POTTERY AND PORCELAIN, MANUFACTURE OF. The object of the present article is to add a few further observations to the manu facturing details given under EARTHEYWARE; and to apply those details to the two kinds of ware which form the extremes of the series, namely, fine or porcelain, rind coarse or pottery : earthenware occupying a medium position between the two.
In the beginning of the last century, it was ascertained by experi ment that the earths alone are infusible, but that when silex is com bined with aluminous earth, and subjected to a strong fire, it unites chemically with the clay, as an acid unites with a base ; and that when a third earth, having alkaline properties, as lime, magnesia, or barytes, is added, a more complete change is effected, and a vitrified body is pro duced, resembling in density and fracture many natural gems. When the earthy materials are free from metallic matter, a perfect porcelain is thus produced. It was found that seven parts of silex, six parts of aluminous earth, and two parts of an alkaline earth, would produce such a body. On this principle the Germans and French founded their porcelain manufactures. In England the enterprising spirit of Wedgwood wrought a complete revolution in the art ; and by bold experiments, guided by his knowledge of mineralogy and chemistry, he laid the sure foundation for all those improvements which have raised the fictile manufactures of this country to so high a level.
The materials are reduced to the consistence of cream, in which state they are called slips (slops) ; this fluidity is necessary to ensure the perfect mixture of all the ingredients, and their mutual chemical action in the fire. The basis of the composition is a clay from the plastic-clay formation in Devonshire or Dorsetshire, to which is added ground flint, which gives whiteness and solidity to the goods. For the better kinds, a portion of China-clay, or decomposed felspar from Cornwall, is added, together with a email quantity of ground white granite. The pro cesses are conducted as described in EARTHE:VWARR, until a smooth and homogeneous mass of plastic clay is produced.
1 t may be well here to give the proportions of ingredients for various kinds of ware or bodies, derived from the actual practice in Stafford ahire Drab-coloured Ware is made of the fine gray marl fonnd between the coal strata, which burns to a cane-colour in the oven ; or a white body as above, is stained with oxide of nickel, which gives it a greenish drab tint ; or with oxide of manganese, These mixtures give a fine white body for ornaments, which may be stained blue by the addition of one per cent. or less of oxide of cobalt, according to the strength of tint required ; or a green jasper may be produced by staining with protoxide of chrome.
Turquoise bodies.—A body called turquoise has been manufactured to a great extent. It is nothing more than a good white body, stained with a mixture of oxide of cobalt, combined with a large proportion of oxide of zinc ; when glazed, it ha. the peculiar milky tint of the gem after which it has been named.
In this manner, by a proper combination of metallic oxides, either as existing in earthy materials, or prepared from their metals, an infinite variety of coloured bodies may be produced. They are usually prepared on slip-kilns of small dimensions,.the bottoms of which are covered with a layer of plaster of paris, if used for the finer colours.
Referring to EAaTUENIVARE for an account of the manner of fashion ing circular vessels on the potter's or throwing wheel ; turning them to the proper thickness, and working mouldings, &e. ; fixing on handles and spouts; and making the peculiar variety called dipped ware : we shall add a little to the details there given concerning ornamented ware and flat ware.
Thrown and turned goods are sometimes ornamented with figures in relief, which are made out of flat moulds by children, and fixed upon the ware by workmen, who, having carefully adjusted each figure to its place, run a little water under it with a camel-hair pencil, which unites it to the surface. Goods of an oval or angular shape, which cannot be turned, are made by pressing clay into plaster moulds, which give the outside form to the vessels. For this purpose the mould is made in two parts, and each is separately filled by laying in a cake of clay that has been beaten out to the proper thickness on a wet plaster-block ; it is pressed into the mould by repeated blows from a ball of wet sponge, then squeezed into all the angular parts with the fingers, and smoothed with sponge, wet leather, and horn. When both sides of the moulds are thus lined with clay, they are joined together ; and the workman lays a roll of clay along the inside of the joining, which he works down until the whole is smooth and solid. The mould is then carried into a stove-room ; and when it has absorbed the moisture, so as to release the clay, the work is carefully taken out, and the empty mould returned to the stove previous to being filled again. Tho seam which remains on the outside of the vessel is removed by scraping and burnishing with wet horn ; the handles and other appendages are then attached. This branch of the trade is called hollow-ware pressing or squeezing. Plat-ware pressing is per formed by giving the shape to the goods by moulds which fit the inside of the vessel ; plates, dishes, saucers, cups, and hand-basins are made on this principle by the English potter, with a dexterity which must be seen to be understood, and which excites the surprise of foreigners 'more than any other brInch. The workman stands at a bench provided with a whirling-table similar to the thrower's, but which has its motion given by a horizontal pulley or jigger, turned by his young assistant. Close to his left hand is a batting-block of wet plaster, and the mass of well-beaten clay from which he is working ; immediately behind him is the stove-room, in which the moulds are ranged on shelves. All being ready, the platesnaker cute his clay Into lengths with a wire, and tears off a piece which ho bats ost thus upon Ilia block by a stroke or two of his tatter, or plaster mallet, and polishes the surface by pressing the side of a long smooth knife across it. During this process, which occupies but a few seconds, the boy has brought a mould, placed it upon the whirler, and taken his place at the handle of the jiver. The man places the clay upon the mould, and as it whirls, presses it down close with Ilia hand ; a profile, or earthenware tool which gives the form to tho bottom of the plate, is pressed upon it as it revolves, the superfluous clay being cut off by a wire ; the boy instantly catches hold of the mould thus covered with a coating of clay, runs with it to the stove, places' it on a Pilaf to dry, and returns with an empty mould ; by which time his master has his clay prepared for another plate. When sufficiently hardened, the bottom of the plate 13 polished; and when taken off the mould, the edges are smoothed and polished with moist leather. Cups, saucers, and hand-basini are now made by this process, instead of being thrown and turned as formerly. By this means they are supplied at a cheaper rate •, one man and a boy being able to manu facture the articles which by the old mode requires the work of three men and three females.