Notes
There are many different types of plaster, they vary mainly in setting time, strength, and 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. The shelf life precautions are especially important in wet climates or where stock is being kept in damp warehouses (in dry climates plaster can be stored much longer).
USG Pottery #1 (2000 psi) is an example of a general-purpose material for high water absorption. It is normally mixed at a 70 consistency (70 water to 100 plaster). Of course, this ratio can be compromised by adding more water (for greater absorbency), but that comes at the cost of mold life and surface durability. Puritan plaster is similar emphasizes hardness at the cost of porosity, it is used in production applications where molds must be more durable (e.g. for use in jiggering machines).
Jiggering benefits from a plaster with more surface hardening additives (i.e. USG's "Puritan" 66:100 water:plaster). 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 at the expense of absorbency" are required (they require much less water).
The optimum compromise between durability and absorbency are best achieved with the proper water-to-plaster ratio. Manufacturers recommend weighing the water, adding the plaster to the water, soaking for 2-3 minutes, then mixing well using a propeller mixer. It is important agitate in such a way that the air bubbles break at the surface during mixing. It is especially important to use the same water-plaster ratio and procedure when making separate mold pieces that must fit and work together (so they mate properly and have the same absorbency).
Gypsum cements are not the same as pottery plasters. They are designed for optimum density, hardness and dimensional stability. Hydrocal, UltraCal and Hydrostone are examples.
Plaster dust is not harmful. It is calcium sulphate, or Selenite. Many raw clays contain small chunks of natural Selenite, it normally just gets ground up with the rest of the clay or converted to calcium oxide during firing (that subsequently reacts with surrounding particles to form calcium silicates and aluminates). If there is a significant amount of calcium sulphate you may smell sulphur during firing (but with ventilation in modern kilns this is highly unlikely). If any larger particles of plaster get into a clay body they can absorb water from the air or surrounding body (if sufficiently porous) to become calcium hydroxide. This reaction causes expansion that can be enough to pop out a piece of the clay.
A continuous plaster mixing machine as available from: Hoge Warren Zimmermann Co., 40 West Crescentville Rd., Cincinnati, OH 45246 FAX 513-671-3514
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