The alpha form of gypsum (calcium sulphate hemihydrate; (CaSO4)2H2O) made by heating gypsum under pressure to 120–150°C. It is frequently coloured yellow. It has greater compressive and tensile strength in comparison with plaster of Paris, due to its regular crystalline structure. It is used extensively as a model material. When used as a denture mould, it minimizes the dimensional changes which occur during the polymerization of acrylic resin. See also stone.