DEFINITION AND GENERAL PmworPtEs.—Every ware made of clay, or of a mixture in which clay is the claief ingredient, and hardened by heat, may he regarded as a apeciea of " pottery." There are many varieties of clay (see Clay, pp. 635-40), all of which have been formed by the disintegration of felspathic and officious rocks, and consist of hydroua aluminio silicate mixed with email and varying proportions of other materials derived from the same sourcea. A clay adapted to the rnanufacture of pottery must be plastic, and must become hard under the influence of heat. Plaoticity is an attribute of hydrous aluminic silicate, and is developed by the mechanical mixture of this .body with a limited quantity of water. Clay is insoluble in water, but may be diffused through it in a state of extreme aubdivision, and regains plasticity when the excess of water is removed. If a clay be exposed to a high temperature, artificially produced, and be rendered anhydrous by the removal of water previously held in chemical combination, it can never regain plasticity by mechanical mixture with water.
Hardening is produced (1) by the removal of water mechanically mixed with the clay, (2) by the removal of water, and sometimes of carbonic acid, chemically combined with the clay, (3) by the closer juxtaposition of the particles of the clay, due to the fusion of a part of the ingredients. Some clays, when exposed to the full heat of a pottery-kiln, fuse readily throughout their subatance, owing to the presence of other materials in addition to the alumiuic ailicate. Aluminic silicate is by itself practically infusible, but when exposed to an intenae heat in the presence of free silica, together with calm, sodio, potassic, magnesio, ferric, or ferrous oxides, it unites, wholly or in part, with the silicates formed from these ingredients, to create a readily fusible glasa. Most natural elaya ceutain free hydrous ailica, together with one or more ef thc oxides mentioned, and the quantity of aluminio ailicate whiell can he rendered fueible is determined by the quantity of ailicate-forming ingredients incorporated with it. Felspar, which is a natural glass, and from whose decompositiou, certain ails are formed, ia built up of equivalent parte of aluminic ailicate and of potassie, sodic, or caleic silicate. Tho fusibility of a clay is greater or leas as its compoaition approaches or recedes
front the proportiona obaervable in felspar.
Solidification neeesaarily impliea contraction. Pure aluminic silicate, when artificially heated, shrinka exceaaively, and aplita into fragments. The purest clays are the most infusible, and at the same time are the most liable to fracture and distortion under the influence of heat. Very few clays in their natural atate are free from intermixture with iron. The form in which iron commonly appeara is that of the yellow or brown hydroua ferric oxide. The presence of iron in an unburnt clay is often concealed by organic colouring matter ; but exposure to a moderate heat destroys the organic matter, aud diacovera the pink or red colour of the anhydroua ferric oxide.
Certain impure natural claya may be employed in the manufacture of the coarseet descriptions of pottery, coherency being produoed either by tho plasticity of the clay and the simple removal of mechanically mixed water, aa is the case in oriental bun-baked ware ; or by the removal of both mechanically and chemically mixed water, together with the incipient fuaion of part of the ingredients, provided theee reaulta are attained at a low temperature. Re.sistanee to high tempera tures, regularity of form, impermeability, purity of colour, and translucency can only be gained by the me of mixtures ao oonatituted that the qualities, which are not aupplied by the natural clay, are yielded by materials artificially introduced. A perfect mixture must be sufficiently plastic, when water ia added, to facilite.te tnanipulation ; aufficiently infusible to resiat collapse by fusion, when expeacd to the heat requiaite to produce hardnesa ; aufficiently stable to reaist excesaive shrinkage and distortion ; sufficiently fusible to become impermeable, and, in Borne came, trana lucent ; and eufficiently free from iron when the colour obtained from iron is not wanted, to be colourleaa or almoat colourleaa after burning. A mixture for pottery ia nt the best a well-balanced mechanical arrangement, and cannot be regarded or repreaented as a chemical compound.