Whatever the form of these kilns may be, the fire is always oxidising in them during the filling of the kiln and at the beginning of the "grand feu," for during these periods the quantity of fuel is small in comparison with the air entering by the open doors, and the latter is in excess in the products of combustion. But during the "grand feu" the doors of the furnaces are closed, and air is no longer in excess, especially at the end of the firing, when all the combustible gases are not always completely burnt. The firing then begins in an oxidising atmosphere and concludes in a reducing one.
Covered kilns make better use of the heat than open kilns, and they allow of the products being well fired, especially round reverberatory kilns. But the loss of heat is still considerable, for the gases go to the chimney conduit at the temperature of the baked products, that is to say, at a high temperature.
For a long time attempts had been made to use this waste heat for warming cold products by placing two or three communicating kilns side by side. While one of these was in full swing, its neighbour received the hot gases, which produced "enfumage" and raised its products to the early red stage, so that, when the first kiln completed its firing, the second immediately entered on the "grand feu" stage, while the third was "en enfumage." There was lacking in this system the principle of continuity, which has been introduced into brick-firing by MM. Hoffmann and Licht of Berlin.
Before describing continuous kilns we will estimate the cost of the different kilns which we have just described. To speak frankly, only one of these is to be recommended, - the reverberatory round kiln. In France this is reserved for the firing of porcelain, of faience, and stoneware, but in the United States of America it is extensively used for all kinds of pottery.
A model built by Mr. Pike (of Chenoa, Ill., U.S.A.) is recommended in France by M. Lacroix, grantee of the American maker. M. Chambrette-Belon also recommends a kiln of this system. As we have said, anyone is free to build a kiln of this kind as he wishes, the patent having expired.
All the types offered by different builders only differ in detail.
In the following estimates, we shall make the same remarks as to the prices we take, as we have already done in speaking of drying - places (p. 182). For purposes of comparison, and in order to show the differences more clearly, we shall suppose a baking of five to six millions of bricks per annum.
We shall make this estimate for two kilns placed side by side and each containing about 160,000 ordinary bricks, which corresponds to a cubic content of about 312 square metres with the following dimensions: height 6 metres, length 8 metres, breadth 6 1/2 metres.
Earthwork, 850 cubic metres at 1 fir. 50=
Masonry, 250 cubic metres at 30 fr. . =
Sundries . . . . . =
Roofing of kilns.....extra.
Note. - The kiln being built on the brickworks, it often happens that the clay extracted is used for making the bricks, which diminishes the cost.
For filling, cooling, and discharging such a kiln, about three weeks will be required: supposing work to continue for seven months, that is to say, 210 days, it is seen that 10 charges can be fired in the same kiln, i.e. 16,000,000 bricks: we must have, then, at least four similar kilns to reach the desired six millions.
We shall take a kiln 6 metres in diameter and 4 in height, giving 84 cubic metres of space and corresponding to 40,000 or 42,000 bricks.
Earthwork, 80 cubic metres at I fr. 50 =
Masonry, 200 cubic metres at 30 fr. =
Fire-bars, furnace doors, etc. .
A fortnight is taken for filling, cooling, and discharging; a single kiln will then fire annually from 800,000 to 1,000,000 bricks (the kilns being covered and working nearly all the year round).
We shall then want six similar kilns to fire six millions, and not including the building in which the kilns stand. As we shall see, a continuous kiln costs less and has as good a production, and the reverberatory kilns should be reserved for small works producing a million to two million bricks. In this case the cost of one or two kilns and their chimney will not exceed 10,000 to 16,000 francs.
6 x 6500=39,000 fr.
A chimney 30 to 35 m. in height
Conduits, registers, etc.