GENERAL - GYPSUM
a. General. A number of gypsum products are used in dentistry. Plaster of Paris and artificial stone powder are the ones most used as cast materials. A general understanding of the chemistry of gypsum products will enable the dental specialist to use them wisely and increase his knowledge of why they react as they do. Gypsum is composed mainly of calcium sulfate dihydrate. A dihydrate is a material consisting of two parts of water to one part of the compound. Calcium sulfate dihydrate, therefore, is one part calcium sulfate and two parts water.
b. Properties. In the manufacturing process, gypsum is converted to plaster of Paris and artificial stone by a process called calcining. The gypsum is first ground to a fine powder of particle size. Plaster of Paris is derived when the gypsum is subjected to heat in an open vat. Artificial stone is produced when the gypsum is processed by steam heat under pressure. In both products, the reaction converts calcium sulfate dihydrate into calcium sulfate hemihydrate by the removal of 75 percent of the water molecules. Chemically, the plaster and artificial stone are identical. However, the plaster particles are rough, irregular, and porous, and the artificial stone particles are prismatic, more regular in size, and dense. When the plaster or stone is mixed with water, a hard substance is formed and the process described above is reversed. In the setting reaction, crystals of gypsum intermesh and become entangled with one another, giving the set material its strength and rigidity.
PLASTER OF PARIS
a. Uses. Plaster of Paris is used for pouring casts, making matrices for prosthodontic restorations, for attaching casts to articulators, and general use in the dental laboratory where strength is not important. The crushing strength for plaster of Paris is 2,600 psi.
b. Mixing. Water-powder ratios must be used as stated by the manufacturer. Before mixing, the can containing the material should be agitated to evenly disperse all elements in the powder. A clean, dry rubber bowl and plastic spatula are used to manipulate the materials. First, the water is measured and poured into the rubber bowl. The powder is weighed and sifted into the water to avoid trapping air bubbles. Then, with a spatula, the mix is stirred (spatulated) for 30 to 60 seconds in a knifing or stirring motion, making sure to include all powder from the sides of the bowl. (Whipping the mix will entrap air and should be avoided.) Before the mixed material is poured, it should be vibrated to remove any trapped air bubbles.
c. Setting Time. The initial setting time for plaster of Paris is 5 to 10 minutes. In this stage, the plaster loses its glossy appearance and is hard enough to hold for carving. The final setting time is approximately 45 minutes. In this stage, the plaster achieves a dry, hard condition. The setting of plaster can be hastened by using less water, by mixing longer, by using chemical accelerators, or by using warm water (up to 85o F (29o C)). Reversing these processes or using chemical retarders lengthens the setting time. The most satisfactory results will be obtained by following the manufacturer's directions.
a. Uses. Artificial stone is used in making master casts and dies and for general laboratory use when a very hard, strong product is needed. Artificial stone particles are nonporous. Therefore, the finished product is hard and dense. This provides an excellent master cast for the fabrication of prosthetic restorations. The crushing strength of artificial stone is 7,500 psi.
b. Mixing. Artificial stone is mixed much like plaster of Paris. The average mixing ratio is 30 cc (cubic centimeters) of water to 100 grams of stone powder. This ratio may vary with different manufacturers. The required amount of water is placed in a rubber bowl. The stone powder is added slowly. (Incorporate all of the powder with water before spatulating.) Spatulation should be thorough without whipping the mixture. Whipping can trap air bubbles, thus weakening the cast. The bowl should be vibrated during the mixing to make air bubbles rise to the surface. Spatulation should be completed in 30 to 60 seconds; after that, the bowl should again be vibrated. The use of mechanical spatulation helps to reduce air bubbles.
c. Setting Time. The initial setting time for artificial stone is usually 8 to 10 minutes. The final setting time is 25 to 45 minutes depending on the type of stone mixed. The surface hardness of artificial stone can be increased by soaking the cast for several hours in a solution of borax.