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Pottery (To manufacture English)
To manufacture English Stoneware.
Tobacco-pipe clay from Dorsetshire is beaten much in water; by this process the finer parts of the clay remain suspended in the water, while the coarser sand and other impurities fall to the bottom. The thick liquid, consisting of water and the finer parts of clay, is further purified by passing it through hair and lawn sieves of different degrees of fineness. After this the liquor is mixed (in varions proportions for various ware) with another liquor of the same density, and consisting of flints calcined, ground and suspended in water.
The mixture is then dried in a kiln, and being afterwards beaten to a proper temper, it becomes fit for being formed at the wheel into dishes, plates, bowls, etc. When this ware is to be put into the furnace to be baked, the several pieces of it are placed in the cases made of clay, called seggars, which are piled one upon another, in the dome of the furnace; a fire is then lighted, when the ware is brought to a proper temper, which happens in about 48 hours, it is glazed by common salt. The salt is thrown into the furnace through holes in the upper part of it, by the heat of which it is instantly converted into a thick vapor, which, circulating through the furnace, enters the seggar through holes made in its side (the top being covered to prevent the salt from falling on the ware), and attaching itself to the surface of the ware, it forms that vitreous coat upon the surface which is called its glaze.
To make Yellow or Queensware.
This is made of the same materials as the flintware, but the proportion in which the materials are mixed is not the same, nor is the ware glazed in the same way. The flintware is generally made of 4 measures of liquid flint, and 18 of liquid clay; the yellowware has a greater proportion of clay in it. In some manufactories they mix 20, and in others 24 measures of clay with 4 of flint. The proportion for both sorts of ware depends very much upon the nature of the clay, which is very variable even in the same pit. Hence a previous trial must be made of the quality of the clay, by burning a kiln of the ware. If there be too much flint mixed with the clay, the ware, when exposed to the air after burning, is apt to crack, and if there be too little, the ware will not receive the proper glaze from the circulation of the salt vapor.
To manufacture English Porcelain.
The iron-stone, which contains a portion of argil and silex, is first roasted in a common biscuitkiln, 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 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 slipkiln, 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.
The These having been mixed together with water by the slip-maker, are again evaporated on the slip-kiln to the proper consistency for use. 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 commonware and glass. Chinese porcelain is composed of two ingredients, one of which is hard-stone, called petunse, which is carefully ground to a very fine powder, and the other, called kaolin, is a white earthy substance, which is intimately mixed with the ground stone. yield a true porcelain by being burnt, and the Several compositions of mingled earth may 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. Felderty to infusible clay. spar and gypsum, if added, may give that prop- When porcelain is to be made, the clay is prop-