PLASTER OF PARIS. _\ cementing stance which is prepared by partial calcination or dehydration of gypsum. This mineral is cally a hydrated sulphate of lime and contains 46.6 per cent. of sulphuric acid, 32.5 per cent. of lime. and 20.9 per cent. of water. ing to the formula 2Hp. When heated to a temperature above I28 C. but less than gypsum loses a definite portion of its water corresponding to one and lents, and then has the composition +11,0. The material thus treated is known as plaster of Paris, or commonly plaster, and has the property of hardening or setting when water is added. If the caleination is conducted at a heat greater than however, the gypsum loses all of its water and becomes inert. The set of plaster of Paris is due to its uniting with water again to form gypsum which assumes a crystalline form.
In the manufacture of plaster of Paris, the gypsum may be quarried and burned directly in kiln:, which is the method usually practiced in Europe. or the gypsum may lie ground to a fine powder and then calcined in kettles, this being the general practice in the United States. The improved form of kiln consists of a cylindrical chamber with brick or stone walls and a at the bottom. The gypsum blocks are looselv placed around and above the so that the gases may circulate through all parts of the chambers before passing out of the top of the kiln. Coal is used as fuel. In the plaster mills
of the United States the gypsum is ground in a buhrstone mill or s(one form of disintegrator and is then run into a calcining kettle set in stonework. The kettle has a diameter of about eight feet and holds about seven tons of ground gypsum. During the process of calcination the powder is stirred by revolving arms so that the whole mass may be burned evenly. Coal and oil :ire used as fuel. For the purpose of delaying the set of gypsum plaster, which ordinarily quires from six to ten minutes, it is mixed with sonic form of such as glue, soda, ghum, etc., and is then sold as wall The time required for hardening these spe cial plasters varies from two to six hours.
The principal uses of plaster of Paris are for the manufacture of stall, which is employed in the construction of temporary buildings. and for wall plaster. owing to its greater hardness and durability, it is much superior to lime for struction purposes. The production of plaster of Paris in the United States in Inol was 412.908 short tons, valued at $1,377.570, the greater hart heing made in Kansas, Iowa, Michigan, and New York. The domestic industry is exceeded only by that of France. Consult Grimsley, of in The Mineral Industry, vol. vii. (New York, 1899). See GYPSUM.