Albert T. Salt
The fact that plaster of Paris is capable of reproducing the most minute design upon a pattern has caused it to be used for casting metals when the most exact impression of the pattern is desired. There is no substance known which will do this in the manner that plaster of Paris will do it.
The best results are obtained by the use of a mixture, of asbestos and plaster of Paris. The asbestos acts in the same manner as the hair which is put into mortar and holds the plaster sufficiently to prevent its cracking when the hot metal strikes it. It will read ly be seen why asbestos is adapted to the work. Its fibrous nature acts admirably for holding the plaster together, and as there is no question about its ability to withstand the heat, asbestos is used with the accompanying good results.
It will readily be appreciated that the more plaster of Paris that can be used for making the mold the more delicate the impression will be. Hence it is desirable to use just as little asbestos as possible. Experience has shown that the lower the melting point of the metal which is to be cast the less quantity of asbestos that is required. For the very lowest melting metals very little or really none is required. For the very lowest melting metals very little or really none is required, although with the lead and antimony alloys a little asbestos mixed with the plaster gives more uniform results. For gold, silver, bronze or german-silver, the following proportions of asbestos and plas-ter of Paris should be used:
Plaster of Paris 1 part.
Asbestos 1 part.
Plaster of Paris 1 part.
Asbestos 2 parts.
The kind of asbestos to use is the short fiber and not the long, stringy kind that is used for packing, board or similar materials. This long fiber asbestos interferes with the impression as the fibers are apt to get between the pattern and the plaster. By the use of the short fiber, however, this difficulty is avoided and the impression is so nearly like that of the plaster alone that it can scarcely be distinguished. The proper kind of asbestos has an appearance similar to freshly fallen snow, while that of the long and unsuitable variety has an appearance of curled hair. These points should be borne in mind when an asbestos is selected for plaster casting. There is no difficulty in obtaining the right kind of asbestos from the manufacturer of asbestos goods. To say that short fiber asbestos is desired is usually all that is required. It should be ascertained, however, that the asbestos is free from sticks or other foreign matter, as these are apt to interfere with the impression of the pattern.
As for the plaster of Paris to use, it may be said that this occurs in commerce in two kinds: Ordinary plaster of Paris used for the manufacture of walls, stucco work, center pieces, or similar work, and dental plaster. This latter is excellent and is in a very finely divided condition and free from foreign matter, but is quite expensive when compared with the other kind. For ordinary work, however, the common grade of plaster of Paris is perfectly suitable and is that which is usually employed. It will be found that it is filled with lumps, sticks, paper, or other foreign matter, but these substances may be removed by riddling. When such a purification has been done, the plaster is excellent.
Let us take for an example of an article which is to be cast in plaster, a bronze medal which has been struck with great care from, a steel die. In plaster every detail may be exactly reproduced so that, with a little trimming of "fins," one would actually believe that it had been struck with a die. It is necessary for the plaster molder to have the detail reproduced but it is possible for him to do so if he understands his business. It must be carefully borne in mind, however, that all the requirements of draught, gating, pouring, etc., which are necessary in sand casting are likewise necessary for casting in plaster. The method is practically the same, and unless a pattern will draw out of a sand mold it cannot be used for plaster. The same rule applies to pouring a gating, for the result is the same in either case. It will be taken for granted, therefore, that the reader of this article understands the art of sand molding. Unless he does it is feared that he will not be able to make a good job with plaster.
Taking for an example of plaster of Paris molding, the bronze die-struck medal, let us proceed to reproduce this in plaster. The first thing to do is to see that the medal is clean and free from dirt which would interfere with the molding. After this has been done, the surface is given a light coat of olive oil by means of a brush and tuft of cotton. This is done to prevent the plaster from adhering and to allow it to be removed after the plaster has set, without tearing taking place. A light film of oil is all that is required and too much will prevent an accurate impression from being taken.
For a bench, a marble slab is excellent, for the reason that it is flat, water does not rust or warp it, and the plaster may be easily removed from it. It is advisable to rub it over with a ligtt coat of oil before beginning the work, as the plaster will then leave it equally well as it will the pattern. Now place the medal upon the marble slab and place four pieces of wood around it in order to hold the plaster when it is poured in. These pieces of wood should be thick enough to stand by themselves.
The regular 3/4-in. pine is perfectly suitable. The width of these pieces will depend upon the pattern and it should be sufficient to leave plenty of thickness of plaster outside of the pattern. If this is not done the mold is much more apt to crack. It is unnecessary to state that the edges of the pieces of wood should be straight and smooth, so that they will lie flat upon the marble slab and also adjoin one another with a tight joint. This is necessary on account of the semi-liquid nature of the piaster, which will tend to run out if the joints are not fairly tight. When all done, this arrangement is a simple box-like appliance for holding the plaster.