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Gypsum is used to create:

Impressions of edentulous arches (inelastic thermoset)
Study models

Composition of Dental Gypsum Products

Extracted from the mineral gypsum

Chemically all dental gypsum products are derived from calcium sulfate dihydrate (In its pure form is white in color)

Gypsum Composition

calcium sulfate dihydrate
Ca SO4 • 2 H2O


All dental gypsum products are produced through a process of heating gypsum and driving off part of the water of crystallization

The result is the driving off of 1 1/2 H2O

Products differ in characteristics due to the type of calcination process used


CaSO4 • 2H2O + heat forms CaSO4 • 1/2H2O
(Calcium sulfate hemihydrate)

Impression Plaster defined

AKA Type I gypsum product;
(for edentulous patients; usually mint flavored)

Plaster defined

AKA Type II gypsum product; beta-hemihydrate; model or lab plaster

Stone defined

AKA Type III gypsum product; alpha-hemihydrate; Trade names: Hydro cal, Capstone (also called Class I stone) Lab stone

Die Stone defined

AKA Type IV gypsum product;
modified alpha-hemihydrate;
improved stone; TM Density; Die keen (also called Class II stone) Die stone

Plaster Calcination

Heated in an open container

Stone Calcination

Steam under pressure in a closed container

Die Stone Calcination

Method uses a calcium chloride solution

Plaster characteristics

Powder is made up of porous, irregular particles

Stone characteristics

Powder particles are more regular and uniform in shape

Die stone characteristics

Powder particles are cuboidal in shape; densest of the products

Plaster strength

Weakest of the 3 products

Stone strength

Stronger than plaster

Die stone strength

Strongest of the products


least expensive

Die Stone

most expensive

Density of 3 gypsums:

equal weights, but different volumes.

Plaster uses:

Study models; to attach casts to the mechanical articulator

Stone uses:

Study models; casts for the fabrication of partial and complete dentures, and appliances like athletic mouthguards

Die stone uses:

Casts and dies for the fabrication of inlays and crowns

Powder + water + heat

All 3 of the products - when hydrated change back to the dihydrate

Plaster setting expansion

expands the most

Die stone setting expansion

expands the least

Reason why more water is used for setting reaction:

for manipulation of the material

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