After manipulation, the wares are gradually dried, preparatory to burning. The ovens em ployed at Sevres are each divided into two kilns, au upper kiln for burning biscuit ware, and 14 lower kiln for burning the ware which has been dipped in glaze. The biscuit- and glaze-kilns are separated by a perforated floor, through which, the surplus beat from the glaze-kiln passes into the kiln above. The heat and draught of the kiln aro very intense there is little smoke, and the ash is entirely dissipated. The fuel principally used is wood, and every precaution is taken to prevent dust.
Tho wares, whether for the biscuit- or glaze-kiln, are placed in saggers, and rest within the Baggers upon flat ground slabs, mado of a refractory ()lay dusted over with flint or fire-clay slip, and bedded evenly in sand. The saggers are built up by plumb line, with every precaution to secure the wares resting perpendicularly. The firing is regulated by the removal at stated intervals of small test-pieces of both biscuit aud glazed ware, of the same composition as that used throughout the kiln, which have been previously placed in saggers from which they eau be readily withdrawn from the outside. There are three stages in the management of the kiln : the gradual heating for tho removal of moisture held by the body or glaze ; the actual baking of the body, and fusion of the glaze; and the annealing of the glass in the body of the ware, as well as in the glaze. The firing may last 30-45 hours, and the kilns may remain closed after the firing, to allow of the gradual cooling of the ware, for a period of 5-8 days. The entire shrinkage of hard porcelain amounts to about 17 per cent.
Glaze.—The material used for the glaze is a natural mixture of felspar and quartz, and is known as pegmatite. Its average composition is silica, 74.3 ; alumina, 18.3 ; potassie oxide, 6.5 ; °idol° oxide, 0.4 ; magnesie oxide, 0.2 ; water, 0.3 ; and it may be approximately represented by the formula 2(Al208,3Si02)+K20,3Si02. It is therefore an ordinary glass, to which d second equivalent of aluminie silicate has been added, and the transparency of which is destroyed by the excess of infusible material.
Each fresh supply of pegmatite is tested in order to ensure a constant result. For use, the pegmatite is first crushed under vertical grinding-wheels turning upon a revolving base. It is then ground with water iu a mill with stone runners, and when reduced to a sufficient degree of fineness, is drawn oti, sifted, agitated in the presence of magnets, in order to remove particles of iron, passed into a receptacle, and maintained in suspension by constant agitation.
During the long process of grinding with water, great care must be taken to prevent a sudden precipitation of the material, either through the slackening or sudden stoppage of the stones. The tendency to precipitation may be retarded by mixing a small quantity of acetic acid with the water. Tho various mills at the Sevres works are moved by wider-power. Into the suspended pegmatite,
the biscuit ware is dipped, care being taken that no part of one piece remains in the glaze longer than another, and that the thick wares shall be dipped in a thin glaze, and the thin in a thick. The parts of ware which have been held in the dipper's hand are retouched with a brush dipped in the glaze. The wares are replaced in saggers, and the saggers are placed in the lower division of the oven, the heat of which is more intense than in the biscuit-kiln. The entire absence of lead renders the glaze when fused exceedingly hard and durable; it is bluish in tint, and cold to handle. The grey tint of the body and glaze is due to the reducing action of the atmosphere of the kiln. The glaze is transparent, and rather more fusible than the body, but becomes thoroughly incorporated with it, and, from its similarity of composition, expands and contracts uniformly with the paste. The bases of ware when removed from the saggers are rubbed smooth with sandstone. .0vving to the difficulty of manipulating the paste, it is customary to build up elaborate vases from distinct pieces, which are joined together by metallic fittings; this especially applies to feet and handles.
Part= and Belleek.—The nearest British representatives of true porcelain are Parian and Belleek. The raaterials generally used for Parian are kaolin, felspar, and small quantities of Cornish stone and ball-clay : the analysis of an average sample of Parian gives—silica, 54.8 ; alumina, 36.21 ; alkalies, iron, and other ingredients, 8.8. In some cases, the composition and preparation of Parian approaches closely to that formerly employed at Sevres in the production of the pcite tendre. A glass is first formed by the fusion of a mixture of sand, felspar, Cornish stone, and potassic car bonate ; this is run into water whilst still hot, broken up by the action of the water, and ground with water in a mill with stone runners. One part of the glass is mixed with about three parts of ground felspar and three parts of kaolin. A hard Parian may be made from a mixture composed of 60 parts felspar, 30 kaolin or china-clay, and 10 ball-clay. The Parian mixture is used in the liquid state, and the ware is fashioned by the absorption of the water from the mixture, and the consequent deposition of the paste upon the inner surface of dry plaster of Paris moulds. Vases and statuettes of the greatest delicacy may be produced by these means. The contraction of Parian in the process of solidification by heat is greater than in any other ware, and amounts to -1--1 of the entire mass. It is worked in a state of perfect liquidity, and is rendered quite vitreous by fusion. The shrinkage is greater in the height of the ware than in the width, owing to the influence of gravitation. In making models for the moulds in which the wares are cast, provision must he made to counteract the inequality of contraction, and to bring the contraction to a common centre.