The desired thickness having been obtained, it becomes a question of allowing the paste to descend and at the same time to support the piece by air pressure. The flange spoken of above is quickly cut, and the paste is made to rise again for the last time, in order to form a new flange, but one that this time will be extremely thin; then a perforated disk designed for forming the top joint, and acting as a conduit for the air, is placed upon the mould. This disk is fastened down with a screw press, and when the apparatus is thus arranged the eduction cock is opened, and the air pump maneuvered.
If the flange did not exist, the air would enter between the mould and the piece at the first strokes of the piston, and the piece would be inevitably broken. Its object, then, is to form a hermetical joint, although it must at the same time present but a slight resistance, since, as soon as the liquid paste has flowed out, the piece begins to shrink, and it is necessary that at the first movement downward it shall be able to disengage itself, since it would otherwise crack.
As soon as the piece begins to detach itself from the mould the air enters the apparatus, and the pressure gauge connected with the air pump begins to lower. It is then necessary, without a moment's loss of time, to remove the screw press, the disk, and the upper part of the mould itself, in order to facilitate as much as possible the contraction of the piece. Finally, an hour or an hour and a half later, it is necessary to remove the lower part of the mould, this being done in supporting the entire affair by the middle. The piece and what remains of the mould are, in reality, suspended in the air. All these preparations are designed to prevent cracking.
The operation by vacuum follows the same phases as those just described. It is well, in order to have a very even surface, not to form a vacuum until about three hours after the paste has been made to ascend. Without such a precaution the imperfections in the mould will be shown on the surface of the object by undulations that are irremediable.
The first flange or vein must be preserved, and it is cut off at the moment the piece is detached.
Moulding by vacuum, aside from the advantages noted above, permits of giving the pieces a greater thickness than is obtained in the pressure process. According to Mr. Renard, when it is desired to exceed one inch at the base of the piece (the maximum thickness usually obtained), the operation is as follows: The piece is moulded normally, and it is supported by a vacuum; but, at the moment at which, under ordinary circumstances, it would be detached, the paste is made to ascend a second time, when the first layer (already thick and dry) acts as a sort of supplementary mould, and permits of increasing the thickness by about ⅖ of an inch. The piece is held, as at first, by vacuum, and the paste is introduced again until the desired thickness is obtained.
Whatever be the care taken, accidents are frequent in both processes. They are due, in general, to the irregular contraction of the pieces, caused by a want of homogeneousness in the plaster of the moulds. In fact, as the absorption of the water does not proceed regularly over the entire surface of the piece, zones of dry paste are found in contact with others that are still soft, and hence the formation of folds, and finally the cracking and breaking of the piece. The joints of the moulds are also a cause of frequent loss, on account of the marks that they leave, and that injure the beauty of the form as well as the purity of the profile.
Mr. Renard has devised a remedy for all such inconveniences. He takes unglazed muslin, cuts it into strips, and, before beginning operations, fixes it with a little liquid paste to the interior of the mould. This light fabric in no wise prevents the absorption of the water, and so the operation goes on as usual; but, at the moment of contraction, the piece of porcelain being, so to speak, supported by the muslin, comes put of the mould more easily and with extreme regularity. Under such circumstances all trace of the joint disappears, the imperfections in the mould are unattended with danger, and the largest pieces are moulded with entire safety. In a word, we have here a very important improvement in the process of moulding. The use of muslin is to be recommended, not only in the manufacture of vases, but also in the difficult preparation of large porcelain plates. It is likewise advantageous in the moulding of certain pieces of sculpture that are not very delicate, and, finally, it is very useful when we have to do with a damaged mould, which, instead of being repaired with plaster, can be fixed with well ground wet sand covered with a strip of muslin.
When the moulded pieces become of a proper consistency in the mould, they are exposed to the air and then taken to the drying room. But, as with plaster, the surface of the paste dries very quickly, and this inconvenience (which amounts to nothing in pieces that are to be polished) is very great in pieces that carry ornaments in relief, since the finishing of these is much more difficult, the hardened paste works badly, and frequently flakes off. In order to remedy this inconvenience, it suffices to dust the places to be preserved with powdered dry paste. - Revue Industrielle.