As we have already stated, the object of this process is the removal of the water contained in green products before they are subjected to a high temperature. In continuous kilns the "enfumage" is effected by the hot gases coming from the compartment" en grand feu"; as these gases progress towards the channel leading to the chimney, they cool and at the same time take up moisture produced by the combustion of the coal and the complete desiccation of the products over which they pass. As long as these damp gases pass over bricks which are already heated to a high temperature there is no condensation, but we must avoid their reaching cold products like those in the compartment from which the register has just been removed, for the moisture will be deposited on them, and produce a damaging effect, giving the fired bricks an irregular colour, yellowish or whitish in places and disagreeable to the eye.
These accidents, however, are only persistent in lightly fired products they disappear under strong firing.
Several means of obtaining a good "enfumage" have been recommended. In the Simon kiln two large cast-iron stoves are placed over the compartment and isolated between two registers; they are joined by a pipe to the heating holes. The trap of the compartment is slightly opened to make a moderate draught. Two stoves are placed near the outer wall and one in the middle of the row of holes farthest from the air trap; thus the hot gases pass over the greater part of the compartment. The progress of the "enfumage" is observed by raising the lids; as long as it is not complete damp soot will be observed on the cast-iron, and when this disappears the "enfumage" is completed. An average of twenty-four hours is required for one compartment.
Attempts have been made to carry out "enfumag " by means of the heat given out by the fired products when cooling, a heat which is far from being all absorbed by the air necessary for combustion. The arrangements consist of a series of air channels placed over the central channel and communicating with the heating holes. The orifices of these channels are closed by a valve or trap to each compartment. This kind of installation is very troublesome; it increases the dimensions of the kiln, and to a certain point diminishes its solidity. It is evident that the more hollows there are in the masonry of a kiln, the more is the resistance to the effects of dilatation decreased.
Moreover, some manufacturers contend that when hot air is borrowed from cooling products for the purposes of "enfumage," the progress of the fire is slackened, and the heat is greater in the discharging compartment.
M. Fillard, the well-known pottery expert, resolutely rejects "enfumage" by stoves or any other similar method. According to him, a separate "enfumage" is in opposition to the continuity of the kiln; he effects it completely, by a properly managed draught. For instance, directly the register is set fire to, a new compartment is put into communication with the others, the trap of the compartment is very slightly raised, and the draught passes through the two or three preceding ones.
Thus the water-vapour does not condense on the cold products; the latter are slowly warmed by contact with the hot walls of the kiln and the very slight draught going to the last air hole. When no more deposit of water is observed in the neighbourhood of the burnt register (this is easily verified by passing a cold iron rod through a heating hole), the opening of the traps previously raised is reduced, and that of the last increased. By these precautions all accidents due to the deposit of water-vapour on cold products are avoided; we get rid of the trouble of removal and maintenance of the fire in the stoves, and we economise fuel.
The importance of only firing dry products is known to all potters; not only do they behave better under firing, but the cost of fuel is considerably diminished. In fact every kilogramme of water to be evaporated means 637 calories, that is to say, about 100 grammes of a coal giving 7000 to 8000 calories; for, besides the heat of vaporisation, we must take into account the heat necessary to raise water-vapour to 300° or 400° C, the temperature at which water of combination is given out.
If the bricks contain 2 or 3 per cent, of hygrometric water, as is generally the case, the expense of fuel on this head will be, for every 1000 kilos of green products:
1000 x 2 x 0.1 = 2 kilog. of coal. If the quantity of water is as much as 10 per cent., 10 kilos of coal will be required, representing 28 kilog. per 1000, if we take the weight of 1000 green bricks to be 2800 kilog. It is therefore highly advantageous to dry the products well.