Alternate Names: Gypsum, Selenite
There are many different types of plaster and they vary mainly in setting time, strength, absorbency. Manufacturers provide instruction booklets on how to mix and use their materials. For example, common precautions with reference to pottery plaster are: Don't stack pallets 3 high to avoid hard chunks, use before the shelf life of 120 days expires (if longer then extend mixing time), don't mix at higher than 105 degrees for proper set.
USG Pottery #1 (2000 psi) is an example of an all around material. It is normally mixed at a 70 consistency (70 water to 100 plaster). Jiggering benefits from a plaster with more surface hardening additives (i.e. USG's "Puritan" 66:100 plastic:water). For carving USG "Moulding Plaster" works well. USG Ultracal (5000 psi) and Hyrdostone (10,000 psi) are very hard materials and ideal for case molds where hardness and the expense of absorbency are required (they require much less water).
The optimum hardness and absorbency of the final product are best achieved with the proper water to plaster ratio. Manufacturers recommend that you weigh the water, add the plaster to the water, soak for 2-3 minutes, then mix well using a propeller mixer. It is important not to mix in air bubbles, but to agitate in such a way that the air bubbles break at the surface during mixing.
Gypsum cements are not the same as pottery plasters. They are designed for optimum surface hardening, dimensional stability, and very low expansion.
An continuous plaster mixing machine as available from: Hoge Warren Zimmermann Co., 40 West Crescentville Rd., Cincinnati, OH 45246 FAX 513-671-3514
Plaster dust is not harmful. It is calcium sulphate, or Selenite. Many raw clays contain small chunks of natural Selenite, it just gets ground up with the rest of the clay. If there is significant amounts then you will be able to smell sulphur during firing (but with ventilation in modern kilns this is highly unlikely). If any plaster contamination particles get into your clay the issue you will experience is called "popping". After firing, as a piece absorbs water from the air over time, the particles of gypsum close enough to the surface can expand and pop out a piece of the clay to relieve the pressure.
Normally a mixture of varieties of lime and gypsum which when wetted and dried remains hard though water absorbent. Typically, it contains a calcium sulfate which limits its use to casting molds, and cannot be used in clay or glaze formulas unless first calcinated sufficiently to burn off the prejudicing sulfur (in combination with water and heat develops sulfuric acid)
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