The art of making vessels from earth. In the earliest ages upon record pottery was manufactured. The chief establishments in England are in Staffordshire, in a district called The Potteries, at Worcester, Derby, Coal-port, and Liverpool. The potteries in Staffordshire employ many thousands of persons, and the value of their produce was estimated at 800,000/. per annum. The essential material of all pottery is clay, which of itself possesses the two requisite qualities of being in its natural state so plastic, that, with water, it becomes a soft, uniformly-extensible mass, capable of assuming and retaining any form, and, when thoroughly dried, and having undergone a red heat for a time, of losing this plasticity, and of becoming hard, close in texture, and able, more or less, perfectly to confine all liquids contained within its hollow. The most important circumstances requisite to be considered in selecting the materials for pottery are plasticity, contractibility, solidity and compactness after drying, colour, and infusibility. Wedgewood was the great improver of this manufacture.
The processes employed at most of the manufactories are very similar, which may be classed under the following heads: - Preparation of raw material, moulding and turning, firing, printing, glazing, and painting. We shall describe these consecutively, as they are conducted at Spode's establishment.
In the preparation of the raw material, a powerful steam-engine performs many of the processes formerly carried on by manual labour. The bodies of earthenware are composed of Kent flint and West-of-England clay. The flint is first calcined in kilns, similar to those in which lime is burnt; it is then broken by revolving hammers, put in motion by the steam-engine, and afterwards conveyed into the pans, paved with stone, to be ground with water. In the centre of the pans there is an upright shaft, from which several transverse arms branch out, having very heavy stones placed between them: these stones, moved horizontally by the steam-engines, grind the flints, until they form a cream-like liquid, which is let off into the wash-tub, where the coarser particles are separated from the fine; the latter runs off into reservoirs, and the former is carried back to the grinding-pan. When the ground flint is wanted for use, it is conveyed from the reservoir by a pump, worked also by the steam-engine. The process of preparing the clay, and mixing it with the flint, is this: - The clay is drawn up into the upper chamber of the slip-house, and there thrown into an iron box, in which moves a shaft, with knives fixed in it, to cut the lumps into small pieces.
The clay is now laid in a cistern with a proper quantity of water, where it softens, and is then put into the -plunging-tub; in this tub the water and clay are stirred until they become thoroughly mixed. The liquid is now drawn off into another cistern, from which it passes through a silk sieve into a third cistern; then into a fourth, through silk sieves still finer; the ground flint and other ingredients are now brought and mixed together, and the whole .passes through sieves of a greater degree of fineness into a fifth cistern: in this is a pump, that throws it into a trough for conveying it into the drying kiln. All these various operations are worked by the steam-engine, and there are fourteen sieves in motion at one time. After the clay has been dried it is taken from the kiln and laid together in large heaps, and, before it is worked into the vessels for which it is destined, the air bubbles are disengaged from it: this is done by a machine turned by the steam-engine. The machine is an iron box, shaped like an inverted cone, with an upright shaft in its centre, to which are affixed knives to cut the clay which is put into the box, by their rotatory motion, and, at the same time, so arranged as to force it downwards to a square aperture at the bottom; it escapes through this in a sufficiently compressed state for the workmen, and is then cut into square pieces of a convenient size, to be distributed in the manufactory.
Near the steam-engine are workshops for those branches of the trade which require the aid of machinery; and in this building there are eight throwing-wheels and twenty-five turning-lathes. Underneath these shops are drying-houses, heated by steam, in which the ware is dried, previously to its going to the oven to be fired; above the workshops is a single room, capable of holding 200 workmen.