The firing of pottery is the most delicate and the most important process in its manufacture. On the success of the operation depends the good quality of the products which have already required so much care and time. For a very long time firing was effected by burning wood in simple ovens, but afterwards coal was substituted for wood in countries possessing mines.
As the means of communication were developed, this substitution became more common, but the method of firing bricks underwent little change. Nevertheless the old Flemish process was gradually replaced by the use of various kilns which simplified the work, and in 1865 a great step in advance was made by the invention of the continuous kilns of Hoffmann and Licht of Berlin. Since then the substitution of gas for coal heating has been another improvement. We shall divide this part of the subject into several paragraphs according to the various methods of firing bricks: -
Firing in clamps.
,, in intermittent kilns
,, in continuous kilns
A. With solid fuel.
B. With gas.
Intermittent kilns are those which are allowed to cool after firing, to permit of the removal of the bricks.
Continuous kilns, as the name shows, are always active throughout the season or even the whole year. The heating and cooling of the products are effected methodically.
We must not think of describing all the kinds of kiln used; every manufacturer has his own style; but, generally speaking, the differences between the numerous types of kiln are of no great importance. We shall content ourselves with describing one example of each system, specially mentioning those which seem to us most to be recommended on account of their economical working and the beauty of the products obtained.
This is undoubtedly the simplest method; but it is neither the most economical, nor the one giving the best results. We shall study it in detail because, in an improved form, it is still the one most widely used in the numerous brickworks of moderate importance.
This system consists of placing the bricks in regular layers intersecting one another, and scattering between them finely divided coal, the combustion of which causes the baking of the bricks. In order to ignite this coal the foot of the kiln is arranged as follows: -
Upon a smooth piece of ground, slightly elevated so that rain-water may not rest on it, rows of fired bricks are laid down on edge (Fig. 188). The number and length of these rows depend upon the quantity of bricks to be fired, which may be from 200,000 to 600,000. The second layer (Fig. 189), placed like the first, cuts it at right angles, the third and fourth are arranged as in Figs. 190 and 191. At distances of a yard are placed furnaces as shown in Fig. 199; they go from one end to the other of the stack, and, before the fifth layer is placed, are filled with wood. Then the vaults are closed, and above, among the bricks, are placed parallel rows of large lumps of coal which will be kindled by the flame from the wood, and by their slow combustion will set fire to the fine coal of which we shall speak later on. When once the foot of the kiln is finished, the "cuiseur" or stoker proceeds to light the fires. When the coal is thoroughly kindled, the "enfourneurs" begin their work; there may be one or two, according to the size of the stack, and they are assisted by "entredeux," who receive the bricks from the hands of the barrow-men. The "en-fourneur," the "entredeux," and the necessary number of barrow-men form a gang. The early part of the work is unpleasant for the "enfourneur," who has to lean continually over burning coal surrounded by the gas and smoke coming from it, therefore he cannot remain long over the furnace; another takes his place, and the work continues. When the clamp is finished, the stoker sprinkles finely divided coal over it. This he does with special baskets which are handed to him by the gang; with a quick motion and without stooping he sprinkles the coal in a uniform layer over the bricks. Then in the same way he sprinkles a little sand to prevent the bricks from sticking together. When the operation is finished, the stacking is recommenced, and a new "clamp" is formed at right angles to the first.
Figs. 188 to 200. View of a Furnace, Elevation, Section, and Plans.
When this is finished, it is sprinkled with coal and sand, and another is begun; in this way six or eight clamps are built in a day, according to the progress of the fire.
During the stacking of the bricks, the stoker dilutes some clay, and when the paste is sufficiently liquid, he plasters over the facing of the four sides of the clamp. After the fire is well alight he closes the entrances to the furnaces with bricks, which he also coats with clay. The fissures which form in the clay plaster permit the entrance of the air necessary for combustion.
Besides, if it is necessary at any time to stimulate the fire at a given point, one or several furnaces may be opened again. The fire passes from clamp to clamp, and the skill of the "cuiseur" consists in guiding it as regularly as possible. Experience alone can give the necessary practice, but a few general hints as to the course to be adopted may be offered. We have stated that a slight layer of finely divided coal as uniform as possible is thrown over all the clamps; it is by this that the fire is communicated. Experience teaches the amount of coal which should be scattered, but the points at which the fire requires to be retarded and stimulated are shown by indications that should be carefully observed. Thus, if we see the bricks of the last clamp becoming white or yellow, it means that the flame is near, and consequently that it is moving too fast in that neighbourhood. Although we might not think so, this will be the place where more coal must be added. When, however, no signs of combustion appear, the fire is sleeping, as they say; therefore no more fuel is added. And finally, in order to retard the action of the fire where there is too much activity, not only is much coal added, but the spot is sanded so as partially to fill up the interstices of the bricks. The last clamp built on the previous day should be alight in the morning; this is a sign that all is going well. The "cuiseur" quickly scatters coal and sand, then the "enfoumeur" who follows him hastens to make another clamp; but, in spite of these workmen being accustomed and inured to heat, they cannot remain more than a quarter of an hour, sometimes less, and others must take their place.