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Page:Popular Science Monthly Volume 21.djvu/324

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THE POPULAR SCIENCE MONTHLY.

more plastic as it grows older. The qualities required of a good paste may be communicated by diluting it and stirring it with water and decanting, or by prolonged beating and manipulation. By treading it out or beating it we not only give it complete plasticity and homogeneity, but we also clear it of air-bubbles which would otherwise swell out in baking and cause much damage.

The next thing is to give our ware the form which has been determined for it in a design previously made. This requires a knowledge of the whole process of fabrication. It would be a mistake to suppose that porcelain can be baked in any desired form. It becomes soft in baking, and has to be supported; and, as it is to be covered with a glass that melts at the same moment, places which need not be enameled must be found for fixing the supports in every piece, unless we are willing to risk having it spoiled. We may thus comprehend one of the difficulties in the manufacture of porcelain, and one of the points in which it differs most from delf.

Articles of porcelain may be shaped without molds or with them. By the former method all the shapes are obtained that may be produced by turning. The clay is first shaped on the wheel by the hand into a rough block of the general shape which the object is destined to assume, and is then left to dry, slowly and with care, to keep it from cracking. When it has been dried to a suitable degree of consistency, it is put upon the wheel again and carefully worked into the exact shape desired, with the moldings and ornaments called for by the design, and by the aid of the most simple instruments.

Articles whose shape does not adapt them to manipulation on a revolving wheel, such as objects of statuary and many lighter objects, may be shaped by molding them. The mold is made of plaster of Paris; to it, when dry, is applied a layer of the porcelain paste, which is pressed into it carefully and as evenly as possible; the earth espouses all the details of the sculpture, and, after a few moments, the plaster having absorbed the water from the paste in contact with it, a shrinkage takes place, thanks to which the proof detaches itself almost spontaneously. The operation is sure to be successful in the case of simple forms whose outlines offer no impediment to taking them from the molds. If, however, we purpose to obtain objects in relief, statuettes, groups of figures, or sumptuous vases, the sculptural decorations of which constitute their chief ornament, the process becomes more complicated. In this case the molder has to divide his pattern into a number of parts, the superficies of which must be determined by the possibilities of taking off the molds; then he must make as many molds in plaster as he has parts of his model; these molds will in their turn serve for the reproduction of each of the parts, which have afterward to be joined and cemented by the aid of the paste diluted in water. After this the seams at the junction of the parts must be rubbed away, and the whole work finished up by a restorer who must