CaSO41/2H2O. A fine white powder composed of calcium sulphate hemihydrate. Plaster of Paris obtained its named because it was primarily prepared from gypsum quarried in the Montmartre district of Paris. It is produced by heating gypsum, alabaster or selenite, which are composed of calcium sulphate dihydrate, to temperatures below 2508C (1508 - 1608C) to drive off some of the water of crystallisation. The resulting calcium sulphate hemihydrate is stable in air, but readily combines with water to form a paste that rapidly sets (in approximately 30 minutes) to an impenetrable solid. It generates heat with setting and may expand slightly. When he set is complete the plaster returns to its original size. Setting can be accelerated by the addition a small amounts of salts (aluminium sulphate, potassium sulphate) or retarded by the addition of organic materials (blood, glue, casein). Plaster of Paris has long been used for moulds, sculpture and casts, and for the decoration of walls and ceilings. Because plaster of Paris is pure white and highly porous, the surface is readily soiled and was therefore often coated. Materials such as tea, gum arabic, waxes, shellac, lime wash and oil-based paints have been used as coatings. Plaster of Paris is also frequently painted and oil-gilded. A sealant, such as animal glue, gum arabic or shellac, is applied first to reduce the porosity of the plaster. Distemper and oil-based paints are commonly used. It is a component of alum-gypsum. Synonyms: calcined gypsum; burnt gypsum; partially dehydrated gypsum; plaster; calcium sulphate hemihydrate. Wittenburg (1999); Museum of Fine Arts, Boston (2000); Trench (2000). [J. Cassar, R. de Angelis]