plaster water paris
GYPSUM, the hydrated sulphate of lime, CaSO4.2H20, is a mineral substance occurring in various rock formations, especially in Tertiary deposits, in very considerable abundance and under varying conditions. In its transparent crystalline state it is known as selenite ; when it presents a finely fibrous opalescent appearance, it is termed satin spar ; and the name alabaster is reserved for the pure milky white massive varieties. Gypsum is very generally disseminated, the most famous locality for the finer qualities worked into alabaster vases and figures being Castelino, about 30 miles from Leghorn ; while Montmartre, Argenteuil, and other places in the environs of Paris, and in the neighbourhood of Derby in England, furnish inexhaustible supplies chiefly for the preparation of plaster of Paris and for agricultural use, Sze. It is also found in large quantities in Nova Scotia, New York, Virginia, and Michigan. The application of gypsum as a manure is referred to under AGRICULTURE, vol. i. p. 351, and its employment for ornamental purposes is described under ALABASTER, vol. i. p. 439. The preparation of plaster of Paris, so called from the fact that the industry chiefly centres in several Parisian suburbs, is the principal primary object of the quarrying or mining of gypsum. By the application of heat gypsum begins, at a temperature of about 175° Fahr., to part with its combined water. An increase of temperature causes the desiccation to proceed with great rapidity, and for manufacturing purposes the best results are obtained at from 230° to 250°. For making plaster of Paris, gypsum is burnt in kilns at about the latter temperatures, and subsequently it is powdered and ground to a fine uniform flour. So prepared it possesses the valuable property of recombining with water when mixed with it, and setting, from a thin paste, into a solid mass, the phenomenon being accompanied with some expansion and the evolution of heat. It is to this property of recombining with water that the value of plaster of Paris is principally due. When, however, gypsum is burnt at a temperature of 480° and upwards, the sulphate rehydrates only with great difficulty, and at still higher temperatures it loses all power of absorbing water, and in this respect it then resembles anhydrite, the natural water-free sulphate of lime. Plaster of Paris is largely used for obtaining copies of statuary figures, coins, medals, sculptures, and carvings, and also for taking casts from natural objects. It is also employed as the material for moulds for electrodeposits, and for the manufacture of embossed and pressed pottery ware. Still more extensively is it consumed in the finishing of internal plaster work in houses, and for making cornice mouldings and other architectural enrichments in positions sheltered from the weather. Plaster of Paris work is, like gypsum, soluble in water at a temperature of 32° to the extent of •205 per cent., rising to a maximum solubility of •254 at 95° Fahr. Plaster casts made simply with water are soft, porous, and easily injured, and various plans have been devised for producing a harder and more compact body with plaster. Keene's cement, which may be taken as a type of the hardened plasters, is made by treating the burnt gypsum with a solution of 1 part of alum to 12 of water at a temperature of about 95°. After about 3 hours the plaster is removed, dried, and rebaked in the furnace, and then thoroughly ground and powdered. Thus prepared, the plaster needs comparatively little water to slake it, and it sets much more slowly than the ordinary plaster, while the comparative tenacity of the two varieties is as 1.5 to 1. Parian cement is plaster hardened with water containing 10 per cent. of borax ; and stucco is plaster rendered tenacious by being prepared with a strong solution of glue. When water containing lime or a solution of gum-arabic is used to slake burnt gypsum, a hard plaster is also obtained, which by smoothing, colouring, and subsequent polishing with oil assumes a marble-like surface. A fair imitation of meerschaum is also made in hardened plaster by polishing, tinting the surface with a solution of gamboge and dragon's-blood, and treating it with either melted paraffin or stearic acid. It is understood that the cheaper " meerschaum " pipes and cigar-holders are thus prepared.