Porcelain Moulding

The Encyclopedia of Founding (1892)

Not many branches of industry are of higher antiquity than that of the potter, as the plasticity of natural clays and their hardening when exposed to heat are properties which were known in the earliest ages. There is a marked difference between true porcelain and earthenware, the body of the former ware being very compact and translucent, and breaking with a conchoidal fracture, which shows there has been partial fusion. The glaze which is applied to porcelain to produce the smooth surface evidently blends with the substance of which the body is composed. This cannot be said of earthenware, which, when it is broken, shows an open and somewhat earthy fracture, and the glaze can be readily detached. The clay employed in porcelain-making is derived from decomposed feldspar, no other kind being pure enough; this is white, and free from ferric oxide. To diminish the contraction which this substance undergoes in the fire, finely divided silica made from pulverized flints is added, together with a proper proportion of feldspar or other fusible material, also reduced to an impalpable powder. The ware is moulded in plaster-of-Paris matrices, or formed by means of a vertical spindle and table (potters'-wheel), and is partially dried in the air, still more by heat, and then converted into what is termed biscuit by exposure to a higher temperature. This porous biscuit is in a condition favorable for receiving its glaze, which is either ground feldspar, or a mixture of silica, gypsum, and some porcelain clay diffused through water. After dipping in this mixture the water sinks into the ware, leaving an evenly spread surface of the powder; after it has again dried it is fired at an exceedingly high temperature. The manufacture of porcelain outside of China is of modern origin, but the Chinese have practised the art from the beginning of the seventh century, their work being in some respects unequalled. The materials employed by them are kaolin, or decomposed feldspar; petuntze, or quartz reduced to fine powder; and the ashes of fern, which contain potassic carbonate. See POTTERY-MOULDING.