Plaster Casts.
We have already mentioned the electrotype, and may now come to a branch of art which is much associated with it, namely, the Stereotype.
As many of my readers may know, types are very valuable articles, and must not be wasted. If, therefore, a book should be thought likely to have a steady sale, much of its value would be lost if the types were kept standing, inasmuch as they could not be used for any other work.
In such cases the Stereotype is employed. Omitting minute details, the process is as follows:—
The type, ready set up, is carefully oiled. Plaster of Paris mixed with water is then poured into a shallow trough, and the type pressed into it. In a short time the plaster hardens, and the type is withdrawn. The plaster mould is then baked, to drive off all moisture, and type metal is poured into it. Thus a solid mass is procured, instead of a number of separate pieces, so that there is no danger of disturbance, and the whole block can be multiplied ad libitum if needed. This process sets free the types, which can be broken up and used again.
The ordinary method of taking plaster casts is nearly the same as that which has been described. The object to be cast is oiled, and plaster of Paris carefully applied to it. When it is “set,” the plaster “mould” is removed and dried. The process is then reversed, the interior of the mould being oiled, and plaster poured into it, so as to produce an exact reproduction of the original.
In Nature we have almost exactly the same process, although it is necessarily conducted in a much slower manner.
All who have tried their hand at practical geology must be aware of the multitudinous casts of perished beings which are found in various strata. Sometimes the casts are those of vegetables, the original material having been decomposed, and stony matter taken its place. Sometimes there are casts of fishes or echini, while shells, and even insects, are found to have been cast almost as perfectly as could be done with plaster of Paris at the present day.
As might be anticipated, the chalk deposits are peculiarly rich in these casts, the fine particles of the chalk taking the place of the plaster of Paris.
In the illustrations are shown examples of casting in Art and Nature. On the right hand is a cast of fruit and leaves, which may afterwards be reproduced in plaster, wax, papier-mâché, or electrotype. On the left is shown one of the shells so common in the chalk, the upper figure representing the shell itself, and the lower the mould that has been formed around it.