Underglaze coloured decoration may be placed upon biscuit ware by the brush, or by a process of transfer printing. Only those oxides can be used for producing colour which are sufficiently stable to resist the heat of the glaze-kiln, namely those of chromium and cobalt. The pattern to be transferred ifil etched upon copper plates with steel gravers. When in use, the plates are kept warm by placing them on the top of a covered stove heated by steam, gas, or coal. The printing medium containing the colouring oxide is made up of a mixture of thickened linseed-oil, rosin, tar, and other ingredients, and is kept in a semi-liquid glutinous condition by exposure to heat. The ink is applied to the plate, and all the superfluity is dexterously removed by a scraper. A sheet of unsized linen tissue-paper, saturated with soft soap and well damped, is next spread upon the plate, and passed through the printing-machine.
The printing-machine, Fig. 1161, consists of an iron framework supporting two rollers, the upper one of which is partly wrapped with thick flannel. Between the two rollers, is a planed iron table, upon which, the copper plate is placed. By depressing the handle, the upper roller revolves, and causes the table to carry the plate between the two rollers. The resultant pressure transfers the ink from the copper plate to the paper. The printed paper is removed from the plate, and the margins are cut away. The paper pattern is applied to the absorptive surfac.e of the biscuit ware, and is rubbed over with a roll of flannel.
The ware is then placed in water, the paper is removed, and the pattern is found to be accurately printed. The printed ware is now dried, placed upon shelves in a kiln, and exposed to a red beat, in order to burn off the oily ingredients with which the metallic oxide was mixed. If the oily matter he not iemoved, the parts of the ware so covered will be non-absorptive, and unable to receive the glaze. After the ware has been gradually cooled, it is ready for dipping in the liquid glaze. By this process, outlines to be filled in by hand-painting, or patterns in one tint, can be printed. In order to print different shades of colour, a process known as " block-printing" must be adopted. In printing a leaf by this process, the different cohrurs of the shadow, fibre, and ground are successively applied by separate plates upon the same paper, but care is taken that no two colours shall overlap. The pattern is transfemd to the ware in the same manner as already described. The highest form of coloured decoration upon the glaze is hand-painting. Transparent coloured glasses and opaque enamels are used by the artist in the same manner as ordinary pigments, with the exception that all have to be fired by heat, and allowance must be tnade for the changes in tint which the flring may produce. So great skill has lately been devoted to the preparation of these colours, that almost every known tint may be aatisfactorily representecL Raised effects and ornaments in relief may be executed in colour by modelling with a clay paste upon the surface of the ware, exposing to heat, and then painting with coloured enamels. Photography has not as yet been employed directly as a decorative process. There are two
mechanical processes, in addition to ordinary printing, for producing coloured decoration upon the glaze, namely " ground laying" and "bat printing." The first is employed when an even coloured surface is reqitired. The parts to remain white are painted by band with a mixture of potassie carbonate and some adhesive vehicle, and the whole is then coated with oil. The coloured glo.ss, or mixture of metallic oxide and flux in an exceedingly fine powder, is dusted over the whole surface, and adlierei to the oil. The ware is then dipped into water, and the alkaline stencil reacts with the oil immediately in contact with it, causes it to saponify and peel off, carrying. the colour with it, and leaving the space white. The ware is then dried and fired. " Bat printing" is used when exeeedingly sharp outlines are required, as for instance for erests and monograms. The pattern is very finely etched on a copper plate, to which oil is applied, but almost entirely removed by friction from the operator's palm. Films of gelatine or glue are applied to the copper plate, and absorb the residue of the oil remaining in the lines of the engraving. The gelatine is applied to the ware, and transfers the pattern in oil to its surface. Powdered enamel is dusted on, and adheres to the lines printed in oil by the gelatine film. Printing may bo executed on the glaze in the same manner as OD biscuit, by incorporating a flux with the ink, aud moistening the paper with essence of turpentine before its removal ; or by removing the paper by heat, and dusting an °Immo] colour upon the adhesive outline which rernains. In the latter ease, the ink contains no colouring oxide, but is only an adhesive mixture. The construction of small mills, for the reduc tion of gold and other colouring ingredients to the state of fineness required for painting or printing, is of considerable importance. The following are descriptions of two which are considered serviceable. In one, a number of glass " mullers " fixed in a frame, to whielt o, horizontal eccentrio motion is communicated, press by means of springs upon a slowly revolving glass table. In the second, a single oval glass muller moves over a 2-ft. ground-glass slab, which is eaused to revolve in an opposite direction to the muller. The motion of both is so arranged that the muller ahall suceessively pass over the entire surface of slab. The muller is grooved on its base, to prevent suction, and carries a scraper, which directa the substance to be ground between the grinding surfitees.
The author desires to express his acknowledgments for special essistanee rendered by the following well-known practical authoritiea :—T. Minton, C. Wedgwood, R. W. Binns, E. Stiff, Copeland, L. Arnoux, Abrahams, Brown, Westhead & Moore, Boulton, and A. Pete.
Bi/diegraphy.—A. Beckwith, 'Pottery ' (New York : 1872); P. Bonneville and A. & L. Jaunez, 'Les Arts et les l'roduits Cdramiques ' (Paris : 1873); 'Modern Pottery Machinery ' (London : 1876); Artizans' Reports on the Paris Exhibition ' (London : 1878). H. J. P.
(See Clay; Glass).