The iron-stone, which contains a portion of argil and silex, is first roasted in a common biscuit-kiln, to facilitate its trituration, and to expel sulphur and other volatile ingredients which it may contain. A large earthen crucible is constructed after the exact model of an iron forge, a part of the bottom of which is filled with charcoal or cokes; these having been previously strewed with ore and about 1/3 part of lime, are raised to an intense heat by a strong blast of air, introduced under the cokes at the bottom. By this heat the ore is fused, and the fluid iron drops through the fuel to the bottom; then follows the scoria, which floats upon the top of the fluid iron. This latter scoria, or, as the workmen call it, slag, is the material used in the manufacture of china, and is much impregnated with iron, and of a compact and dense structure. The slag is next let off, by a hole through the forge, into a clean earthen vessel, where it cools. This last vessel is then broken, in order to detach the slag from it, with hammers. The scoria is next pounded into small pieces and ground in water to the consistence of a fine paste, at the flint-mills of the country. This paste is then evaporated to dryness on a slip-kiln, well known amongst potters. Thus evaporated to dryness, it is used with the other ingredients in the following proportions, viz.:
Prepared iron-stone, 3 cwt.; ground flint, 4 cwt.; ground Cornwall stone, 4 cwt.; Cornwall clay, 4 cwt.; blue oxide of cobalt, 1 lb.
These having been mixed together with water by the slip-maker, are again evaporated on the slip-kiln to the proper consistency for use. The clay, thus prepared, is of course used in the usual manner in the fabrication of the several kinds of vessels.
_To make Porcelain, or China._
Porcelain, or china, is a semi-vitrified earthenware of an intermediate nature between common-ware and glass. Chinese porcelain is composed of two ingredients, one of which is hard-stone, called petunse, which is carefully ground to every fine powder, and the other, called kaolin, is a white earthy substance, which is intimately mixed with the ground stone.
Several compositions of mingled earth may yield a true porcelain by being burnt, and the porcelains of various countries differ in their mixtures. But the principal basis of any true porcelain is that kind of clay which becomes white by baking, and which, either by intermingled heterogeneous earth, or by particular additions undergoes in the fire an incipient vitrification, in which the true nature of porcelain consists. Feldspar and gypsum, if added, may give that property to infusible clay.
When porcelain is to be made, the clay is properly selected, carefully washed from impurities and again dried. It is then finely sifted, and most accurately mingled with quartz, ground very fine, to which then is added some burnt and finely-pulverized gypsum. This mass is worked with water to a paste and duly kneaded; it is usually suffered to lie in this state for years. The vessels and other goods formed of this mass are first moderately burnt in earthen pots, to receive a certain degree of compactness and to be ready for glazing. The glazing consists of an easilymelted mixture of some species of earths, as the petrosilex or chert, fragments of porcelain and gypsum, which, when fused together, produce a crystalline or vitreous mass, which, after cooling, is very finely ground, and suspended in a sufficient quantity of water. Into this fluid the rough ware is dipped, by which the glazing matter is deposited uniformly on every part of its surface. After drying, each article is thoroughly baked or burned in the violent heat of the porcelain furnace. It is usual to decorate porcelain by paintings, for which purpose enamels or pastes, colored by metallic oxides, are used, so easy of fusion as to run in a heat less intense than that in which the glazing of the ware melts.
Return to The Household Cyclopedia of General Information