GYPSUM (Lat., from Gk.-yNnn,gypsos, chalk, probably of Semitic origin, cf. Ar. jibs, gypsum, from jabasa, to walk haughtily). A hydrated calcium sulphate composed of lime 32.56 per cent., sulphuric acid 46.51 per cent., and water 20.93 per cent. It crystallizes in the monoclinic system, has a hardness of 2, and a specific grav ity of 2.31. The crystals are found in beds of massive gypsum, in veins, or scattered through beds of marl and clay. When transparent they are called selenite. Alabaster is a massive white, opaque variety of gypsum, and satin spar is a fibrous variety. A loose, earthy gypsum found in Kansas passes under the name of gypsite. The color of gypsum may be white, red, green, blue, gray, or brown. When gypsum is heated to a temperature of 120° to 130° C. about 14 per cent_ water of crystallization is driven off and the product left has the composition of (CaS0,), the mineral is then easily reduced to a powder, in which state it is known as plaster of Paris. Plaster of Paris when mixed again with
water forms a hard mass, and this setting is due to the recrystallization of the calcium sulphate. If the gypsum is underburned its setting power is injured, and the same effect may be produced by over-calcining, for the reason that the latter so comminutes the gypsum particles that none are left to serve as nuclei around which the crys tals can begin to form when the setting takes place. Fine grinding does not have any effect on the setting power. Underburned gypsum is tough and soluble in cold water to the extent of about one part in 461, and it is scarcely more soluble in acids or warm water.