POTTERY AND PORCELAIN upon the degree of fusibility of the mass and upon the heat employed in baking. When the materials are white, and in baking are brought to that state of fusion which produces vitri- fication and translucency, the product is a porcelain of more or less hardness and fine- ness. When the materials are coarse and con- taminated with metallic oxides so as to render the ware opaque, common stoneware is pro- duced, of a texture resembling that of porce- lain, but much coarser. The various kinds and grades of earthenware, such as the cream- colored (c. c.) ware of Staffordshire, and fine stoneware, are distinguished by the granular or chalky surface of the fracture, instead of the vitreous fracture of porcelain, the former resulting from the absence of the degree of fusion necessary to produce vitrification. They are also distinguished by the more fusible and superficial glaze, which is applied at a lower heat than is employed for glazing porcelain. In ordinary stoneware vessels, such as butter, to prevent the free passage of the clay, and it is found more practicable to remove the stones, &c., by hand. In this country common stone- ware is usually made from clay alone, but in Europe a little fine sand and feldspar and some- times chalk is used. The mass when tempered FIG. 14. Pug Mill. preserve, and pickle jars, the clay, after it has been exposed to the weather and treated with water sufficient to form a plastic mass which can be readily moulded by the hand, is simply worked up in a kind of pug mill similar to that used for mixing clay in a brick yard. A vertical shaft armed with knives, placed with the planes of their blades in a spiral direction, is made to revolve by horse or other powei within a stout cylinder having a funnel-shaped top. From the inside of the cylinder knives also project, and by the action of both sets the moistened clay is reduced to a homogeneous mass and forced down to the bottom, from which it issues through a rectangular orifice 8 or 10 in. in. diameter. It is projected upon a platform, when an attendant cuts it into thin slices, and removes with his fingers stone and other solid substances. Some potters USP coarse wire grating through which the clay i forced, and which retains the solid particle that are too large to pass through the meshes The meshes however soon become filled so a FIG. 15. Potter's Lathe.
- o the proper consistency is taken to the pot-
her's lathe or "throwing wheel,' 1 to be formed, nto ware, or "thrown," as the operation is
- echnically called. The potter's lathe is one
of the most ancient machines, and was used in Egypt 4,000 years ago. Its oldest form was that of an upright shaft about 3 ft. high, which turned in a frame, having a small horizontal wheel at the top and a larger one at the bot- tom 3 or 4 ft. in diameter, and also horizontal, by which it was made to revolve by the action of the workman's foot. A treadle like that of an ordinary turner's lathe is more commonly used, or the form shown in fig. 15, which re- quires the help of an assistant. In large pot- a FIG. 16.-8tages of Formation. teries steam power is employed A common stoneware vessel having a circular horizontal fecS is "thrown" in the ff owingm^ The workman takes a mass of the pla and throws it with a smart blow *P a ad cular block of gypsum which forms the
