The stacking is done as in other continuous kilns, but heating wells are not used. When we come to a row of holes, we place the "chandelles" underneath, the joints arc luted with clay, and the stacking is continued, the space between two "chandelles" being left free, so that there is throughout the section of the kiln an empty space of the thickness of the "chandelles"; it is in this space that the combustion of the gas will take place.
The igniting of the kiln offers no special feature, except that the gas generator should work when a sufficient quantity of brick is raised to a red-heat capable of setting fire to the gas. The distribution of the latter is effected by the sheet-iron conduits of which we have spoken, and which are placed over the kiln. The number of rows of "chandelles" lighted, depends upon the degree of firing and the nature of the products to be baked; this number generally varies from two to six. The supply of gas is regulated by stop-cocks placed in the movable conduit above the "chandelles." It is better not to introduce the gas into the compartments until the bricks contained in them have almost reached the proper .degree of firing. In this way the gas which is not burnt in the first layer will be burnt in the others, and no useful part of the fuel will escape into the chimney. The progress and height of the fire in the kiln is observed through little openings in the roof.
Gas Kiln With Generator. (Scale of 5 millimetres to the metre).
Fig.228. Horizontal Section.
Fig. 229. Horizontal Section of the Kiln, and View of a Generator from Above.
The action of a gas kiln is the same as that of one with solid fuel. The air enters through the doors of the cooling compartments, passes over the fired products, drawing heat from them, and comes in a very hot state into contact with the "chandelles" which are emitting gas. The latter burns with a brilliant flame behind the fire where the air is in excess, but the flames are duller and longer in front where the air contains less oxygen. The firing thus takes place in an oxidising atmosphere; moreover, this is the method to be preferred in most cases; but if it is necessary, the atmosphere of the kiln may in a few moments be made a reducing one by diminishing the draught and increasing the volume of gas introduced. In order to avoid the loss of the excess of gas, which would escape by the chimney, this excess is burnt by introducing a little air in front of the fire. There is no disadvantage in the fact that the atmosphere at that point is an oxidising one, as the "enfumage" of all pottery takes place in the midst of excess of air.
The gas being easily distributed, it may be seen that it is possible to stimulate the fire at a given' point by opening wide the stop-cocks regulating the entrance of gas into the "chandelles"; in the same way it may be moderated by lowering these valves and even extinguishing one or two "chandelles" by closing them entirely.
The temperature which can be attained in a gas kiln is very high, and stoneware pastes fire well in them; these pastes may even be melted by stimulating the fire to its greatest power, which, however, can hardly be considered as a normal state.