Introduction
White plaster, also called laboratory gypsum or model plaster, is the everyday gypsum of the dental laboratory. It is the cheapest gypsum product and is used, for example, to mold heat-cured acrylic resin.
Composition
White plaster is beta-hemihydrate of calcium sulfate (CaSO₄·½H₂O). It is made by firing crushed gypsum rock in open kilns at 110 to 120 °C and normal pressure for 24 to 36 hours. The rock loses part of its crystal water and forms irregular, spongy crystals.
Spongy crystals need more water to wet them, so white plaster is mixed at a water/powder ratio of 0.4 to 0.5. This equals a powder/water ratio of 170 to 200 g of powder per 100 ml of water.
Preparation
- Dose. In the filling method, pour the water into a clean rubber bowl and sprinkle the powder in slowly until no free water and no dry powder remain. Do not touch or tap the bowl during this step. Weighing the powder and measuring the water is more accurate.
- Mix with a metal spatula in circular motions.
- Remove air by tapping the bowl and placing it on a vibrating table. A vacuum mixer gives the best result.
- Pour the mix into the impression on the vibrating table.
Air left in the mix forms pores, which weaken the model and spoil its surface.