The wood strips are now held down by some lead weights or other suitable methods. The nowel, which this really is, is now ready for the plaster of Paris. The asbestos is mixed with water to the consistency of cream and kept in this form. It never becomes hard, so that it may be kept indefinitely in this form. If it is preserved in such a manner, the air gradually works out of it and there is less danger of bubbles forming upon the pattern where the plaster of Paris with which it is mixed, sets. How much plaster to use for the mold will have to be left to the judgment of the operator, and he will have to' be sure that he has plenty. Like many other things, this part is a matter of experience, but is not at all difficult to learn.
For the mixing of the plaster and the asbestos with the necessary amount of water an enamelled kettle or pan is quite suitable. It does not rust like a tin pan. Fill it partly full of water and sift in the plaster of Paris until it just shows above the top of the water. Now add about one-half as much of the cream asbestos as there is of plaster mixture. This proportion is taken if it is desired to cast gold, silver, or bronze. If german-silver is to be cast, an equal amount of each is taken. After the asbestos has been added, the whole is stirred and the pan or kettle which contains it is brought near the mold.
Dip a medium stiff brush in the plaster mixture and "dab" the pattern all over so that all parts of the surface are covered with the plaster. This will ensure a perfect impression being made, and als0 free the pattern from air bubble. This will usually take from two to three minutes. When the plaster begins to set on the pattern, or medal, which we have used pour on the rest of the plaster mixture so that the wood frame used for the flask is filled to the top. Now wash out the pan before the mixture has set hard. If this is not done it will be difficult to remove it and a dirty pan will be had for the next operation.
As this setting is hardening, another flask may be prepared. This setting, however, usually takes place in a few minutes to a sufficient degree to allow the top to be scraped off with a steel scraper so that it is flat. This operation should be done when it is just barely set, as the plaster is then quite soft and the scraping is an easy operation. The weights may now be removed and, when the plaster has sufficiently set to allow it, the strips may be removed. If the strips of wood are varnished with shellac the removal is much facilitated and the plaster does not adhere as tightly as it does to the unvarnished wood.
Now turn the mold over so that the face side is up. This will bring the pattern, or medal, into view. If the plaster has run under it a little trimming, so as to leave the face uncovered will render everything in satisfactory shape. Now cut four conical holes in the corners of this mold so that when the cope has been cast, there will be four pins to serve as guides in closing the two parts. These will answer the same purpose as pins on a regular molders' flask. The conical holes may be made with a knife or any suitable tool. They should be deep enough to prevent any abrasion taking place when the mold is being closed. The depth, of course, depends upon the depth of the pattern, but any molder knows that a flask pin must be suited to the pattern, and so it is in this case.
Now soak this plaster mold in water until it will take up no more. This operation is necessary for the prevention of the other side or cope from adhering when it is set. Usually fifteen or twenty minutes are necessary for the complete soaking of this mold, and care should be taken that it is thoroughly soaked, otherwise the two halves will not pull apart as they should. While the soaking is going on, a third side may be prepared.
When the soaking is completed, the plaster mold is put back down upon the marble slab and wood strips placed around it as before. In this instance, however, the strips must be twice the width of those used before so as to allow the cope to be case of the same thickness. As the strips which were used before were one and one-half inches in width, they must be three inches in this case. The surplus water should be removed from the pattern and, if necessary a little more oil rubbed in it. The pattern should not be removed at all until both sides have been cast and the mold parted.
The operation of mixing the plaster and asbestos is carried out in the same manner as before; but, in this instance, the operator can probably gauge the quantity which is required with more exactness than in the other case. The experience which has been gained with the other side will give an idea of how much to use. Dab the pattern with the brush as before and then, when the plaster which has been put on begins to set, pour on the remainder. It is usually advisable to let the plaster rise a little way above the wood strips, as it can then be shaved down to a flat surface with exactness. When this side has set sufficiently, the top may be scraped off as before. The whole mold is now cast, and when set so that the mold will allow it, the strips may be removed and the mold allowed to set hard. Give it plenty of time to do this or difficulty will be experienced in getting the cope and nowel apart. Five or ten minutes will usually suffice for this and when it has been ascertained that the mold is hard enough to stand the opening operation, the halves may be pried apart by some hardwood wedges. These are inserted between the halves or at the joint. A table knife is also handy. The halves are simply pried open like a book.
If everything has been done properly, there will be produced a perfect mold and with the detail of the pattern exactly reproduced. It is better to cut the gate before the pattern has been removed, and this may be done in the same manner as in sand. Vents should be provided at the top of the casting, as there is no opportunity for the air to escape unless this is done. When the gate has been cut the pattern may be removed by means of a draw-nail or sharp pointed tool. Where to pry it out must be carefully considered, as a blemish will usually result where it is done. There are but few patterns, however, where there is not some spot which is better adapted for such a purpose than others, and this position should be used.
The backs of the cope and nowel may now be scraped so that they will be flat. This will allow an even pressure to be exerted when the halves are clamped together. The mold is now ready for drying. It will be seen that all of the water which the plaster has taken up must be driven off before the casting can take place. This is accomplished in an oven heated by gas. The heat which is best is one which is not extreme but gradual. The large excess of water should be driven off slowly and at the last a higher heat may be used. When all the water has been driven out of the mold, it will be found to be very light. As the plaster absorbs so much water, the drying is necessarily a slow operation, but it should not be hastened at all, otherwise the mold is liable to crack or blister. Take plenty of time if good results are desired. In drying, the mold should be set upon its face or back, as there is then more surface exposed for the drying and the mold is not apt to warp.
The mold, when dried, is ready for casting. The pins which have been cast on it serve to guide the cope into place, and when it has been done a hot iron plate is placed on the top and bottom and the mold clamped together. The plaster mold is also used when hot, as this insures the metal running in better shape and also obviates the absorption of moisture which a cold mold would be liabla to do. The mold is now ready for casting and is usually poured on end as the castings, which are made in plaster molds, are generally thin and require the pressure of a gatehead to insure a sharp impression.
One of the requisites for successful plaster casting is the drying of the mold. As there is so much water in the mold, it dries very slowly and there is always a temptation to hasten it as much as possible. This should be avoided, as a mold which is dried quickly will warp and crack when it is being clamped.-" The Brass World."