After "running the moon" into all the arches of the kiln, the latter are allowed to wait, or cool, for twenty minutes or so, when the arch first mooned is fired from both sides at the same time. The amount of coal thrown uniformly through each arch varies with the condition of each particular arch. An arch that is very hot is not fired so hard as one that is cooler.
The usual amount of coal thrown into each arch in these settling fires is about from thirty-two to forty shovels full for a kiln containing 200,000 brick, that is, from sixteen to twenty shovels full for each door on each side of the kiln. Before these fires are given, the doors in the ash-pits are closed, and kept closed for about five minutes after the last arch is fired. Any "cold" place in the kiln can now be detected by the black smoke not freely issuing from it, which can be seen from the top of the kiln. A few shovels full of coals are now thrown into the arches under these places.
The doors to the mouths of the arches are closed soon after the fires are crossed ; if an arch is too hot, the door is opened a little, which is called " cracking the door." It is a difficult question to determine when and how much air to admit into a kiln during the process of burning. Air in proper quantities may often be admitted advantageously above the burning fuel. After the gases are superheated in their passage through the heat chambers, then near where the heat enters the kiln proper there should be admitted as much oxy gen as the heated air will absorb without reducing the heat, as it enters the kiln, below the degree required for the burning of the brick. This can be regulated by admitting more or less air and a careful observation of the heat. One skilled in burning brick knows at a glance whether he has the proper degree of heat or not. After the proper amount of air has been admitted above and below the grates in the furnace, the amount ad mitted below being regulated by the ash-pit door, then again near the point of contact with the brick, admit just as much as it will bear. We have then done all that can be done in this direction ; the quantity is to be determined by the burner and adjusted to suit the quality of the fuel and the condition of the kiln at the different stages of burning.
The "settling fires" are given to the kiln about every two hours, unless it happens that the wind and rain keep the heat down in the arches, in which case the firing is delayed until the arches are cool enough to receive them.
After the kiln is " burned off," all the doors and all the cracks are plastered, and the kiln remains closed for five days.
If the arches are fired too hot, they will " run " or stick to gether. Some kilns have very high stationary roofs, others have movable roofs that slide on railroad tracks from one kiln to another ; but the majority of open or Dutch kilns have only temporary roofs, which are taken off when the kiln gets "hot," which is, as has been stated, about the fifth day.
No rule can be laid down to determine when the kiln of brick has settled sufficiently, that is, sunk ; the proper amount of settling is known only by experience with the clay ; but for moderately strong clay, it is about seven-and-one-half per cent. of the height.
For instance, say a kiln of brick is made of moderately strong clay, and set forty-two high in the kiln, and the brick measure four-and-one-half inches in width, the total height would be one hundred and eighty-nine inches, and seven and one-half per cent. would be a little over fourteen inches.. Many classes of weak clays for the same sized kiln could be settled only about seven inches, and still make good building brick.
For information as to the nature of clays the reader is referred to Chapter II., which treats of the different varieties.
The kiln which has been described in burning is the ordinary open Dutch kiln. We selected that class of kilns for describing the process because they are, unfortunately, the ones that are generally used. There are various other kinds of kilns, many of which are decided improvements, especially in the economy of fuel.
Many of the annular kilns are very economical in the con sumption of fuel ; but kilns of this kind have usually other dis advantages which offset the saving of fuel. The over or down draught kiln is an excellent improvement.
There are other methods for burning brick, such as by com binations of gas and air, combinations of air and gaseous fuels, and by the use of natural gas and petroleum for fuel, and other devices, which, although they are all good, require a highly scientific knowledge of heat, its mechanical action, and many other things.
The amount of coal required to burn a kiln of hand-made brick is usually about one-quarter of a ton to one thousand brick ; but for dry-clay brick a larger quantity is 'required, the amount being about one-third of a ton to one thousand brick, which applies in both cases to the open top or Dutch kiln, but when more economical forms of kilns are employed the con sumption of fuel is much less.