Plaster of Paris is a well known material, obtained by exposing the purer varieties of gypsum or alabaster to a heat a little above that of boiling water, when it becomes a fine, white dry powder. Sometimes the gypsum is first reduced to a fine powder and then heated in iron pans, and in this case the operation is sometimes called "boiling" plaster, because the escape of the water, with which crystalline gypsum is always combined, gives to the fine powder the appearance of boiling. Plaster of Paris, after being boiled, rapidly deteriorates when exposed to the air, consequently when plaster is required for making cements or for other purposes for which a good article is needed, care must be taken to secure that which is good and freshly boiled. The Italian image makers always use a superior quality of plaster, and it may generally be obtained from them in small quantity.
The employment of gypsum in casting, and in all cases where impressions are required, is very extensive. A thin pulp of 1 part gypsum and 2 1/2 parts water is made; this pulp hardens by standing. The hardening of good, well-burnt gypsum is effected in one to two minutes, and more quickly in a moderate heat. Models are made in this substance for galvano-plastic purposes, for metallic castings, and for ground works in porcelain manufacture. The object from which the cast is to be taken is first well oiled to prevent the adhesion of the gypsum. When greater hardness is required a small quantity of lime is added; this addition gives a very marblelike appearance, and the mixture is much employed in architecture, being then known as gypsum-marble or stucco. The gypsum is generally mixed with lime water, to which sometimes a solution of sulphate of zinc is added, After drying, the surface is rubbed down with pumice stone, colored to represent marble, and polished with Tripoli and olive oil. Artificial scagliola work is largely composed of gypsum.
There are several methods of hardening gypsum. One of the oldest consists in mixing the burnt gypsum with lime-water or a solution of gum arabic. Another, yielding very good results, is to mix the gypsum with & solution of 20 ounces of alum in 6 pounds of water; this plaster hardens completely in 15 to 30 minutes, and is largely used under the name of marble cement. Parian cement is gypsum hardened by means of borax, 1 part borax being dissolved in 9 parts of water, and the gypsum treated with the solution. Still better results are obtained by the addition to this solution of 1 part of cream of tartar.
The hardening of gypsum with a water-glass solution is found difficult, and no better results are obtained than with ordinary gypsum. Fissot obtains artificial stone from gypsum by burning and immersions in water, first for half a minute, after which it is exposed to the air and again for two to three minutes, when the block appears as a hardened stone. It would seem from this method that the augmentation in hardness is due to a new crystalization. Hardened gypsum, treated with stearic acid or with paraffine, and polished, much resembles meerschaum; the resemblance may be increased by a coloring solution of gamboge and dragon's blood, to impart a faint red-yellow tint. The cheap artificial meerschaum pipes are manufactured by this method.