Gypsum Products: Plaster, Stone and Die Materials Guide

Gypsum products are the plaster and stone materials poured into impressions to make solid models of a patient's teeth. All come from calcium sulphate dihydrate, heated to drive off part of its water so the powder hardens again when mixed with water.

These powders are the laboratory's backbone: almost every crown, denture and brace starts as a gypsum model. Soft plaster crumbles under a denture flask, while costly die stone is wasted on study models, so correct type, ratio and handling save money and remakes.

Key Concepts

• Calcination: heating gypsum rock to about 110 to 130 degree Celsius drives off three-quarters of its water, producing calcium sulphate hemihydrate, the reactive powder in all dental plasters and stones.
• Type II, dental plaster: porous and weaker, for study models and denture flasking. Water-powder ratio about 0.45 to 0.55.
• Type III, dental stone: denser and stronger with a smoother surface, for working models. Water-powder ratio about 0.28 to 0.32; compressive strength around 21 MPa.
• Type IV, high-strength (die) stone: hardest and most abrasion-resistant, for dies that crown margins are finished against. Water-powder ratio about 0.22 to 0.24; strength about 35 MPa.
• Setting expansion: as hemihydrate reverts to dihydrate, growing crystals expand the mass slightly (0.1 to 0.3 percent for plaster, under 0.1 percent for die stone), keeping the model true to the impression.

How It Works

1. Weighed powder is sifted into measured water (never the reverse) and soaked about 20 to 30 seconds so every particle wets evenly.
2. Hand or vacuum spatulation for about 1 minute mixes the slurry and drives out air bubbles that would pit the model.
3. The mix is vibrated into the impression so bubbles rise to the surface.
4. Dissolved hemihydrate re-precipitates as interlocking needle-like dihydrate crystals; the mass warms slightly and hardens, with initial set in roughly 8 to 16 minutes.
5. Models are separated after about 1 hour and reach full strength over 24 hours as residual water evaporates.

Worked Example

A technician pours a working model with Type III stone. The water-powder ratio is 0.30, and 150 g of powder fills the impression and base:

1. Water needed = 0.30 x 150 g = 45 mL (1 mL of water weighs 1 g).
2. 150 g of powder is sifted into 45 mL of water, soaked 30 seconds, vacuum-mixed 60 seconds.
3. The mix is vibrated into the impression; initial set begins at about 10 minutes.
4. After 24 hours of air-drying it reaches about 21 MPa, plenty for wax carving.

Common Misconceptions

• More water makes pouring easier with no harm. Wrong. Extra water leaves pores, and stone mixed at 0.45 instead of 0.30 can lose half its strength.
• Hot water speeds setting safely. Wrong. Water above about 37 degree Celsius can retard the set or give a chalky model. Use room-temperature water around 20 to 23 degree Celsius.
• Plaster and stone are interchangeable. Wrong. Plaster's soft surface wears away when wax margins are carved on it; die stone holds a crisp edge for precision crown work.

Real-World Applications

• Orthodontic study models are poured in Type II plaster for diagnosis and records.
• Crown and bridge models use Type III stone, with the prepared tooth poured as a Type IV die for margin accuracy.
• Complete denture processing uses plaster for flasking the wax denture before acrylic packing.

Calcined hemihydrate, mixed at the correct water-powder ratio, sets through crystal regrowth into plaster, stone or die stone of increasing strength. Measure carefully, mix under vacuum, pour promptly and dry fully for crisp, durable models.