Page:Popular Science Monthly Volume 21.djvu/325

Rh necessarily be an artist. I can not leave this part of my subject without mentioning the property which porcelain has of shrinking when baked. The shrinkage amounts to about ten or fifteen per cent.

A third method of shaping porcelains is by casting, which was discovered at Tournay, 1784, and in which Brongniart has made numerous improvements. Nothing is more simple than the manufacture of a small object by this process. Thus, if we take a plaster mold of a cup, and pour into it a quantity of barbotine, or porcelain-clay mixed with water, the mold will absorb the water from the clay in contact with it, forming a shell less liquid than the rest of the barbotine, and which sticks to the plaster. When this shell has attained a suitable thickness, the rest of the barbotine may be poured out: what remains in the mold constitutes the cup. We leave it to dry, and in a little while it will have gained consistency enough to be taken out of the mold without being deformed. Ware thus made is extremely delicate; the slightest pressure with the lingers may destroy it. This process is used at Sèvres for large pieces, but special manipulations are required for such work; for the weight of the shell which should adhere to the mold when the liquid is poured out, and which should be thicker and heavier in proportion to the greater size of the vessel, is very apt to cause it to separate from the plaster and fall. The least awkwardness might destroy the piece, and this should be avoided at any cost. MM. Milet and Delacour have devised a method, which has been used at Sèvres since 1857, for avoiding such accidents by turning compressed air against the interior of the mold at the moment the barbotine is poured out, to take the place of the liquid and hold the porcelain shell against the plaster. M. Regnault has simplified this operation by exhausting the air on the outside of the mold, which effects the same purpose and is more convenient of execution. The absence of seams, the purity of the outlines, and the clearness of the surfaces obtained by this process, make it one of inestimable value when we wish to get an object of art, and lift it far above the process of molding. The details of the operation are very exacting, but none of them should be neglected. Their importance may be realized by reflecting that a hidden fault in the interior of a large piece, a bubble of inclosed air, a lack of homogeneity in the paste, or other flaw, is not perceptible till after the baking, when the vessel has been decorated, and may perhaps have become of very great value. The slightest defect in the casting may destroy this value.

The objects, having been properly shaped and fitted, have next to be transformed into porcelain by the action of fire, the function of which is to combine the different elements of the paste and determine the fusion of the glazing. The baking is done twice. In the first operation, when the temperature is relatively only moderately high (1,800 to 2,160), the earth is converted into what is called biscuit baked porcelain; it becomes very tough and sonorous, and extremely