The processes employed in making plaster Paris moulds and casts are very simple, and easily learned, even by one who has had no previous knowledge of the subject. To be sure, a certain degree of intelligence and skill is necessary in the operator; but we are not writing for the edification of duffers who do not know how to use their hands, or follow plain directions.
The first thing to understand is the difference between a mould that will "draw," and one that will not. A mould may be made on one side of a base-ball, and it will draw off the object at once, because there is no point on the ball behind which, or under which, the plaster can catch, and hang fast until something breaks. A mould of one full side of an apple will not draw, because the apple has a hollow at each end, and when these are set full of plaster the mould and the apple are held firmly together.
A hollow or a protuberance on an object which would prevent a mould from drawing away makes what is called an "undercut," and necessitates the making of a separate piece in the mould. To cast several copies of a human head and neck necessitates the making of a mould in several pieces, all fitting very nicely together, with countersink joints, to accommodate the undercuts behind the ears, under the chin, the hollows of the eyes, etc.
A mould made in more than two pieces is called a "piece mould." It may have any number of pieces, of course, according to the nature of the object. Perhaps the most remarkable piece mould in existence is that used by Professor H.A. Ward in making casts of the tail of the great fossil armadillo called the glyptodon. The tail is a mass of big conical tubercles, and the mould contains, as nearly as I can remember, about one hundred and twenty-five pieces, all fitting into a big "jacket" of plaster which holds them all in place while a cast is being made. In the case of a piece mould the cast is not taken out of the mould, but the latter is dissected and taken off the cast, piece by piece.
The principles involved in making moulds and casts are best explained by brief descriptions of the processes, and if they are learned by carefully following the directions here given, the operator will be able to apply them, indefinitely, to all classes of objects, large or small.
How to Make a Piece Mould.—Let us take a good-sized apple as our subject, and follow out the process, step by step.
1. In making a mould of any kind of fruit, first soak the fruit in water an hour or two, to "plump it up," so that it will not swell in the mould and cause trouble.
2. Wipe it dry, and with a small paint-brush give it a thin coat of lard oil, so that the plaster will not stick to it. Some objects should be coated with clay water, or very thin clay, instead of oil.
[a]Fig. 59.]—Beginning to Make a Piece Mould.