DECORATIVE PROOESSES.—Tha decorative affect of pottery is due to form, surface, or colour. Decorative form may be due to the testa and manual skill of the thrower, or, if the ware be moulded, pressed, or cast, it may originate with the art of the designer of the model for the mould. Ware, after receiving its outline from the thrower, and whilst atill retaining plastioity, may have its form compressed, or indented, and its edges waved or crimped by the fingera, and acoording to the fanoy of the artist. Ware may alao be built up of detached pieces, as in the case of fine baaket-work, and of imitation flowers. In the former, threads of clay, expreseed through a stencil fixed in the base of a cylindrical screw-press, are twisted and cemented together, laid upon a plaster block of the ahape which the interior of the basket-ware ia intended to poesesa, and hardened by burniog ; in the latter, each petal is made separately by hand, and cemented together by liquid slip. The mouths of vessels may be artistically shaped by the insertion of " forms " made of burnt stone-ware or china.
The colour in the body of a ware may be natural or artificial, and may be mechanically or chemically combined. It is due to the presence of a metallic oxide, and the colour may either be the aotual tint of the anhydrous oxide, or the tint which a glass assumes when fuaed with the oxide. The red of terra-cotta is caused by the presence in the natural clay of ferric oxide; and the buff and paler tints, by a smaller proportion, amounting to 1-3 per cent., of the same oxide, or by a larger proportion intermixed with lime or magnesia. The tiut of ivory and cream-coloured ware is due to a glass coloured yellow by a minute trace of ferric oxide, and intimately mixed throughout the body. The mixtures for cream-colour and ivory are respectively the same as those used for earthen-ware and china, with the difference that claya are used containing naturally a slightly higher proportion of ferric oxide, and that sand is sometimes substituted for flint.
In order to facilitate the colouring of stona-ware by the presence of artificially e,oloured glass, as for inatance in making a blue ware by the addition of cobaltie oxide, it is necessary to add Cornish stone to the clay, so as to render the body vitreous, and to supply a flux for the colouring oxide. Wedgwood'a fine " jasper" ware is an artificial mixture of the same character, but con
taining a large proportion of baric aulphate. Thia substance serves the double purpose of residing fusion, and of reflecting any colour that may be incorporated in the body of the ware. Au average mixture is, baric sulphate, 57.1 ; baric carbonate, 4.8 ; flint, 9.5 ; clay, 28.6. The ware is unglazed, has a crystalline surface, is vitreous throughout, and may be coloured in the same manner as a glass. The sage-colour is due to chromic and cobaltic oxides ; the drab, to nickel oxide ; and the dove-colour, to cobaltic and manganic oxides. Stone-ware may be tinted by mechanical mixture with certain oxides. Thus ferric oxide gives a red, brown, or chocolate; manganic oxide, a black ; and uranic oxide, a yellovv. Differently coloured clays niay be so kneaded and worked together as to present a good imitation of the grain and colours of marble. The costly Henri-Deux ware may be reproduced by inlaying coloured clays in patterns, damped or engraved in separate bats of plastic clay, adjusting the separate bats in a mould, and uniting thern by a common backing of clay, so that they are made to combine, and assume the form of the mould. The body is generally white ; the inlaid ornament, brown and black ; and the glaze, a warm cream-colour.
A decorative surface may be produced by impression, incision, or application. Impressed decoration may be transferred from the surface of a mnuld, or may be directly produced by a atamp or seal, or by a pattern cut in relief upon the edge of a small revolving wheel ; by the repetition of the impression of a starnp, the effect of a diapered background may be obtained. A moulded pattern may be cut through with a knife, so as to imitate coarse basket-ware ; and the delicate tracery of perforated ware may be produced in the same way, but with finer implements and greater skill. Lattice-work, executed in a similar manner, may be applied over depressions, or upon the surface of vesaela which still retain their plasticity. A coloured clay is sometimes intro duced into moulded depreasions, and the surplus is removed on the lathe.