The drying and tempering of all classes of refractory mater ials is an important part of the manufacture—the more care fully the drying of the clay-wares is conducted the more satisfactory and valuable will be the product. A poor article well tempered is often more satisfactory than a better grade of ware would be if too hastily dried. If there be no overheating of the brick on the dry-floor, there will result no cracking of them.
There should be no unnecessary handling of the green brick, and every precaution should be taken to avoid the chipping and other injuries which the brick have inflicted upon them before they are fired. The best way to obtain a high grade of fire-brick is to put them on a mildly warm floor and allow them to stiffen gradually through and through, so as to be in good condition for pressing the morning following the day during which they were moulded. After being pressed the brick can then be placed on a hot floor and dried, care being exercised to let none but thoroughly dry brick go to the kiln. These precautions insure the production of sound and well-finished brick. In some fire-brick works the brick are taken from the moulder's bench and placed upon a floor which is much too hot, and from which in a few hours they are picked up and pressed and put down again to finish drying. The conse quence of this method is that the brick are pressed Sefore the middle of the brick is sufficiently dry—the outside and edges are hard enough, no doubt, for pressing, but the centres of the brick are much too soft for the application of pressure. Good fire-brick cannot be made after this mode. The custom is to keep the floors hot constantly ; the mass of the body heated makes this easy to be done. The fuel used is coal slack in almost all cases, as its combustion is gradual, and after the floor is once hot, gradual heat is the kind wanted. Brick placed on such a floor dry in twenty-four hours from the tempered, plastic clay to a state so hard that the hand can make no impression on them. Lack of drying-floor constitutes one of the greatest obstacles to an increase of capacity of a factory. Air-drying is usually done in the second story over the ordinary drying floor. If the roof be tight the heat in the second story is quite uniform, and is strong enough to do quite rapid work. The temperature is often too° or naturally, and by using a slat-work floor the capacity is largely increased. The kinds of ware adapted to air-drying are large pieces which the heat of a floor can only attack on one side at a time, which is always done at a risk of cracking.
In the manufacture of fire-brick on a large scale by the soft mud process, when many thousand brick are moulded in a day, it is, of course, necessary to have some expeditious method of drying, and the rule in such works is to have the brick dry and in the kiln within twenty-four hours of the time when they were moulded. Although 'any suggestion that a longer time than
this should be allowed the brick is scarcely practical, yet the fact cannot be controverted that the refractory qualities of all classes of fire-clay wares are much improved by slow drying and long tempering in the dry-house. The various tunnel. dryers now on the market are well adapted to the drying of fire-brick in large quantities. After the brick have been made and dried they are wheeled into the kiln and set for burning.
Burning. The manner of setting the brick in the kilns de pends upon so many circumstances that no general rule can be made part of the work. In kilns which are gas-fired the manner of setting is different from the way in which the brick are set for burning in coal-fired kilns. Special instruc tions on this point are always furnished by the builders of gas fired kilns.
On the subject of setting and burning fire-brick and the construction and operation of fire-brick kilns a large volume could be written.
The Harbison & Walker Co., of Pittsburgh, Pa., use the Dunnachie gas-fired kiln, for which Charles T. Davis, of Wash ington, D. C., is the agent in the United States. For a descrip tion of the kiln see Chapter IX.
The most appropriate form of kiln suited to individual re quirements must be determined by local circumstances, cost of fuel, labor, etc. Gas-fired kilns, however, are more economical to operate in both fuel and labor cost than are coal-fired kilns, and on these accounts are being adopted by the leading fire brick manufacturers of this country and Europe.
Fire-brick require to be thoroughly burned, and if they pos sess any refractory qualities, a high temperature is necessary to accomplish this, in order to bring them to the point where the shrinkage or contraction shall have ceased. When taken out of the kilns, if not shipped away immediately, the brick should be stored in a good water-tight stock-shed, as brick and other refractory products should, if it is possible to avoid it, never be allowed to get wet.
In order to acquire and hold a desirable trade-demand for refractory materials, only first-class wares should be placed upon the market. In order to do this it is necessary to place in charge of fire-brick works only experienced and careful fire brick makers—men who will give their personal attention to all the details in the different processes of mixing the clays and the moulding and burning of the brick. The fire-brick having been made and burned, it is the duty of the manufacturer not to allow the brick to become exposed to the weather, and shipped away full of moisture, as it is expected that all fire brick shall go to the furnaces for use, free from moisture.