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Plas′ter-cast.

A copy of an object obtained by pouring plaster of Paris (sulphate of lime calcined and pulverized) mixed with water into a mold which forms a copy of the object in reverse.

While the art of modeling in clay was attributed by the ancients to Dibutades the Corinthian, about 985 B. C., that of taking plaster casts is attributed by Theophrastus and Pliny (A. D. 70) to Lysistratus of Sicyon, a brother of the celebrated sculptor Lysippus. He first took plaster casts, and from these obtained a second cast in wax. The plastic nature of the latter enabled him to rectify imperfections and form an enduring, life-like copy. The names of Rhaecus and Theodorus of Samos are also noted in this connection.

After being neglected for centuries, the art was revived by Verocchio (1422-88) and by others: the immediate incentive being to obtain copies of the specimens of ancient art which were being disinterred and discovered. The sovereigns of France also took this means to add to the artistic interest and beauty of their capitals and art museums.

In making plaster casts the gypsum is mixed with water, the proportion of the latter depending on the purpose to which it is intended to be applied. The more fluid the mixture is the more easily and perfectly will it enter the finest lines of the mold; but such casts, when dry, are more porous and therefore more tender than those in which a smaller quantity of water has been used. Much also depends on properly mixing the ingredients; the water must never be poured on the powder, but the powder must be shaken into the water, and the best way of doing this is to take up the powder between the fingers and thumb, and then allow it to drop gradually into the water, taking care that it does not run into lumps, and that every particle becomes thoroughly soaked. Proceed in this way till the powder comes to the level of the water, and then stop if a thin plaster is wanted; but, if a stronger one is required, keep adding powder till it stands about a tenth of an inch above the water. When the whole of the gypsum has been put in, allow the ingredients to remain undisturbed for a few seconds, and then stir up the mixture cautiously and gently with a spatula, and as soon as it begins sensibly to thicken pour it into the mold. The great error that amateurs fall into in this part of the process arises from want of patience; they put the powder into the water too hastily, generally employing too much of it, and keep incessantly stirring it till it thickens; The consequence of which is, that it contains air-bubbles, is hard in some places and soft in others, and is not capable of being perfectly united to a succeeding layer of plaster. Good plaster, when mixed with the proper quantity of water, should be of the consistence of cream, and will set or become solid in about ten [1736] minutes, but cannot safely be removed from the mold till about half an hour afterward.

If a mold is to be made of a simple object, it is evident that it must be formed in at least two parts, that we may be able to extract the cast, or allow the mold to deliver, as it is called; and in the case of any regular geometrical figure two parts only are necessary. The same may be said of any figure whatever which has no recesses or projections whose sides do not overlay any portion of the mold in the direction in which it is necessary to withdraw the latter. Should any of them do so, it will be necessary to construct the mold of a greater number of parts, so that each separate part may be removed in the direction of the wider part of the opening formed by the object and the projection. A great part of the art of the molder consists in forming these safe molds, as they are called, in as few pieces as possible. Nevertheless, in intricate pieces of statuary it is said as many as 700 or more separate pieces are required.

Deeble's mode of obtaining casts of leaves is to lay the leaf face upward on a bed of fine-grained, moist sand, and cover it by means of a camel's-hair brush with a mixture of wax and Burgundy pitch made fluid by heat. The leaf, being then removed from the sand and dipped in cold water, is readily withdrawn from the wax mold, which is then placed in sand, as the leaf was, and covered with a thin plaster laid on with a brush and forced into all the interstices of the mold; more plaster is then poured on, which, in setting, evolves sufficient heat to render the wax pliable and allow it to be removed from the cast.

Mr. Fox's method of forming an elastic mold in two pieces capable of delivering undercut surfaces is, after oiling the body to be molded, to cover it completely with a thick mixture of warm melted glue with a little treacle. The glue, when cold, is cut into two parts, and may be removed without difficulty from the object. They are then oiled internally and bound together, and the cavity being filled in the usual way with plaster, affords a perfect cast.

Luster may be imparted to plaster casts by dipping them repeatedly in a hot, saturated solution of alum. The surface of the cast is thereby hardened, and a certain amount of translucency obtained.

Another mode is to dry it, then coat with melted white wax and warm at a fire to effect the absorption of the wax.

1877.

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