The use of 'jiggers' and 'jollies' has greatly increased the rapidity and regularity with which vessels may be shaped. A jigger is a machine carrying a revolving mold in which the clay is shaped by a former which is brought down and held firmly within the mold, the clay having been carefully spread by hand upon the inside surface of the mold. The jigger is used for deep dishes, vases, and the like. The jolly is a similar contrivance used in forming plates and other flat pieces. Jugs and bottles are com monly made in two parts and cemented together before the clay is dry. The clay is usually allowed to dry in the mold, and as the water is drawn off the clay shrinks so that it does not cling to the mold. Such additions as spouts and handles are molded separately and cemented with moist clay before the pieces dry.
Casting is employed in making very fine ware. The plaster-of-Paris mold is filled with liquid clay which is allowed to stand until a thin film is formed the surface of the mold, after which the rest of the day is poured out: this process is used for 'egg-shell' porcelain, but also. in rare eases, for fine, hard pottery. The making of these plaster-of-Paris molds is a matter of great delicacy. The model having been designed, a mold is made from it which is divided so that it may be easily removed from the clay. This is called the block mold. From this there is made a cast which is, of course, a rep lica in plaster of the original model, and from this the working molds are made. After the pieces of pottery have been formed, they are taken 'to the drying-stove, where they are ex posed to a heat of about 85° Fahr. When con sidered dry, they are placed in great earthen ware vessels, called seggars, or saggars, are so shaped that they can be piled one upon another to a great height. The seggars are often made large enough to hold each a number of pieces of the uubaked pottery, and as these would adhere if they touched one another, a number .of curiously shaped pieces of baked clay are used, upon which they rest: these are called watches, cock-spurs, triangles, stilts, etc. Thus, each seg gar forms a small oven by itself, and by this means the unequal heating of the pieces is pre vented, and they are also protected from smoke. A pile of seggars is called a bung, and there may be forty-eight or fifty bungs in the charge of a kiln. When all this is arranged, the furnaces, of which there are several to each kiln, are lighted. The firing requires from twenty-four to fifty hours, after which the ware is allowed to cool very slowly. See KILN.
The articles are now in the state called biscuit, and still require glazing. and perhaps decorative painting and gilding. (For the various kinds of glazing see the articles STONEWARE; FAIENCE; MAJomcA, etc.) Many mixtures are used, the essential thing being that the glaze and the body shall be of the same general nature in order that . they may contract evenly during the firing. The result of uneven shrinking is seen in the cracking of the surface, called 'crazing:' and the crackle of Oriental porcelain and pottery is deliberately produced in the same way. The different glazes are composed of litharge, flint, feldspar, Paris white, and white clay, applied in the form of 'slip.' The glazing materials are
triturated with water, with the same care and by similar means to those employed in forming paste, and are reduced with water to the same milk-like liquidity. Each workman has a tub of the glaze before him; and as the articles of biscuit-ware, either with or without decorations, are brought to him, he dips them in the glaze, so as to insure a uniform coating over them; and, by nice management, he prevents any large drops or accumulations on one part more than another. The porous biscuit-ware rapidly ab sorbs the moisture, and dries up the thin film of glaze on the surface of the articles, which are again placed in seggars, and carried to the glaze kiln, where they undergo another firing, which melts the glaze, and converts it into a perfectly transparent glass, all over the surface, and renders any pattern previously printed upon it very plain. The temperature in the glaze or enamel kiln is increased very gradually, and is kept up for about fourteen hours, after which it is allowed to cool slowly. and the articles are taken out completed. So far, this description has applied to the manufacture of pottery and porce lain on a large scale, for general purposes; but when it is applied to more costly and artistic works, very special arrangements are required.
Decoration by means of painting may be ap plied to pottery on the biscuit, o• on the unfired enamel, or on the already fired enamel, or glaze, of whatever nature. The method is limited in its scope, but very permanent. The difficulty is to find colors that will stand the great heat required for firing the glaze. Practically the only color used for a hard glazed ware is cobalt blue: but for the softer glazes the oxides of many metals, as copper, iron, and nickel, may be used.
The above applies especially to porcelain. Painting on the unfired enamel is used in some of the brilliant modern varieties of faience. Painting upon the glaze, allowing of almost any combination of colors, is the more common prac tice in ceramic art generally. Painting under glaze requires great skill and experience, for the appearance of the pigments as they are laid by the painter is altogether different from the resulting effect after the firing. The decora tion may be applied with a brush-or by a transfer of printing. In the latter process the designs are engraved on copper plates; the colors are specially prepared with a printing oil, and the designs printed on a wet tissue paper, which in turn is laid upon the ware and transfers its pat tern to the surface of the clay. After the paper has been washed off, the decoration may be touched up with a brush, if necessary.
When it is desired to avoid sharpness of outline in painted or transferred designs, the process called 'flowing' is sometimes used. Sups of more volatile liquid are placed in the seggars, and the vapor of this, partially combining with the metallic colors, softens their outlines. Some times the glaze itself is colored, and brilliant and very much admired effects are produced in this way.
In general the term earthenware is used for common pottery, and this carries with it the gen eral idea of a coarser, a softer, and a less care made ware than those specified under dif ferent names.