dental02.jpg (11342 bytes)Fundamentals of
Dental Materials

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Gypsum Products, Dental Waxes, and Impression Materials

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3-1. 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.

David L. Heiserman, Editor

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Revised: May 22, 2017