The ignition of the kiln is effected by heating one compartment by means of firebars placed in front of it; then when the bricks are red-hot, the heating is continued by throwing coal through the holes in the vault. In some kilns the igniting furnaces are placed permanently at one end of the kiln with their doors outside. When a sufficient quantity of brick has become red-hot, and the fire has started properly, the furnaces are extinguished and their openings stopped up by a wall.
In preference to these fixed furnaces we recommend movable ones, which are installed in the kiln, and removed as soon as the fire is started. The construction of these furnaces is very simple.
Three of them are usually arranged in the width of the kiln; their bars are of cast-iron and the doors are formed of sheet-iron plates placed in front.
The length of the latter is determined by the length of the bars. In front of the furnaces a wall of bricks bound together with clay is built so as to completely close in the kiln; in this wall three openings are left for passing the gas to the furnaces. This wall almost touches the green bricks with which three or four compartments are filled. The bricks are thus enclosed between the wall of the furnaces, and the register placed against the last compartment. The whole forms an ordinary flame kiln, the draught passing through the vertical conduits and the central channel. We begin by heating with the "petit feu," then we pass to the "grand feu." The bricks in the first compartment are thus raised to a red-heat at the end of a few days; from time to time we make sure that the heat is sufficient to ignite the coal which is thrown in through the holes in the roof. When the coal burns well, we begin to heat from above while keeping up the fire in the furnaces. At last, when a compartment, or better still a compartment and a half, is raised to a red-heat, we let the fire in the furnaces go out, and heat only by the holes in the vault. To bring coal to the kiln, a small Decauville railway of .4 metre gauge is laid on each side; thus the fuel placed in a waggon is within reach of the stoker, who pushes the waggon forward as the fire advances. He takes the coal up in a round shovel like those in household use, and, raising the lid of the holes with a movable hook or with a handle fitted to the lid, he quickly throws the fuel into the well, allozving as little cold air as possible to enter. He goes in this way from hole to hole, beginning at one end of the compartment and finishing at the other; generally only one compartment is heated at a time. When he has finished his round, he waits a certain time and then begins again in the same order. In order to measure precisely the interval between two heatings, a special clock is placed on the kiln which strikes once at the hour, two strokes at five minutes past, three strokes at the quarter past for beginning again, one stroke at twenty minutes past, and so on. In this way it is easy for the stoker to notice the time which has passed, especially at night. The heating times are generally ten minutes or five minutes apart.
The progress of the fire depends upon the degree of difficulty of baking of the products; an advance of from 4 to 8 metres a day may be expected, that is to say, from one to two compartments. If we take the first estimate, the stoker will, at the end of six hours' heating, begin to put fuel into another row of holes, and will leave one row behind him. As there are four rows of holes to each compartment, it follows that this operation will be repeated four times in the twenty-four hours. In order to remember at what time he began to heat a new row, he writes it down with chalk on the lids. When one row is no longer being heated, the well is closed with two bricks laid flat, and coal is placed on these two bricks for five or six hours. This coal does not fall down, it helps to maintain the heat under the vault; for the fire has always a tendency to descend, owing to the draught passing to the floor of the kiln.
A good precaution in firing is to measure the contraction in height of the mass of bricks with a graduated iron rod, taking the distance from the edge of the cast-iron shoulder at the end of the heating hole.
The draught is regulated by raising or lowering the trap-doors, the threaded rods of which are worked by a small flywheel (Fig. 21 3). The draught exists throughout the "enfumage," the "petit feu," and the "grand feu," but the holes behind the "grand feu" should have a return current. Draught and return current are easily distinguished from one another. When a lid is raised, there is immediate aspiration through the open hole if there is a draught, whereas if there is a return current the hot air is felt issuing from the hole. The stoker regulates his trap accordingly; there are generally one or two traps open, sometimes three, according to the number of compartments in front of the " grand feu".
The draught should come from as great a distance as possible, that is to say, there should be the greatest number of full compartments before the one "en grand feu," so that as little heat as possible may be sent into the chimney.
The greater the distance of the draught is, the more regularly does the fire progress; the obliquity of the path of the gases going from the outer wall of the kiln to the central channel is less great; it rarely, however, advances more quickly towards the central channel than towards the outside wall. It is almost always necessary to keep the fire in this part several holes behind.
Firing and "enfumage" take place in an oxidising atmosphere. The air in fact reaches the firing compartment after having been heated but not having lost its oxygen; consequently the products just baked, and those being baked, are in a very oxidising atmosphere. As the air is in excess, combustion does not absorb all the oxygen, and the compartments in front of the "grand feu " are also in an oxidising atmosphere, but less so than that in the compartment preceding the "grand feu".
In the Hoffmann kiln, then, it is immediately after firing that the products are exposed to the most oxidising gases, while in intermittent kilns the gases are oxidising before firing and somewhat reducing during and after. Attempts have been made to render the continuous kiln reducing and oxidising at will by lowering the draught to its minimum, and stopping it completely for half an hour three or four times in the twenty-four hours. The results are not very satisfactory, and are costly on account of the delay caused in the progress of the fire.