The first thing to be considered when a kiln is built is the stacking of the bricks. Here once more the nature of the ground has to be considered. According to the greater or less difficulty of firing of the products, different methods will be employed. First the foot of the kiln has to be made. This foot consists of longitudinal channels arranged along the floor of the kiln to permit a draught to pass. The width of these channels is small enough to allow of their being covered by a brick placed lengthways; their height varies from 3 layers to 8, that is to say, from a foot to 2 1/2 feet. If the kiln is small, and the goods easy of baking with a fairly moderate fire, we can adopt the arrangement in Fig. 220, which fails, however, if the products are difficult to bake on account of the waste it causes. In fact the foot supports the whole weight of the bricks stacked; if firing takes place at a high temperature, the contraction causes movements which make the whole mass unsteady, and all the bricks at the foot are deformed by crushing, while those of the fourth layer are opened in the middle by the weight they support. Under these conditions we must reduce the channels to the number absolutely necessary, say three, and they are arranged below the heating wells, as shown in Fig. 221.
The foot thus formed is of great resistance and does not give deformed products, but the bricks above the channel are always a little curved or opened in the middle. In order to avoid this inconvenience, which, we repeat, is only excessive in the case of bricks requiring a high temperature for their firing, the width of the channel is diminished and its height increased (Fig. 222).
The foot of the kiln having been formed, the stacking is continued, and then arises the question of heating wells. The heating wells are hollows left between the bricks, below the holes in the arched roof, to receive the burning fuel. Their importance will be understood; with good heating wells a regular and economical firing is obtained; it becomes, on the contrary, bad and troublesome with badly arranged wells. Every brick - maker is said to have his own system of wells. We will mention a few. One (Fig. 220) is to be used with clays which fire easily; another is applicable to clays which bake with difficulty. The first requires open stacking, that is to say, the bricks are separated from one another by a distance equal to the thickness of a finger, the rows crossing at right or acute angles. The well is formed of interspaced bricks, placed one upon the other, and forming "chicane" (these bricks are distinguished in the figure by hatchings). The coal, falling from the hole in the arch, is scattered among these "chicanes"; a little remains on each brick and burns there, the bricks being at a sufficiently high temperature to cause its ignition.
A well-made well should retain the coal at different points of its height without letting a single piece fall on the floor of the kiln. These wells work satisfactorily provided the intervals between the bricks be neither too large nor too small. If too large, they allow the coal to fall into the channel at the bottom, where it accumulates and burns the foot of the kiln without baking the top; if too small, they are liable to be stopped up by an accumulation of fuel, and the effect is reversed : the top fires too much, while the bottom does not fire at all.
Finally, this kind of well does not give a sufficiently regular baking to products which are difficult of firing. We have found the kind of well represented on the right of Fig. 213 answer admirably.
The stacking is solid, that is to say, the bricks are close together and placed in the direction of the length of the kiln. When we come below a row of holes, a hollow is left equal to half the length of a brick. This interval exists throughout the section of the kiln and constitutes the well, which is thus formed of the hollow between two "feuilles." It is evident that under these conditions the coal, finding nothing to restrain it, would fall on to the floor of the kiln. Therefore bricks are placed "en chicane" in this hollow in the following way: the first "feuille," which forms one of the walls of the well, is stacked perpendicularly without any projections; but in stacking the second, bricks are pushed forward here and there which approach the opposite "feuille " and interrupt the continuity. These projecting bricks will retain the coal. Generally they are placed in every other row of the height and at every third brick of the breadth, care being taken that the bricks so projecting are opposite to those of the lower row. The coal, meeting all these obstacles, is uniformly distributed throughout the mass of bricks, and produces a regular heat. In this solid stacking the draught of the channels at the foot would not suffice, and we must make a communication between the different wells so that the air may reach all the places where there is fuel, and cause it to burn. This is done by leaving in each "feuille," at every two or three layers, and at every other brick, small hollows in width about half or three-quarters the thickness of a brick. This hollow is covered over in the following row. As its width is small, the brick or bricks which cover it are not sufficient, especially as they are placed lengthwise. These little channels stretch in a straight line from one well to another, but of course it is not necessary that they should be in a line from one end of the kiln to the other; it is enough if they ensure connection between two neighbouring wells.
For this solid stacking, which gives fine products but which has the disadvantage of making the progress of the fire slower, open stacking is often substituted; the bricks are separated by a small interval, each layer or clamp crossing at a right or acute angle. The wells arc made in the same way, and across the whole width of the kiln. It is not every clay which will bear this method of stacking; there are some which show the mark of the space separating two bricks. However small this space may be it is said that the bricks are cut.
One important observation must be noted in building heating wells; this is that the fire always draws the bricks forward to about half the height, that is to say, that a "feuille" stacked perfectly vertical is found inclined when remove: as the upper part overhangs, it will even be necessary to take precautions in removing it. This movement always takes place in the direction of the draught. This being so, it will be understood that it is better to throw the axis of the well a little behind the axis of the heating holes, since the wells will come forward during firing. It even happens sometimes that, if in stacking, its axis is placed too far in front of that of the holes, when one comes to throw in the fuel the well is invisible, it has passed the holes. This accident is easily avoided: all that need be done is to throw through the fuel holes, into the interval between the "feuilles," a few pieces of brick, which will act as wedges and hold the "feuilles" in their places.
Fig. 222. Various Kiln-Feet.