This is carried out in kilns identical with those used for firing bricks, and mainly in continuous kilns, in the case of mechanical tiles. As tiles are of a more fragile nature, they must not come into direct contact with the fuel. A tile-factory nearly always produces bricks also, and in this case they are arranged round the tiles, which are thus protected as if by a sagger. To do this, however, the production of bricks must be one-third or one-half of the products made. When it is less than that proportion, we must try to find a kiln which, while ensuring the continuity of the fire, keeps the tiles out of contact with the fuel. The Virollet continuous kiln, called also "four a tranches," is one of those most commonly used.
Like other continuous kilns, it consists of a certain number of compartments which are reached by doors, but which are separated from one another by walls pierced below with holes (Fig. 412). On the floor of the kiln, a certain number of firebars 4 to 6 inches broad are arranged transversely. Under these bars, which occupy the whole width of the kiln, is an empty space into which the outer air is introduced through a conduit in the thickness of the wall and issuing at the level of the pavement. The orifices of these conduits arc closed with sand plugs.
The fuel, being thrown through small openings in the roof, falls on the bars and burns there. To preserve the tiles from contact with it, several layers of bricks are placed at the bottom of the kiln, and receive the heat direct from the coal. These highly fired bricks are always of inferior quality and represent a certain loss.
The object of the walls which separate the compartments is to isolate them when the firing is completed, and all that need be "four a tranches" for tiles done is to close the openings in the walls with a layer of sand or a metallic register.
Fig. 411. Longitudinal Section.
Fig. 412. Transverse Section.
Fig. 413. Plan.
As the air necessary for combustion is introduced into each compartment, and not behind the fire, the fired and isolated compartment may be allowed to cool more or less quickly, and is thus similar to an intermittent kiln.
The kindling and management of the fire differ very little from those of the Hoffmann kiln already described.
From the point of view of economy, this kiln does not present the same advantages as the Hoffmann kiln, for the outer air is not heated before it reaches the fuel, nor can we use slack, which would burn badly; but it has the advantage of a more rapid cooling when the products can bear it, and so allows of a diminution in the number of compartments. Above all, it avoids contact between the tiles and the coal.
This latter advantage is found in gas kilns, which, moreover, require no bricks for stacking, as thelites are placed on the floor of the kiln between the refractory clay "chandelles." There is no fear of the ash stains which are produced under a high draught when solid fuel is used; and finally, the degree of firing which can be reached is much higher than in the Virollet kiln.
All these advantages should recommend gas firing to tile manufacturers, and in new installations it is undoubtedly indicated.
The method to be adopted depends upon the shape of the tile, and on the kiln at disposal. The important thing is to avoid warping during the firing by pressing them closely together. Fitting tiles are arranged in pairs and one close against the other, so as to take up less space and preserve the projecting parts.
Round tiles stand upright one against the other. But often several kinds of tile are fired together, and then the method of stacking depends on the special conditions.
If the firing takes place at a temperature near that of softening of the paste, special precautions must be taken to avoid loss of shape; for instance, we may stack in batches, using bricks to construct the divisions.