Gypsum is a valuable mineral of a comparatively soft nature. Chemically it is a hydrated sulphate of lime, . Its specific gravity is 2.31, and its hardness is from 1.5 to 2 of the mineral scale. The massive marble-like variety, which is usually white or delicately tinted and translucent, is called Alabaster (q.v.); when transparent and crystallised it is known as Selenite (q.v.); and when fibrous and with a pearly opalescence it is termed satin spar.
Gypsum occurs in various geological formations, and has a wide geographical distribution. Extensive beds of the common variety are generally made up of irregular, concretionary, nodular masses. In the New Red formation near Derby, at Carlisle, and in some parts of Nottinghamshire, as well as in the Tertiary beds of the suburbs of Paris, it is largely worked for the preparation of plaster of Paris. Productive beds of it are found in numerous localities in the United States, principally in Ohio and Michigan; in New Brunswick, Nova Scotia, and Ontario; and in the Punjab. Gypsum is very frequently associated with rock-salt. See ANHYDRITE.
Gypsum contains 21 per cent. of water, which can be driven off by heat. It is burned in kilns at or a little below a temperature of 250° F., and afterwards ground to a fine powder, which is called plaster of Paris. This recombines with water, evolves heat, and almost immediately solidifies or sets. It is this property which makes it so serviceable for many purposes in the industrial arts. If in the burning of gypsum the temperature is raised as high or higher than 480° F. it loses the power of rehydrating, and is then said to be dead burnt, in which state it will not set when mixed with water. Like gypsum, plaster of Paris is soluble to the extent of rather more than 2 parts in 1000 parts of water at ordinary temperatures, its point of maximum solubility being 95° F. It is therefore unsuited for external work, except in dry climates such as that of Persia. For making casts the plaster of Paris is made up with water to a consistency of thick cream. In this state it is poured into a mould, which is usually also made of the same material, and left to solidify. Some oil, such as olive, is brushed over the mould to form a parting between it and the cast. Plaster of Paris is most extensively used for taking casts of sculpture and architectural details, as well as for casts of small objects such as coins, medals, and engraved gems. For pottery moulds it is also largely employed, and it is used to take a first copy from the modelled clay in the production of metal patterns. Large quantities of it are consumed for the mouldings of the internal plaster-work of houses, and for cornice and other ornaments. For hardened plaster of Paris, such as Keene's cement, see CEMENTS; and for the agricultural applications of gypsum, see MANURES. Gypsum is one of the substances which renders water hard, and such water is useful in the brewing of some kinds of beer. Pearl hardening, used as a filling in the manufacture of some kinds of paper, is an artificial sulphate of lime, precipitated by sulphuric acid from chloride of calcium. Fictile Ivory is plaster of Paris which has been made to absorb beeswax, spermaceti, and stearic acid, in their melted state.—The average annual production of gypsum in Great Britain is nearly 120,000 tons, value about £48,000. In 1888 the production of the United States was about 96,000 tons; of Nova Scotia, 126,118 tons. For the anhydrous sulphate of lime, see ANHYDRITE.