Iiimar OR OVENS, AND MUFFLES.—The two important processee for which ovens are required are (1) the hardening of ware, and (2) the fixing of glaze upon its surface. Wares, after manipu lation, and a preparatory course of drying, are inserted iu deep fire clay trays (" saggers"), which are piled up in columns on the floor of the oven. After exposure to the heat of the oven, the ware is found to be hard, but, at the same time, porous in greater or less degree, and possessing a surface which is rough and usually absorbent. An absorptive or rough surface is well suited to receive certain forms of decoration, and especially tho glaze which is applied in a liquid state. The arrangement of an oven for firing the glaze is similar to that used for hardening the ware, and differs only in size, the hardening-oven being consider ably larger. The latter is techni cally termed tho " biscuit-oven," and the ware after burning is said to be " biscuit-ware ; " whereas the oven for firing the glaze is generally called the " glost-oven." In the glost-oven, the interstices between the Baggers are luted with plastie clay. The old fashioned biscuit- or gloat-oven oonsiate mainly of a dome, situated within large conical chimney or " hovel." Round tho base of the dome, or oven proper, project 8-12 fire-places, with ash-pits sunk below the level of the ground-line. Tbe fire-places are charged from above, and the opeu jugs for charging eau be closed at will. The flame and brat from each fire-place enter the oven by two flues, the one vertical, the other horizontal ; dampers inserted in the outside walls of the vertical flues regulate the introduction of air. The horizontal flues pass under the floor of the oven, and converge to a vertical common central flue, by which the heat enters the oven. The smoke and products of combustion pass through openings in the upper part of the dome, into the chimney or hovel which surrouude it. In Robey's oven, the flame and heat enter by holizontal flues con verging to a central opening in the floor, but the dome being closed during the process of firing, they aro reflec ted back upon the saggers, and the products of combustion pass away by apertures in the floor between the central opening and the wall of the oven, these apertures communicating with a, circular flue, which discharges itself into an external chimney-shaft.
Figs. 1117 to 1119 illustrate the construction of Minton's patent oven. The figures represent respectively a vertical section of the oven, a horizontal section through the fire-places and the ground horizontal flues, and a section of an upright flue where it meets the trial-holes. The bars a have temporary brick partitions to prevent the fuel falling into the ash-pit e; b are the doors by which the fuel is introduced; e, the fire-places and vertical flues by which the flame is directed towards the upper part of the oven f. The vent m being closed, the heat is flected to the floor g, and the products of combustion, passing through several openings in the floor h, into the horizontal flues i, escape through the vertical flues j, into an upper chamber e, and thence into the atmosphere by the chimney h. The small openings m above the charging-doors act as dampers for regulating the mission of air, necessary to ensure complete combustion.
The upper vent m, as well as the flue communicating with central vent h, is opened when the firing has been completed, to assist in cooling the ware. The upper chamber helps to equalize the draughts of the different fire-places. The advantages claimed for this oven are (1) saving of space by doing away with the external hovel, as well as the projecting fire-places, (2) saving of fuel, (3) complete combustion of the fuel, and consequent prevention of nuisance.
For ware which is hardened and glazed in one firing, the glaze being produced by the combina tion of volatilized salt with the material of the surface of the ware, ovens are constructed with ports which can be opened or closed at will, or the openings in the crown of the oven are utilized as well as the fire-places for the introduction of the salt. Wares glazed by this process are exposed with out saggers to the full heat and flame of the fires. Coal with a small proportion of coke is used as fuel. Figs. 1120 to 1123 represent respectively the exterior, vertical section, and horizontal sections through tho fire-places, and through the crown of the ovon, showing the apertures or ports, of an ordinary np-draught salt-glaze oven. In Figs. 1124 to 1126, arrangements are shown by which an
up-draught oven may be converted into a down-draught oven. The burniog-chamber a is enclosed at the top by the arch b, the apertures in which have been closed by tiles, but may be opened for the introduction of salt ; c are the fire-places, the flame entering the oven indirectly through the small apertures in the vertical flues c. Fire-clay blocks, placed on end on the floor of the oven to support the ware, are se arranged as to leave intervals for the escape of the reflected gases into the underground horizontal flues f, whence they escape through the two horizontal flues g into the flue i, which enters the ehironey above a valve e, so as to allow the space D to be used for a drying-room or for burning terra-cotta.
Figs. 1127, 1128, and 1129 illustrate the application of the principle of Siemens' gas-furnaces to the burning of pottery. Fig. 1127 shows a longitudinal Eection through four connected ovens ; Fig. 1128, a sectional plan of the same ; and Fig. 1129, a transverse section of the fourth oven. The objects represented in the ovens are bricks, but pottery can be burnt with equal facility. The four ovens A' A' A' A' are connected with the gas-generator, the air, and the chimney, in the following manner. The bottom of A' communicates with the top of A2 through the passage a' ; the bottom of A2 with the top of A' by a2; the bottom of A' with the top of A' through the passage a' ; and the bottom of A' with top of Al by passages x and a'. Slides cl—c' for dampers are provided in each of the passages a'—a4, but there is only one damper for all the four passages, and it is consecutively placed in each. An underground flue B' (Fig. 1129) passes from the gas-generator along the front of all four ovens, before each of which it opens into the vertical shafts Dl—D4, which are closed at the top, and from which, branch the pipes El—E'. These pipes enter the centre of the side of each oven, and branches c'—e' on the same also enter into the passages already mentioned. All these branehes are provided with valves, actuated by the levers b1-64, by means of which, the gas can either be admitted through the passages al—a4 into the top of the oven, or through the branches ELF" into the middle of the ovens, or the gas can be shut off altogether. When the ovens are filled with ware to be burnt, tubes Fl—F4, with perforations, are placed in them, in such a position that the gas, when entering the ovens through the pipes El—E4, passes into these tubes, and is thus more equally distributed. Each of the ovens is also provided with an aperture Gl—G4 at the bottom, connecting it, by means of a branch, with the flue 11 conducting to the chimney-shaft ; each of the openings can be closed by dampers g'—g'. A large opening I'—I' is provided at the top of each oven, for the purpose of filling and emptying it; eaeh of these is temporarily bricked up when the oven is at work, but the one being open where the oven is being charged, admits air to assist combustion of gas in the oven that is being fired. If ware in Al is being fired, and A' has been recently filled, and is being heated pre paratory to firing, and A' is being emptied and filled afresh, and A' has been fired and is cooling, then the action throughout is as follows. All the passages from the gas-generator to the oveus are closed, except el ; the damper will be placed in passage a', and all communications with the flue H leading to the chimney will be closed except G2 ; gas enters at the top of A' through a' at tbe same time air enters through the open oven Aa, passes through A", and becomes considerably heated ; then passing through the passage X to the oven Al, it mixes with the gas, and causes com bustion. The products of combustion pass through al to the top of oven A2, thence through opening G into flue H conducting to the chimney. Each oven is charged, fired, and cooled in rotation. To facilitate the circulation of the heat, the wares or Baggers are placed upon fire-clay blocks f, separated by suitable spaces.