Because it expands, plaster castings do not lose any detail, and a mold with a glass smooth surface will result in a glass smooth plaster casting. It can be carved, sanded, drilled, cut, textured, added to, reinforced and remain incredibly strong throughout. Unlike practically any other compound, when plaster turns from liquid to solid it does not shrink, rather, it expands ever so slightly as it forms crystals. This remarkable material is stored as a powder, and is mixed with ordinary water into a liquid that gets gradually thicker and thicker until it becomes plastic, then a paste, then a cheese-bodied mass finally turning rock hard in about an hour. After it is cooked (“calcined”), it is easily reduced to a powder, but the addition of water will reconstitute its original hardness, once the material sets. When heated, the mineral loses some water which is chemically bound into it and gives it its hardness. Plaster is made from the mineral gypsum, which is called Calcium Sulphate by chemists (Gypsum in the ground is calcium sulphate dihydrate, after calcining, it becomes calcium sulphate hemihydrate, also known as hydrous calcium sulphate.) It is often called plaster of Paris, because of the large deposits of pure gypsum underlying the French capitol, which were utilized early on by local artisans. In Alt.Sculpture FAQ, Casts and Molds, Concrete and Plaster, Hazards and Safety
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