PLASTER CASTING In plaster casting there are at least three methods of procedure, each of which is determined by the kind of mould used : (I) the gelatine mould, (2) the piece mould, (3) the so-called waste mould. The gelatine, or glue, mould is generally employed when more than one plaster cast of the same model is desired, and this we shall consider first.
After covering the clay or plasticine model, which is to be cast, with paper, a layer of clay about three quarters of an inch is placed over it. Next to this is made the outer shell of plaster which will hold the glue. This shell is made in sections, when the model is in the round, so that it may be removed without disturbing the model. These sections must also be carefully keyed, both to the base and to each other, so that they may be placed in their original position, leaving an even space between the model and this outer shell. This space is filled with the melted casting glue or gelatine, which is poured in through a hole made in the top. Other holes are also made in the shell to allow the air to escape as the melted glue rises in the space, and are closed with clay when the glue reaches that level. When the gelatine, or glue, is sufficiently cooled, the shell is opened and the clay, or plastilene, is cleaned from the gelatine mould. It is then properly greased with a mixture of stearine and kerosene and the mould, with the shell to hold it, is placed together again and plaster, mixed to a normal consistency, is poured into it. Care should be taken to reopen the mould at the proper time so that the heat of the plaster, while in the process of setting, may not melt the gelatine and in this way alter the mould so that the freshness of the original surface of the model is lost in the cast. In re-casting the mould should again be cleaned and greased lightly as before.
This method is employed for the same purpose as the gelatine moulding. It gives a sharper, more ac curate reproduction but requires greater skill and time, as the mould is made in many pieces of plaster, so placed and fitted that they may be freed from what is termed undercuts in the cast. In gelatine moulding, on account of the pliability of the
mould, this difficulty with undercuts is avoided. Each part of the piece mould must be made separately. The soft plaster, placed on the section of the model that can be cast in one piece, is allowed to set, after which it is smoothed and trimmed with a chisel, and the surface against which the next piece will lie, is oiled so that the fresh plaster will not adhere to it. When the model is covered, the tops of the pieces are oiled and plaster placed over them, which, when set, forms a shell to hold them in place while they are being filled with the plaster of the cast. Before casting, however, the mould must be taken apart and the clay washed from each piece. It is then soaped and oiled and put together ready for casting. After each casting the pieces should be cleaned and oiled to prevent them sticking to the cast.
This process is used either when only one reproduction in plaster is desired or in very large work. Proper divisions are first made in the most salient points of the model by small pieces of brass, so that when the mould is finished it may be opened and all the clay removed. The model is now covered with a coating of coloured plaster which is made by colouring the water, with which the plaster is mixed, with blueing or dry colour. In the process of chipping the plaster mould from the cast, this coating warns the moulder that he is near the cast and must chip carefully to prevent cutting into it.
After the first coating of coloured plaster, about three-eighths of an inch thick, a thicker coating of white plaster is applied. When the cast is large, or when there are parts of it that might break easily, it may be reinforced with iron pipes which are fastened to it with strips of burlap dipped in plaster.