One of the simplest kilns is represented in Figs. 202 and 203. In order to economise masonry, and also to resist the pressure of the fire which is particularly noticeable in the lower part of kilns, they are partly made of clay. The foot of the kiln or gridiron, which is pierced with a large number of square holes, rests on Gothic arches, called "arches," three in number. They arc formed of a certain number of vaults, between which arc openings communicating with the three squares of the gridiron. These vaulted galleries serve for the passage of air, and for setting fire to the foot of the kiln by means of wood which is burnt on the pavement of their floors. One or two holes made in the wall, and closed during the firing, give entrance to the kiln.
Wood or coal is used as fuel. The part in front of the kiln, called "avant-four," is generally below the level of the ground; hence, in order to avoid the collection of rain-water which would invade the kiln, a deep well is dug and filled with large stones.
Fig. 202. Longitudinal Section.
Fig. 203. Plan.
(Scale of 4 millimetre;, to the metre).
The kiln is filled with bricks, each layer being separated from the next by sand. The method of filling the kiln is variable, but in kilns holding from 100,000 to 120,000 bricks, about one-half are stacked solid, that is to say, the bricks close against one another in the mass, vertical chimneys being reserved corresponding to the holes in the gridiron. The rest of the bricks are packed loosely, that is to say, the chimneys are omitted, but a certain interval is left between the bricks; the layers cutting one another at right angles, the bricks are always placed on edge. It is well to put at the bottom of the kiln the bricks which will undergo the least contraction, and which best resist heat; that is to say, those made of strong clays, on account of the weight of the mass which they support.
As filling the kiln takes time, and rain may cause damage, - for no fire will guard against the moisture caused by it, - the kiln is covered with a wooden roof which is taken off before firing.
When the kiln is full, we place on the raw bricks a row of baked bricks laid flat and called "platin," then the fire is lighted on the floor of the subterranean galleries called "arches" or "cloches," and is very gently pushed; this is the period of the "petit feu" during which "enfumage" takes place, that is to say, the removal of the hygrometric water still left in the brick.
The "enfumage" being finished, we pass to the "grand feu," which is obtained by actively pushing forward combustion night and day. When the mass begins to turn red, the draught is lessened by covering the "platin" in places with damped clay, especially where the fire is rising too quickly. The progress of the firing is followed by measuring the contraction by means of fixed marks. The fire is kept up near the orifice of the "cloches," and the draught suffices to distribute the heat throughout the kiln; nevertheless, in order to ensure a uniform baking, some fagots are from time to time pushed down into the vaulted galleries with long pokers; this is what is called "pousser au fond".
Sometimes soot fills the chimneys and obstructs the draught. It will be sufficient to let it burn itself out by diminishing the fire in the vault corresponding to the obstructed chimney. From day to day the upper part is more plastered to concentrate the fire in the mass. When a certain degree of contraction is reached, - not the total contraction, for the firing mass will continue to diminish in volume, - the orifices of the "cloches" are stopped up, the top of the kiln is covered with a thick layer of clay, and it is allowed to cool. Wood-firing gives excellent products, well coloured and ringing well, but it is troublesome and difficult to work. For fuel, fagots are used, or, more economically, brushwood.
It is as difficult to estimate, if not more so, than in the case of coal. With brushwood at 15 francs the hundred, however, it may rise to 8 or 10 francs per 1000 bricks. The waste is not great if the kilning is well done. In the chimneys bricks are found which are black and varnished; this effect is due to soot and the fusible products contained in it, which become attached to the bricks when heated to a red or white heat
This is carried out in exactly the same way and with the same care as by the Flemish method. To guard the bricks from rain and wind, a shed is built over the walls of the kiln with a tiled roof high enough to leave unhindered the progress of the firing and the disengagement of gas and vapour.
When several ovens are close together, a movable roof is sometimes constructed which runs on flanged wheels guided by rails placed on the walls of the kilns. When the weather is fine, this roof is pushed over a kiln not in course of firing; if rain comes on, it is brought over the active kiln. In spite of these precautions it is difficult to bake in these kilns during the winter on account of the inclemencies of the weather, which cause loss and render the work troublesome and sometimes impossible; this is because the bricks have to be transported across the open from the sheds to the kiln, and also because much smoke, due to the moisture, is produced as soon as one or two clamps are laid.
Generally two kilns are built one against the other, and this is simply done by dividing a single large kiln into two by a thick wall. The capacity of these kilns is very variable. There are some which can bake as many as 400,000 bricks, but these large kilns are less convenient than those holding 150,000, on account of the enormous number of raw bricks which must be accumulated. The number of kilns in a brick factory depends upon its importance, but there must be at least two, for the bricks are taken out as they are wanted for sale; and if none were being fired while this was being done, the factory would find itself without goods, the kiln being empty.
That mode of firing called "a la flamme" is carried out in a kiln like the foregoing one, but, instead of burning coal placed in thin layers between the bricks, combustion takes place on fire-bars placed in front of the kiln. The fireplaces are three or four in number, and are fixed, each grating being about two yards square. These kilns are not recommended, for, under such conditions, it is better to use covered ovens.