Continuous firing may be attained by causing the base of an oven to pass through fixed zones, in which the ware on the base of the oven is successively warmed, fired, and cooled. For this purpose, the oven is annular in form, and is heated by gas on the regenerative principle. Fig. 1130 is a vertical section, and Fig. 1131 a plan, of the annular oven. In Fig. 1130, T2 is the annular floor or table, having a refractory facing, and being mounted on wheels t, which run on circular rails. On the under side of the table T, is a rack, gearing with a wheel on a shaft W, by turning which, the table is caused to revolve in the direction of the arrow (Fig. 1131). The wheels and axles of the table are protected from excessive heat by flange,s attached to the sides of the table, and dipping into troughs of sand e. The annular chamber in which the table revolves ia divided into four zones, namely, one for introdueing or removing the ware, one for warming, one for burning, and one for cooling the ware before removal. The warming and cooling zones are interchangeable by means of two sets of apertures p, through either of which the gas and air ean be introduced at will. If the gas and air enter by the apertures on the right, the flamo sweeps through the heating zone X, and the products of combustion escape through the apertures on the left, and heat tho regenerator with which they are connected, When the left regenerator is heated, and the right regenerator connected with the apertures on the right is cooled, the gas is introduced by the apertures on the left, and the products of combustion pass into the right regenerator. Whilst the flame passes from right to left in the zone X, the adjacent zone is heated by opening the flue Z, and drawing part of the flame in that direction, the adjacent zone being in this way converted. into a warming zone ; whereas when the flame pasEes in an opposite direc tion, and the flue Y is opened, the zone about Y is converted into the warming zone, and the zone about Z into the cooling zone. The warming and cooling zones are separated from the cold zone 0 by means of sliding valves M. Ware to be burnt is placed upon the part of the table 0, the valves M are raised, and the table conveys the ware away from O. As the table is caused gradually to revolve, fresh wares are introduced, and burnt wares are removed at O. At each movement of the table, both the valves M require to be raised.
Closed Kilns or Dfujles.—Biscuit-ware is very commonly printed with a colour mixed with a medium of a dense oily nature. This oil must he removed from the ware before the liquid glaze can be evenly spread upon its surface. The removal is effected by placing the ware in a closed chamber or muffle, beneath and around which, heat is directed by flues suitably disposed. This process is known technically as " hardening on," and the kiln used for this purpose as a " hardening-on kiln." A smaller muffle and kiln constructed on the same principle is used for fixing enamel, painting, gilding, silvering, and other forms of applied decoration to the surface of a glazed ware. The wares in either case are placed in the muffles without Baggers or other protecticrn, the fronts of the muffles, which are reniovable, are replaced, and all openings are luted with fire-clay, except such as are necessary for observing the course of the fire.
The manager of the Royal Worcester China-works has patented an arrangement for heating a small muffle by means of the ordinary town gas supply. A series of tubes c (Fig. 1132) is connected with the main, and terminates in groups of burners ; tho heat from each group is received into a separate compartment e, formed under the bed of the muffle b, a being the outer wall of the kiln. The heat from the separate compartments e is
conducted by separate flues, passing in various directions about the muffle, into a common chimney above.
Fig. 1133 shows two or more kilrs containing muffles connected with a central chimney-stalk : A is a muffle built of overlapped tiles ; B, the kiln in which the muffle rests, showing the ports for directing the flame and heat from the fire-place beneath to play upon various parts of the muffle ; 0 D, auxiliary flues ; E, the main flue passing into the central shaft H ; F Y, dampers ; I, the entrance to the shaft for the stokers, as the fire-places are charged from within the shaft.
Although it is impossible to divide pottery into accurately distinct species, it will he convenient to classify the different kinds of ware in the following manner :— I. Wares rendered coherent by the removal of the water mechanically combined—Sun-bakcd wares.
II. Wares bardened and rendered anhydrous by artificial heat, but the porosity of which is unaffected, owing to the infusibility of the ingredieuts—Crucibles, Saggers.
III. Wares fired at a comparatively low temperature, and porous in texture—Bricks, Majolica, Teri a-cotta, Drain-pipes.
IV. Wares fired at a high temperature, and dense in texture, but perfectly opaque—Stone-ware, Earthen ware.
V. Wares rendered translucent by the fusion of an incorporated felspathic glass—English china, Parian ware, true Porcelain.
selecting clays for the manufacture of tire-ware, particular attention must be paid both to their chemical nature and to their physical aggregation. Fire-wares are required mainly for three purposes :—(I) To withstand great alternations of temperature, as in the eases of Baggers ; (2) to resist intenseheat, without shrinkage or fusion, and, at the same time, to retain heat with as little loss as possible, as in the case of furuace-bricks ; (3) to resist an intense external heat, 1133.
accompanied by the interna4 corrosion of metals or other substances in a state of fusion, as in the eaee of crucibles. The refractoriness of a fire-clay may be estimated by the result of an analysis. If the proportion of foreign matter, that is to say of the alkaline, calcic, magnesic, and ferric oxides, exceed 4i per cent., the clay is unsuitable for furnace-bricks or crucibles. No faith must be placed in the colour of a flreaday, as the appearance ef whiteness may be due to an excess of calcie oxide. The relative shrinkage of a sample of fire-olay, whether by withdrawal of moisture or by fusion, is a point demanding partioular oonsideration in the selection of clay for the manufacture of furnace bricks and oruoiblee ; it is beet determined by making a brick of the clay under examination, breaking it in half, and burning one half whilst retaining the other for comparison. If the burnt and unburnt halves fit together exactly, the eample may be pronounced satisfactory. The liability to corrosion is best determined by making small experimental crucibles of the clay, and fusing in them such substances as borax and plumbic' oxide. If the corrosion in a given time be exceseive, the ola,y must be condemned as unfit for metallurgical purposes. The importance of physical aggregation reets upon the fact that a coaree, porous, brittle ware withstands changes of tempera ture better than a dense one ; whereas a emooth, dense ware is better fitted to withstand corrosion, and to retain heat. A clay is rendered naturally porous by the presence of an excess of sand ; the same result, however, may be attained or increased artificially by coarse grinding, and by the addition of the coaree powder of burnt, broken, fire-clay ware, or of a foreign refractory or infusible substance, euch as graphite.