GYPSUM PRODUCTS, DENTAL WAXES, AND IMPRESSION MATERIALS
Section I. GYPSUM PRODUCTS
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.