The advantages gained by firing pottery by the combustion of gases were pointed out long ago by Ebelmen and Salvetat, who had specially in view porcelain and faience. It was only much later, in 1869, that eminent manufacturers like Muller, Marie, etc., undertook to apply gas firing to architectural pottery, and especially to bricks. If the question seems, theoretically speaking, simple, and is presented in a seductive manner, it meets nevertheless in practice many difficulties which have much hindered its success. It was in fact necessary to find a kiln which would present evident superiority over the Hoffmann kiln, and which would effect firing as economically, even more economically if that were possible.
The few small disadvantages of the Hoffmann kiln are as follows: -
The presence of ashes, the residue of combustion, which the draught carries into the different parts of the kiln, and which may become attached to the products;
The direct contact of the fuel with the bricks of the well, which always causes loss;
The cooling caused by the entrance of cold air when the solid fuel is thrown in; for however short a time the lid may be opened, as the action is frequently repeated there is a sensible loss of heat;
The difficulty of easily transforming the oxidising action of the kiln, which is the usual action of the Hoffmann kiln, into the reducing action sometimes necessary for certain products; The impossibility of getting a constant heat throughout the kiln, and the difficulty of attaining very high temperatures.
The use of gas kilns does away with all these inconveniences, at the price of some difficulties, however, the most serious of which has been, until very lately, an increase of the expense of firing. That can easily be understood. The problem before us as to the firing of bricks is as follows: how to raise to a high degree of temperature products which we should be able to introduce and withdraw in a continuous manner. If it is admitted that these products should be fixed with respect to the fire which itself moves, we come inevitably to this conclusion: collect the gases made by the gas generator so as to distribute them afterwards to the place of combustion. But, in order that a gas generator may give its maximum of effect, the gases must be used as hot as possible, at 400° or 500° C, their temperature when issuing from the generator.
In the present case this is impossible; they have to pass through the whole length of the kiln, hence there is a cooling which causes a certain loss of heat, and the condensation of the soots and tars carried off by the gas.
This tar at last obstructs the channels; we get rid of it by-burning it in the conduit itself by letting in the air, communication with the generators being interrupted, and thus a stoppage of a few hours being caused in the working of the kiln. Besides, this short stop only takes place every five or six months with a well-managed generator, and may even be altogether avoided.
As for the cooling, it is reduced to a minimum by placing the gas generators at the end of the kiln. The real cause which has so long delayed the application of gas kilns to the firing of bricks is that the more a machine is perfected, the more does its management become delicate and require care and precautions.
The Hoffmann kiln was an advance on intermittent kilns; some time was required to achieve its triumph, and even to-day it is certain that many manufacturers do not profit by all the economy which it may bring, because the possession of a machine is not everything, much lies in its proper handling. Gas kilns are an important advance on the Hoffmann kiln; for that very reason they require more skill in management, and above all a change of habits in the workmen, which is not always easy to obtain. When one thinks of the details on which the success of a commercial operation so often depends, one perfectly understands the difficulties which hinder those who are urging the firing of bricks by gas.
In our opinion, then, the objections made to gas kilns - such as danger of explosions, greater cost of construction, obstruction of conduits by tar, more expensive firing, etc. - have very little, if any, foundation. But we repeat that to succeed in using them carefully, well-trained workmen are required who are capable of ensuring a steady and uniform working of the generator; there lies the whole secret of success.