The methods of making ordinary plaster casts are well known; but there are a few special methods of treating this substance which it may be well to describe. The material employed is plaster of Paris, which is 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 or 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.
Casts are frequently taken from living objects; and a cast of the human face is often taken for the purpose of preserving the likeness of a person. The art is easily acquired, and only demands a little care. Let the person, a mold of whose face is to be taken, lay down upon his back; let the hair be tied back, or otherwise kept back by grease, or by flour-dough placed on it; grease the eyebrows, and, if necessary, the beard and whiskers; also anoint the rest of the face with sweet oil. Then place a quill in each nostril, keeping it there with dough. Tie a towel round the face, and make it fit tight with dough also. The patient being thus prepared, mix up the required quantity of plaster of Paris with warm water, and just as it is ready to set pour it upon the face, taking care that the eyes and mouth are closed, and the outer ends of the quills above the plaster. Use a pallet-knife to spread the plaster evenly over all parts of the face, until a coat is formed half an inch or more in thickness. In about two minutes it will set sufficiently hard to be removed. When dry and well greased, a cast in plaster may be taken from the mold, or, if wetted, a cast in wax may be taken with equal facility. A little warm water will remove the dough, etc., from the face. In this manner casts are often taken of tumors and skin diseases, the wax casts being afterwards colored. For wax casts a good composition is white wax 1 lb.; turpentine in lumps 2 ozs.; flake white 2 ozs.; and vermillion to color the whole.
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 a 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 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 crystallization. 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.
Plaster of Paris treated with paraffine may be readily cut and turned in the lathe, and forms a very pleasant material to work.
When plaster is used for architectural purposes and greater hardness is required, a small quantity of lime is added. This addition gives a very marble-like 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.
Ordinary casts may be rendered very hard and tough by soaking them in glue size until thoroughly saturated, and allowing them to dry.
Casts of plaster of Paris may be made to imitate fine bronze by giving them two or three coats of shellac varnish, and when dry applying a coat of mastic varnish, and dusting on fine bronze-powder when the mastic varnish becomes sticky.
Rat-holes may be effectually stopped with broken glass and plaster of Paris.