From the block mold a reverse is made, called a "case," and from this, in turn, working molds are made in any required number. While it is possible to use the block mold as a working mold, and, if only a few pieces are required this is quite sufficient, yet, as it is often necessary to have a number of molds, the student should understand how to make a case.
A case may be defined as a mold from which a mold is made. If one can imagine the visible half of the vase form as it appears in making the mold, with the templates and ends cemented into one piece, one has a conception of one half of a case. The problem is to make this with permanent but movable ends so as to have a convenient form from which half molds may be easily made.
Fig. 8. Offset plates. F, top plate, front view. F', side view. G G', bottom plate.
Fig. 9. Sectional view of mold ready for casing. A, mold. B B, offset plates. C C, end plates. N N, natches.
The ends are joined to the body by means of offsets and the first step is the construction of these. One half of the block is taken and laid upon its back, being supported by clay so that the face is level and steady. An offset plate is now cut to fit each end. To make these a piece of plaster is selected or made which is true and smooth on both sides. The plates are cut of the same width as the mold and are beveled at the upper edge so as to rise slightly from the mold face. The curve at the end of the mold is cut out to fit and beveled in like manner. Then two end plates are fitted. These should be about two inches higher than the offset plates and are square at the top. Upon each of these two or three clay natches are set, being placed low down near the face of the mold. The mold and plates are well sized and bound together with side walls just as in the making of the mold. Plaster is poured to a height sufficient to well cover the natches and left to set hard. No shaping is necessary. When well set the end plates and offset plates are removed but the vase mold and the case are left attached together. The other half of the mold is prepared and run in the same way, the same offset plates and end plates being used with such slight refitting as may be necessary. The work is now examined and all rough places and scraps of adhering plaster are removed. The two halves of the case, the half molds being still attached, are set up on end, back to back, being separated by a thin piece of plaster or a strip of cardboard which should extend two inches above the top. The top ends are now sized, the natch holes having been smoothed off, a band of paper is tied around and plaster poured on top to a depth of about one inch. When set the whole is turned over and the operation is repeated on the other end. After the final setting the ends are easily removed and by the insertion of a thin knife driven by a light blow, the molds and case are separated. Each half case is now laid on its back and the proper ends are fitted in place. It only now needs the usual side walls to be tied on and molds can be made with ease just as the original block mold was made.
Fig. 10. Mold and case in position. The top ends are lifted to show fitting. The bottom ends are not shown.