The principle of the continuity of firing was stated by Peclet, in his Traitt de la Chaleur (3rd ed., 1860), and various practical applications of this principle had been carried out successfully by Muller and Gilardoni, when Hoffmann and Licht gave to continuous kilns the definite form which made a success of them.
With these kilns only can be completely acquired what had been already sought for in covered and combined intermittent kilns; that is, the utilisation of the excess of heat necessary for firing for the gradual heating of the objects kilned. In continuous kilns the heating is methodically carried out, that is to say, the warm gases after having effected firing are cooled by passing over cold products in the kilns and escape into the chimney carrying a minimum of heat with them. On the other hand, the air necessary to combustion is strongly heated while passing over fired products which are cooling.
Two processes ensure the continuity of the fire while fulfilling the above conditions: one is to have a fixed furnace to which the products are brought to be fired, and from which they are, after firing, removed; the other is to alter the position of the furnace with respect to the bricks which remain stationary.
The first system does not appear to have reached a practical stage in spite of the ingenious attempts of M. Demimuid and MM. Boulet freres, who thought of making waggons, covered with refractory clay and filled with bricks, move before a fixed furnace.
The excess of heat was used to warm the waggons in front.
In M. Barbier's kiln, the reverse took place: the furnace, mounted on a waggon, was moved and came to bake in turn the different batches of bricks arranged in a fixed kiln.
The use of these machines, which date from 1855, has been prevented by various causes, especially the maintenance of the rolling-stock and certain practical difficulties, but more than all by the appearance at the Universal Exhibition of 1867 of the continuous kiln of Hoffmann and Licht, which was to recommend itself to the pottery trade by its logical principle and the excellence of its results, from the point of view of economy as well as of the beauty of the products obtained.
For the last thirty years it has come more and more into use; if its form has been modified in order to simplify its construction, and if some details have been altered, its principle and fundamental arrangements have remained the same. But a new advance has been made. In the Hoffmann and Licht kiln, the firing is effected by a solid fuel which is thrown upon already red-hot bricks and there ignites. It has been thought that the system of gas-generators might be applied to continuous kilns, and that the firing of the products might be effected by the burning of the gas; this presents certain advantages, such as the absence of ashes, the great regularity of the baking, and the high temperature which can be reached. After many attempts a satisfactory result has now been attained, and continuous kilns heated by gas will certainly come more and more into use when their working is rendered quite easy and some practical difficulties are overcome.
Continuous kilns are divided into two classes -
A. Those with solid fuel.
B. Those with gas fuel.