The value of the separate materiala depends as much upon phyaical aggregation as upon chemical composition, and their qualities must be determined both by analysis and hy direct experiment. Analyaes and experimenta muat be conatantly repeated, inaamuch as the inateriala conaist principally of natural products, and not of artificially produced chemicala. Aa the mixtures for different wares must vary according as the phyaical or chemical naturea of the raw materials vary, recipea and even analysea of wares are of little practical uae. For white or light coloured goods, as well aa for those intended to withatand high temperatures, pure claya are med. The diatortion and fracture, due to excessive or irregular shrinkage, to which warea made from pure elaya are especially liablo, may be considerably reduced by the introduction of an infuaible anhydrous eubstanoe, in such proportion as not to interfere tnaterially ith the plastieity of the clay. The substances employed are, for common ware, sand or a proportion of a grittier and leas fusible clay ; for fire-ware, graphite or burnt fire-clay ; for domestic and sanitary ware, calcined flint ; and for ornamental ware, bario sulphate or calcie phosphate. If a mixhire be manipulated in a atate of liquidity, and the reaultant ware be brought to a vitreoua condition by heat, the total ahrinkage may amount to aa much as SO per cent. If wares are required to be impermeable or tranalucent, the infusibility of a pure clay, and the increase of infuaibility caueed by the introduction of an infusible foreign aubstance, muat be compensated by the addition of a proportion of a glasa-forming material of felapathio nature. By varying the proportion of this ingredient, warea may be obtained iu every stage between poroaity and translucency, and proportionately differing from or resembling glees in their physical properties.
The nature of waree depends in a great measure upon the temperature to which they are expoaed. With a gradually inoreaaing temperature, the same mixture may auccesaively aesume the texture and character of aun-baked ware, terra-cotta, etone-ware, porc.elain, and glass. Intenae and prolonged heat will convert the external crust of a Stourbridge-clay (see Clay) crucible into translucent poreelain. In artificial mixtures, the proportion of glass-forming ingredients is purpoaely kept so low that the aurface of the ware, even after expoaure to the full heat of the kiln, remains rough and absorbent. For moat decorative, domestic, and aanitary purposes, it ia necesaary to cover thie surface with a smooth, non-absorbent film ; thia result is practically gained by covering the surface of the ware with an extremely thin layer of glass. For cm tain common waree, whoae composition rendera them unfitted to reaist a high temperature, and which would otherwise remain porous and incoherent, a film of glaaa serves the double purpose of a glaze and a bond.
Warea are coloured by metallio oxides. The common red, brown, sod yellow tints are due to
ferric oxide, whether naturally present or artificially introduced. The colours which may be obtained by the use of iron compounds depend upon the temperature to which the wares are exposed, the atmosphere in which they are burnt, and tbe constituents of the ware. If the temperature be low, and free access of air be permitted, the ware is tinted by the natural colour of tbe anhydrous ferric oxide ; if, however, there be present an excess of calcic or magnesic oxides, the tints are greatly modified. If organic matter be present in large quantity, or if the ware be exposed to a strongly reducing atmosphere, the ware may be tinted black or grey, owing to the conversion of the ferric into magnetic oxide. At a high temperature, in an oxidizing atmosphere, and in the presence of glass-farming materials, the substance of the ware will be tinted yellow by the colour of the glass with which the ferric oxide has combined. If, however, the ferric oxide be reduced to the ferrous condition, the ware will be tinted green. All the metallic oxides which are used for colouring glass (see Glass, pp. 1083-4) may be used for pottery. If the temperature be low, or if no glass be formed in the ware, it will be coloured by the natural colour of the anhydrous oxide ; if, on the other hand, the temperature be high, and if a glass be formed, the ware will receive the same colour as a glass would receive under similar circumstances. Advantage is taken of this fact to neutralize, by the addition of a minute trace of cobaltic oxide, the tints produced by ferric or ferrous oxides. The infusible substances introduced into mixtures in order to reduce shrinkage, being generally of a white colour, heighten the whiteness of the wares produced.
For all manipulative processes, it is necessary to reduce the raw materials to a fine state of sub division, either by grinding, or by diffusion in water. Wares whose different ingredients have been reduced to different degrees of fineness, even though the difference be imperceptible, are rendered more solid, and are better fitted to resist the temperature of the kiln, as well as the changes of temperature to which they may be afterwards exposed. Wares are produced from suitable mixtures (1) in the state of dry or slightly moistened powder, (2) iu a plastic condition, (3) in a liquid state. In all processes of manipulation, care must be taken that the body of the ware be homogeneous throughout. Inequality of pressure, by disturbing the homogeneity of the substance, is a fruitful source of disfigurement and fracture. Although no trace of irregularity may be apparent in the unburnt ware, it will, if present, be discovered by the ordeal of fire. After manipulation, wares are gradually dried, aud solidified by exposure to heat in suitably-constructed kilns. Glazing and the different processes of decoration generally require one or more additional firings.