Plaster-of-Paris, or gypsum, is a sulphate of lime found at places in Cheshire, Cumberland, Derbyshire, and Oxfordshire, in England, and at many places in the neighbourhood of Paris, France, hence, one of the names given to it. It is also found in Germany, Switzerland, Italy, Spain, and North America. According to Burnell, it occurs " either in contemporary strata of great thickness (as near Paris) in the tertiary formations; or in the iridescent marls of La Meuse, or the Aveyron; or in masses of a subsequent date in different secondary rocks." The latter kind, being generally in contact with igneous rocks, is associated frequently with the dolomites, rocksalt, bitumen, and sulphur. The better qualities of gypsum have almost the hardness of calcareous stones, but after the evaporation of the water of crystallisation by burning they are easily powdered. On being moistened with water gypsum reassumes the hydrate form it possessed before it was burnt, and it crystallises on and around the substances between which it is placed, recovering its original density and strength. It is for this reason that gypsum is so extensively used in building. Gypsum is quarried underground and in the open either by cutting with picks and wedges or by blasting with explosives.
The gypsum stone is broken up fairly fine and conveyed to the kilns, which are primitive structures, consisting of three brick walls supporting a tiled roof in which are openings to allow the escape of steam; one side of the kiln, which really is but a shed, is open. The gypsum is piled up in the form of arches, the larger stones being at the bottom, near the fireplace formed by the vaults of the arches. In the latter a wood fire is lighted, the flames rising through the crevices left between the stones. A greater heat than 2008C. over-calcines the gypsum, which then loses its power of combining with the water and reassuming its hydrous sulphate form, A better kiln than the shed form is that with its chimney passing round and round the gypsum, which thus does not come in contact with the smoke or fuel; the latter in the ruder form of kiln discolours the calcined article. Perhaps a still better method is the one in which advantage is taken of the fact that steam at very high temperatures is a gas possessing great affinity for water. The finely broken gypsum is subjected to the action of steam of the temperature of 205° O, and a pure anhydrous sulphate of lime is produced. The calcined gypsum is powdered in a mill, and is then ready for use.
It is necessary to pack it very carefully, as in contact with a damp atmosphere it will rapidly spoil.