The method pursued in making pottery and porcelain is the same in principle every where; we shall, therefore, give the general outline of the process without entering into the minute differences which distinguish the variations produced by different.manu faeturers in this and other countries. Pottery and porcelain differ chiefly in this, that the superior quality of the materials used in making the latter, gives it a peculiar trans lucency. For pottery, inferior materials are used, and a considerable admixture of cal cined flint., bone-ashes, or native phosphate of lime, are added to the clay. The use of the calcined flint was said to have been first adopted by a Burslem potter named Astbury, who. while traveling to London on horseback, in the year 1720, bad occasion in passing through Dunstable, to seek the assistance of a hostler in consequence of some disease in his horse's eyes. He noticed that the man took a piece of flint, burned it, and then reduced it to a fine powder, which lie blew into the horse's eyes. Astbury noticing the beautiful whiteness of the powder, conceived the idea of using it in his pottery; and did so with great success.
The ingredients, such as the clay and calcined flints, are ground by separate means; the former in the pug-mill. This is an upright, iron-hound, wooden cylinder, with an , axis turned by machinery; projecting from the are 7 arms, each of which has 3 knives fixed in it, with the points outward, and so arranged that they spread over the largest amount of space in the interior; and altogether they are placed in a spiral manner, so that when in motion, the clay which is thrown in lumps into the hopper shaped upper part of the vat, is worked down, and is so cut and kneaded by the knives that it is forced out at an opening at the bottom, in the state of soft pap. This is aided by the knives on the lower part of the lowest arm being connected together by a plate, which prevents all settlement at the bottom. This pap-like clay passes into a large wooden tank, in which it is agitated with water until quite incorporated so as to resemble milk in color and consistency. In another mill, of a different construction, the Cornish granite and calcined flints are being reduced to a somewhat similar state. This mill is very strongly constructed, and consists of a iu the center of which turns an axle moved by machinery; in the bottom of the vat is a thick stone-bed, consisting either Of chert or horn stone. From the upper part of the axis 3 strong arms project like the spokes of a wheel; and strongly attached to these are stout beams pointing down ward, and nearly touching the stone-bed. As the axis, with its arms and beams, turns round. the beams push some large masses of the Cornish granite or of chert stone round with them, and these triturate the calcined flints and other bard materials, and stir up the water with which the vat is kept constantly supplied, while it overflows in a milky state, charged with the finely divided materials, into a cistern, where it is kept stirred until it is sufficiently supplied with the solid materials, and the thickened milky liquid is then drawn off, in proper proportions, into a vat to which the prepared clay is also passed. The mixture of the two is then allowed to subside until the water is nearly clear, when it is drawn off; and the sediment is deprived of its surplus moisture, either by evaporation, or in the best works, by a pneumatic-exhausting apparatus, which does it very quickly. The composition is then a fine plastic material of the consistency of tough dough, and is ready for the potter's use. In preparing the finer materials for porcelain many other operations are required, all, however, having the same object, viz., the extremely minute division of the substances used.
The prepared clay is taken to the or potter's lathe, which is repre seated in fig. 1. This consists of a fixed table A, through which passes the axle B; and rises a little above its surface, and having on, its upper end a disk C, which revolves with it. The axle is put into rapid motion by turning the flywheel 1), either by hand or machinery; and this causes a rapid revolution of the disk C, upon is placed the soft mass of clay to be molded. At B is seen an upright,
with a small sliding-bar regulated by a screw; this is the guide for the potter to regulate the height of the vessel he is making. When the lump of clay is revolving, the potter, with his bands or with proper tools, fashions it -into any shape he pleases; his management of this requires considerable skill, as nearly every article requires a different configuration. But some articles are formed In molds, the molds being made of plaster of Paris. This answers well for fine porcelain intended to be very thin, because plaster-mold absorbs much of the moisture in the paste, and thus partially dries it, so that it admits of handling, which mu a softer state would be very difficult. The paste is used so liquid that it can be poured into the molds. It is usual, in casting, to have a mold for each part.
For nearly flat articles, such as din ner-plates, a plan is adopted which combines both processes: a mold, usu ally of plaster, Fig. 2, a, is placed on the disk of the throwing-wheel, b, and a thin layer of the paste is pressed on to it, so as completely to take its form; then to the guide-post, c, is at tached an arm, d, with a small brass plate, e, on its lower side. This plate is cut to the outline of half the plate, or dish; as it revolves, this pares down and shapes the clay to its own outline, and to the thickness to which it is set, there being an arrangement on the arm of the guide-post by which this can be effected. Sometimes, us in the case of deep vessels, molds are used for the exterior, and the interior is formed by the hand. This process insures certainty of size and shape, which is important in making large numbers of similar articles, as tea-cups, etc. The mold is lined with a thin cake of clay, and when placed on time revolving disk, it is fashioned inside by band, and finished off with a wet sponge. Sometimes metal or horn tools are used for pro. clueing moldings and other raised ornaments, or for grooves, when the turning or throwing wheel is used. If the articles made require handles or other similar accessory parts, they are always molded, unless of very simple Rums, and are attached whilst they and the body are still soft enough. They are joined by a thin fluid paste called a slip, and the junction is smoothed over with time wet sponge, which is one of the most useful of the potter's tools. Being formed, the articles, of whatever kind, are now taken to the drying-stove, where they are placed on shelves, and remain there some time, exposed to a heat of about 85° Fahr. When quite dry, they are next taken to a workshop near the kiln, and they are here carefully packed in coarse earthenware ves sels, called seggars, which are so made that they can be piled upon one another to a great height in the kiln, as S'oen in fig. 3, in which some of the seggars are seen in section, for the purpose of making the arrangement more intelligible. As the seggars are generally made large enough to hold a number of articles, which would, when highly heated, adhere if they touched, a number of- curiously-shaped pieces of burned clay are used for placing between them, so as to make them rest on points; these are called watches, cock-purs, triangles. stilts, etc. Another object is gained by this, in burn ing fiat articles such as plates: these, if placed one upon another, would not be fired equally; but, when they are lucid apart, the heat affects all parts alike. The seggars ate so piled in the kiln that the center is hollow, and there are free spaces between them through which the fire can ascend; props, a, a, a, fig. 3, being so placed as to keep them from immediate contact with the sides all round. Thus each seggar forms a small oven, in which one or more pieces of pottery or porcelain are baked, and the seggars prevent any unequal heating of the pieces, and also protect them from smoke. A kiln has gen erally eight furnaces, and it is usual to raise six piles of seggars between every two fur naces, or rather between their fines, which rise to a considerable height in time kilns.