problem by altering the moisture content of the air. All gypsum products should be stored in closed, sealed containers in a dry room. A plan for the systematic withdrawal of stock from the supply room should be standardized to expend old supplies first. This will help to minimize overlong periods of storage. Unduly large quantities should not be stocked because of the danger of deterioration. Large quantities should not be stored in a plaster bin. A 2-day supply in the bin is sufficient, with the remainer stored in airtight containers in a dry room.

c. In the storage of casting investment mater

ials, the heavier constituents tend to settle to the bottom of the container. This upsets the proper balance of the investment material and changes its working properties. This is very apt to occur when investment material is stored on the same bench with a vibrator. Vibration tends to separate the materials. To insure that the investment material is homogenous throughout, the container must be tumbled before use.

3-6. Types of Gypsum Products

a. Plaster of Paris. Plaster of paris for dental use is processed in a precise manner to provide a product of highest purity and suitable working properties. These properties must be consistently uniform throughout a particular batch and from one batch to another. Plaster of paris must harden within definitely prescribed periods of time and its setting expansion must be within reasonable limits. It is used in the laboratory to make casts and to attach casts to articulators. The initial setting time for most dental plasters is 7 to 13 minutes, and the final set takes about 45 minutes.

b. Impression Plaster. This plaster is especially compounded by the manufacturer for use in the mouth. Its characteristics differ slightly from those of plaster of paris. It must set more rapidly to reduce the time it must be held in the patient's mouth by the dental officer. Sometimes it must be broken into pieces to be withdrawn from the mouth. It cannot be too strong and must be brittle enough to fracture cleanly so that the pieces can be accurately reassembled. It must have very little setting expansion. If the impression expands appreciably, it will not be accurate. It may be flavored for the patient's comfort.

c. Soluble Impression Plaster. When a positive cast is made by pouring a gypsum product against a plaster negative impression, there may be difficulty in separating the two hardened materials. Therefore, some impression plasters are made water-soluble by the addition of starch. Such a plaster can be dissolved from a hardened cast with hot water and there is no danger of damaging the cast. Soluble plaster can be made in the laboratory by adding 21⁄2 parts of potato or corn starch to 71⁄2 parts of impression plaster, both proportioned by weight.

d. Artificial Stone (Hydrocal). Chemically, this material is very similar to plaster of paris, but variations in the manufacturing process cause differences in the physical properties. Under the microscope, the plaster of paris particles appear long, needle-like, and quite porous; whereas the artificial stone particle appears smaller, much

more dense, and relatively nonporous. For this reason, artificial stone requires less water in mixing and sets more slowly. Once set, it is harder and much more dense, and has a higher crushing strength. These properties make it preferable to plaster of paris as a master cast in complete and partial denture fabrication. It withstands more pressure in flasking and packing and is less apt to be scratched or damaged in handling. Artificial stone is used for preliminary and master casts and dies, and in flasking procedures. It is colored by the manufacturer to make it easily distinguishable from plaster of paris. The initial set of artificial stone is 13 to 17 minutes. The final set is about 45 minutes.

e. Die Stone. This is a specially processed form of artificial stone used to make crown and inlay dies. It is harder and more dense, and has less setting expansion than artificial stone. Die stone is usually colored to differentiate it from plaster of paris or artificial stone. Since the amount of setting expansion is critical, it is important to use the water-powder ratio recommended by the manufacturer when making a die with this type of stone.

f. Casting Investment Material. A casting investment material is a gypsum product in which a wax pattern for a prosthodontic restoration is enclosed. When the wax is burned out from the investment, a mold space remains, and it is into this mold space that molten metal is forced to form the restoration.

(1) Properties. In addition to the general physical properties common to all gypsum products, investment materials must possess other special characteristics because of the particular procedures in which they are used. These materials must have a smooth surface when set. A smooth surface on the mold will result in a smooth surface on the metal which is cast into the mold space. Casting investment materials must be able to withstand high temperatures without cracking or distorting. If the investment breaks down under high heats involved in the casting procedure, the molten metal will run into the flaws and produce "fins" on the casting. This material must have a degree of expansion which is readily controlled. It should not yield any byproduct which would contaminate the metal being cast. Carefully controlled heating to burn out the wax pattern before casting will satisfy this requirement. Higher temperatures than those recommended by the manufacturer will cause the investment to break down and give off sulfur gas. This gas will combine with the molten metal and yield a brittle casting. Although the investment must have a

smooth surface, it must be porous enough to allow gases to escape from the mold space during the casting procedure. As the molten metal enters the mold space, it compresses the gases. If gases cannot escape, their pressure will increase to where the metal will be prevented from filling the mold and the casting will be incomplete.

(2) Composition. The above properties are obtained by careful preparation and blending of ingredients incorporated by the manufacturer. The two main ingredients of any investment material are the binder which holds the material together and the refractory substance which resists heat. Investment material formulas contain combinations of 25 to 35 percent powdered gypsum (either plaster of paris or artificial stone) as the binder; 65 to 75 percent silica (either quartz or cristobalite) as the refractory; minute quantities of modifiers to slow or hasten the setting time (borax or potassium sulfate); material to prevent surface oxidation of the cooling metal (graphite), and oxide pigment powders to color the investment material.

(3) Expansion. When molten gold alloy is cast into a mold, it cools and solidifies. As it cools, it shrinks. The amount of shrinkage is approximately 1.25 percent. If nothing is done beforehand to compensate for this shrinkage, the casting will be too small. The mold space must be enlarged so that the molten metal is cast into a space that is 1.25 percent too large. As the molten metal solidifies and shrinks, the casting will attain the correct size. When the investment is heated, the mold space expands 1.25 percent and the molten metal is cast into the enlarged space. During solidification it shrinks to the proper dimensions. Gypsum products contract when they are heated. The amount of contraction is directly related to the amount of water present in the material. Therefore, the required amount of expansion to compensate for the shrinkage of the cast metal must be provided by other ingredients added by the manufacturer. These ingredients furnish the required expansion in one or both of the following ways:

(a) Thermal expansion. Thermal expansion is the amount that a material will expand when it is heat-soaked at high temperatures. With investment materials, this expansion is directly related to the type and amount of silica present. Casting investment materials of the thermal expansion type have a setting expansion of 0.1 to 0.5 percent when mixed according to directions. To compensate exactly for the shrinkage of the solidifying gold alloy, the remainder of the required expansion takes place as the mold is heated

to 1250° to 1290° F. This thermal expansion prepares the mold to receive the molten metal.

(b) Hygroscopic expansion. Hygroscopic expansion is the amount that a material will expand due to absorption of water when it is placed in contact with water while it is setting. In the use of hygroscopic investment material, the ring is placed in a container of water while the investment hardens. The hygroscopic technique requires a special investment material and cannot be used with a thermal expansion type. The amount of hygroscopic expansion is inversely proportional to the amount of silica present, and it can be increased by using less water in the mix, by immersion before the final set, or by decreasing the size of the powder particles.

(4) Casting chrome-base alloys investment materials. Investment materials used in casting chrome-base alloys are similar to those used in casting gold alloys except that the binder must be able to withstand a much higher range of temperatures. The gypsum in a standard gypsumsilica investment will break down at temperatures above 1300° F., giving off a sulfur gas which will cause brittleness in a casting. The investment must be compounded so that its combined expansions exactly compensate for the subsequent shrinkage of the alloy, which is approximately 1.7 percent.

g. Soldering Investment Material. Soldering investment materials are similar to casting investment materials except that the gypsum-silica ratio is different. They are coarser than casting investment materials and not as strong. They are compounded to give adequate resistance to the thermal shock incident to heating with a blowtorch and to permit their expansion along with the metal being soldered. Soldering investments do not expand enough to compensate for the shrinkage of molten gold alloy and should never be used as casting investments.

3-7. Rules for Manipulation of Gypsum Materials

a. Always use a clean mixing bowl and spatula. Hardened particles left in the bowl from a previous mix will alter the setting time and weaken the material. As little as one-tenth of 1 percent of hardened particles in a mix of casting investment material will reduce the setting time and alter the thermal or hygroscopic expansion and may even cause a shrinkage instead of the desired expansion.

b. Always add the powder to the water, never the water to the powder. Place the proper amount

f. After using the mix, form the habit of cleaning the bowl and spatula while the plaster is soft and easy to remove.

g. Never use a scarred or cracked plaster bowl. Minute particles of material lodged in the cracks can spoil the mix.

h. Use mechanical spatulation in preference to hand mixing.

i. Never allow water, not even one drop, to fall into a bin containing gypsum material. One drop of water can weaken the entire contents.

j. Make it a rule to know the age of the material being used. Old material will not produce the desired results. It is good technique to test routinely each new container of material before using it for the actual fabrication.

3-8. General

An impression is a negative imprint of a given area. Into this imprint a second material is poured and, when the two are separated, an accurate, positive cast of the desired area is obtained. Each of the many types of impressions made in the dental clinic requires a material specifically compounded for a particular purpose. The first step in the fabrication of a complete denture requires a material which accurately registers every detail of the area upon which the denture is to rest. The impression material used in the fabrication of partial dentures must be elastic enough to spring around tooth and soft tissue undercuts upon removal from the mouth and return to its original contours and dimensions. In some procedures, more than one impression material is used in a sequence, taking advantage of the most favorable properties of each to build up a composite final impression which is accurate in detail. Although many impression materials are used primarily by the dental officer at the chair, the dental prosthetic specialist must know how to care for and work with them in the dental laboratory. 3-9. Modeling Plastic (Impression Compound)

a. Modeling plastic is composed of shellac, talc, and glycerides derived from tallow oil. The material is described as thermoplastic, since it softens when warmed and hardens when cooled, all without change in its chemical makeup. The temperature range at which softening occurs depends upon the proportions of the ingredients contained in the material. Modeling plastic is supplied in

cakes, wafers, or sticks, and in various colors to aid in distinguishing between products of different softening (fusing) temperature ranges.

b. Any of the gypsum products can be poured against a modeling plastic impression without the use of a separator. Modeling plastic is one of the few impression materials against which an amalgam die can be packed. Although no separating material is required in either procedure, extreme care must be used in drawing apart the impression and cast or die, since both materials are hard and relatively unyielding. Therefore, the modeling plastic is softened in warm water (120° F.) and removed gently, so that the cast is not damaged.

3-10. Impression Plaster

See paragraph 3–66.

3-11. Soluble Impression Plaster See paragraph 3-6c.

3-12. Hydrocolloid Impression Materials Hydrocolloid impression materials are gelatinous substances developed for use in dentistry because they can accurately reproduce details in a dental impression. Unlike plaster, which must be broken to remove it from an undercut, or modeling plastic, which is distorted when removed from an undercut, hydrocolloid impression material can elastically deform momentarily so that it can be removed. After removal it will snap back to its former shape. There are two basic types of hydrocolloid impression materials: reversible (agar) and irreversible (alginate). They differ chemi

cally and physically, and they require different handling. The use of a separating material is not required when pouring a cast in an impression made from either type.

3-13. Reversible (Agar) Hydrocolloid

This impression material is a gelatin-like substance which is softened by heating and hardened by cooling. A physical, not chemical, change is involved. Since this process can be performed many times, the term "reversible" is used to describe this material.

a. Composition. The basic ingredients of this material are water and agar-agar, an extract from seaweed. The strength of the material is increased by the addition of a hardener such as borax. This also increases the viscosity of the material so that it can be handled easier. When a gypsum product is poured against a hydrocolloid impression, its setting time is retarded by borax and its contact with the gel itself. Consequently, an accelerator such as potassium sulfate is added to counteract the action of the borax and the gel. The impression can be immersed in a solution of the accelerator before the cast is poured, but it is better to have the accelerator incorporated into the material by the manufacturer. The strength, viscosity, and rigidity of the impression material are improved by the manufacturer's addition of filler material such as wax, clay, or other inert powders. It is essential to comply strictly with the manufacturer's instructions, especially those concerning the prompt pouring of the cast. The dimensional stability of hydrocolloid impressions depends upon a balance between the processes of losing water by exudation (syneresis), which causes shrinkage of the impression, and gaining water by absorption from the air, which causes swelling. Casts must be poured promptly.

b. Use. Reversible (agar) hydrocolloid impression material is heated in a double boiler or special syringe, depending on the amount required, until it softens to a jelly-like consistency. Then it is carried to the patient's mouth in a special tray and water-cooled to set. The cast should be poured immediately. No fixing solution or separating material is required unless specified by the manufacturer. This material is used mainly for making impressions for the fabrication of partial dentures, crowns, or inlays. It can be used in the dental laboratory for duplicating purposes if duplicating material (para 3-43) is not available. 3-14. Irreversible (Alginate) Hydrocolloid

This impression material is supplied as fine

powder, either in bulk or premeasured units. It is used primarily by the dental officer to make impressions in the patient's mouth. When it is mixed with the proper amount of water, a chemical reaction occurs and the jelly-like mixture gradually hardens to the desired elastic rigidity. Since the reaction is chemical and cannot be reversed, the material is called an "irreversible" hydrocolloid and can be used only once.

a. Composition. The basic ingredient of this material is a soluble alginate derived from an extract of seaweed. Other ingredients are added by the manufacturer to produce the desired physical properties. Since the basic alginate is soluble in water, it must be changed to an insoluble compound which will gel and hold its shape when removed from the mouth. The chemical used most to produce this change is calcium sulfate (reactor), which reacts with the soluble alginate to yield insoluble calcium alginate. To prevent this chemical change from occurring before the material can be placed in the mouth, a retarder such as trisodium phosphate is included in the powder. The reactor acts first upon the retarder. When the available supply of retarder is exhausted, the reactor acts upon the alginate and the gel sets. The manufacturer adds inert filler powders to increase the strength, firmness, and hardness of the gel.

b. Effect on Gypsum Materials. Irreversible (alginate) hydrocolloid materials, like the reversible (agar) hydrocolloid, retards the setting time of gypsum materials poured against it to form casts. It softens the surface of the cast. To prevent softening, the manufacturer sometimes adds a hardener, usually a fluoride compound, to the powder which hardens the surface of the cast. If the alginate powder does not include a hardener, the manufacturer will supply one separately with which a hardening solution can be made. The dental prosthetic specialist immerses the impression in the solution before pouring the cast.

c. Pouring the Cast. The dimensional stability of irreversible (alginate) hydrocolloid impressions is similar to that of reversible (agar) hydrocolloid impressions. Dimensional stability depends upon the balance between the processes of syneresis and imbibition. Therefore, the impression must be poured immediately.

d. Manipulation. The constituents of the irreversible (alginate) hydrocolloid impression material are compounded by the manufacturer to give the desired properties. Directions for mixing should be followed carefully, with no changes being made in the powder-water ratio or the spatulation time. The best method for controlling