![]() ![]() ![]() These were originally made from animal gelatin, which was superseded by vinyl hot-pour, which in its turn was made obsolete by the modern synthetic rubber molding materials based on silicone and urethane. While plaster can be cast into plaster molds if they are sealed and well-soaped, it works best in flexible molds. In moldmaking, plaster is used to make mold casings (mother molds), waste molds, casting and retouching masters. At one time, virtually all decorative moldings in houses were of plaster and, of course, plaster is the principal material of pattern making and foundry processes. Plaster can be “screeded” with a template to form straight and curved patterns with any cross-sectional contour. Plaster is used extensively in the manufacture of pottery, and plaster is used to cover the walls in quality homes. Plaster absorbs water and can therefore be used to make molds for casting porcelain and ceramic slipware as well as latex forms such as those aliens and monsters used in the Star Wars movies. 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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