Plaster of Paris is a white powder that, when mixed with water to form a paste, will turn hard in a few minutes. This substance is used for casting small statues, for making models and molds, for surgical casts, and for other purposes. It is made by heating gypsum, a mineral composed of calcium sulfate and water. When the water is partly driven off, the gypsum softens and is easily crushed to form a powder. When water is added again, the mass hardens to a stonelike substance.