Calcium sulfate (CaSO4) occurs in nature in both the anhydrous and the hydrated form. The former is found primarily as the mineral known as anhydrite, while the latter is probably best known as alabaster or gypsum. Calcium sulfate also occurs in forms known as selenite, terra alba, satinite, satin spar, and light spar.
Gypsum was well known to ancient cultures. Theophrastus of Eresus (about 300 B.C.), for example, described its natural occurrence, its properties, and its uses. The Persian pharmacist Abu Mansur Muwaffaq is believed to have first described one of the major products of gypsum, plaster of Paris, around 975 A.D.
Calcium sulfate occurs as a white odorless powder or as crystals that may be tinged with color by impurities. It has a melting point of 2,642°F (1,450°C) and is only slightly soluble in water. When heated, the hydrated forms of calcium sulfate lose 1.5 molecules of water and form the hemihydrate, CaSO4 • 1/2H2O, commonly known as plaster of Paris. When added to water, plaster of Paris forms a hard mass used in making plaster casts, quick-setting cements, molds, wall plasters and wall board, and inexpensive art objects. Neither the anhydrous nor the hydrated calcium sulfate will react with water as does the hemihydrate.
Among the many other uses of calcium sulfate are as a pigment in white paints, as a soil conditioner, in Portland cement, as a sizer, filler, and coating agent in papers, in the manufacture of sulfuric acid and sulfur, in the metallurgy of zinc ores, and as a drying agent in many laboratory and commercial processes.
See also Calcium.