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One of the most important parts, and perhaps the most difficult, is to mix the plaster and water properly. The manner is as follows :-As much water as is necessary to be used is put into the bowl; the plaster is then shaken into it from the hand or a spoon, without stirring, till no more can be moistened by the water. In this situation it is suffered to remain about one minute, until the plaster is free from air ; this can be known by the mixture ceasing to bubble; by this, small holes are prevented from occurring in the casts, which is very desirable. The contents of the bowl are then stirred together and used. In casting hollow figures, the mixture should have the consistency of cream ; for making moulds it should be thicker. The plaster must be used as soon as it is well mixed ; if too thick, it can be diluted, but this must be done when it is first stirred.
The mode of making moulds is different, according to the objects to be copied. For instance, anything soft and pliable, like parts of the human body, can be easily taken at once, as the object can be delivered from the cast without injury to either ; whereas, from a hard body, as a marble bust, many pieces are required. Each manner will be described separately.
In making a mould from a soft and yielding substance, it is necessary first to lubricate the object well with a mixture of oil and tallow. The oil is used to prevent the plaster, when hardened, from adhering too tightly to the object to be copied; and, without the slight adhesive power of the tallow, the liquid plaster would be apt to run off as fast as put on. After having put the oil and tallow on delicately and thoroughly, with a brush, the plaster must be mixed with the water, and be put on. At first, a thin coat only had better be put on, so as to prevent distortion, which would ensue, were much of the heavy material to be heaped on. After the plaster has become somewhat firm, enough should be put on to make the mould have a tolerable degree of strength. When hard, the object must be carefully removed, after which it is ready to be seasoned, which process will be described in its proper place.
Moulds from flat surfaces of hard bodies are made in precisely the same way.
Moulds from medallions are best made of sulphur. The mode is this :-First, the medal, if it is of plaster, must be seasoned ; then a rim of sheet-tin, of about half an inch in width, should be placed around it, and be kept in place by a cord. The medal must next be delicately oiled with a little olive oil. Thus prepared, it should be held to the fire till it is quite warm, after which, the melted sulphur is to be poured upon it, and allowed to get nearly cold. Just before it is cold, a little freshly-mixed plaster had better be cast on top of the sulphur, which causes it to separate itself from the medal, and, besides, gives strength to the mould. Moulds thus made require no seasoning, and give beautiful impressions.
Moulds from hard and irregular bodies, as from figures, busts, &c., consist of many pieces, and are more difficult to make than those above described. They are made by pieces, in the following manner :-First, one piece must be made as above, care being taken that the mould will separate from the object without damage to either. This piece, having been made, must be trimmed smoothly at the edges, and a few holes, or, as they are technically called, keys, should be made in the edges, so that the next piece may make a good joint with it, and the pieces of the mould be more easily kept together. The piece thus made must be placed on the object exactly in its first posi. tion, and its edges must be well greased, to prevent its adhering to the piece to be cast next. Another piece is now to be cast, and served in the same way, until the mould is completed. One example will give a much better idea of the process. Let A (Fig. II,) be a bust, from which a mould is to be taken. In the first place, the bust (A) must be well brushed over with oil; the plaster (B) is then put on by a spatula or spoon, and, as it grows hard, is heaped on, until the piece is sufficiently thick to be strong. This piece, (B) as can be seen from the figure, covers a part of the bust extending from F to H. After having become hard, this piece is removed from the bust and trimmed, as stated above. Another piece (D) is made on the opposite side of the bust, extending from 6 to 1. When hard, this piece is also removed and trimmed smooth. Small round depressions (c) are made with the instrument described above, and shown in Fig I, in all the sides trimmed, (except the bottom in this case) to serve the purpose of keys. These two pieces, thus made, are to have the edges which have been trimmed well greased; they are then put upon the bust in their former places, and are to be secured firmly by a cord. A third piece (E) must then be cast, and trimmed as the pieces B and D, care being taken not to move the pieces B and D, as they are to form a water-tight joint with the piece E, when hardened. This third piece will have small round protuberances on the extremities which come in contact with the other pieces. These eminences correspond to the depressions in the pieces B and D, and keep them all firmly together, when put in their proper position without the bust. Small depressions must now be made in the front and back edge of the piece E, at the places marked c. Having proceeded thus far, the three pieces must be greased and placed upon the bust, which will then have the appearance that is represented in Fig. II. To finish the mould, the front and back, respectively, must be greased, and plaster then cast upon them. When dry, these pieces must be removed from the bust, and put together without it, and in this situation they will form a hollow mould, which must be seasoned, as will be mentioned hereafter, before it will be ready for use.
It sometimes happens that internal pieces, as B, F, and G, of Fig. III, are required. These must be made first, and a loop of wire be placed in each of them while soft. The keys, (cc and 11) must not be forgotten. These pieces, when hard, must be removed, trimmed smooth, and well greased. A little glazier's putty must next be put around the loops of wire, and the pieces be replaced. The piece NNN must be cast next; when hard it must be removed, and the holes DD and kk must be bored entirely through it, where the loops of the internal pieces will come, or rather, where the putty made depressions. The internal piece (B) is fastened to the piece n, by passing a string through the loop of wire and through the piece n, and by tying the string to a stick at E. The pieces F and g are fastened in a similar manner by the strings Lt, which form a knot at m. The letters aan of the figure represent the object from which the mould described is supposed to have been taken.
In making moulds of plaster it is often necessary or convenient to make the sharp angles of wax. To do this, the place to be occupied by wax must be first filled with some soft substance, as lard or soap. The piece of the mould above this must then be made of plaster. When this piece has hardened, it must be removed, trimmed, and a large hole bored through it;- remove the lard which was used to fill the cavity, and replace the piece of plaster. Thus prepared, the melted wax, which must not be hot, must be poured through the hole of the piece just made, and, on cooling, it will form the piece required. If a little resin is mixed with the wax, the corner will be harder.
Among the articles for making moulds, isinglass and common glue were mentioned. They can be used as follows:-Dissolve the isinglass or glue in brandy, by a slow fire. The article to be copied must be oiled, and the solution be poured upon it, and suffered to remain until it is hard. This method is used in copying medals, seals, and coins, but is an inferior mode of doing it.
Moulds are made from tin-foil and block-tin, by pressing the material upon the article to be copied. This requires great pressure, and must be done with a machine. Silver and bronzed medals, which would be defaced by hot sulphur, are copied in this manner.
After moulds have been well made, the next thing is to season them, so that they may not absorb the oil that is put on them to prevent their adhering to the figures which are to be cast in them. This process is quite simple, and is performed in the following manner :--The plaster mould is allowed to become perfectly dry. Two or three weeks are required for this, unless artificial heat is used. When perfectly dry, the mould is to be well soaked with boiled linseed oil, put on very delicately with a brush or sponge. In this state the oil is suffered to dry in the pores of the plaster; and when sufficiently dried, another very delicate coat must be put on, and perhaps a third ; and when these last coats of oil have become dry, the pores will be all filled, and the mould sufficiently seasoned. Every time the moulds are used, they must be lubricated with the mixture of oil and tallow. When the mould must be broken to pieces, as is the case in taking casts of living heads, the mould need not to go through this course of seasoning, but may be prepared in the following manner : - A thick lather, made from common brown soap, must be