As this kiln is no longer constructed of the primitive round shape, and as the rectangular form given to it by M. Simon, M. Hamel, and others, has been preferred, we will show its working by the diagram in Fig. 212.

The kiln consists of a circular tunnelled gallery called firing-chamber, which communicates with the exterior by means of doors, P P (there are twelve in the diagram). These doors are used for charging and discharging, and are hermetically sealed during firing; the distance between them is called a compartment. The gallery also communicates by conduits, c c, with a circular channel B, which leads direct to the central a chimney C. The orifices, o o, by which the conduits open into the central channel, are closed by covers or traps which can be raised at will by some kind of mechanism: screw, chain, pulley, etc. All these traps being lowered, there remains no communication between the gallery and the chimney; if one of them be raised, a draught is established through the conduit leading to that trap.

The arched roof of the gallery is pierced with a certain number of holes closed above by cast-iron stop-valves which fit into a groove filled with sand and hermetically joined. These holes are intended for the passage of the coal, which, when cast upon the red-hot bricks, will ignite on contact and bake the mass.

Let us now consider how the kiln works. We will suppose it to be in full swing. At a certain point in the gallery is a sheet-iron or paper register, R, fixed against the kilned brick and closing the gallery completely. On one side of this register the gallery is filled with bricks, on the other side is an empty compartment where raw bricks are being placed; the following compartment is quite empty, and the third, still on the same side of the register, is being discharged; thus, of twelve compartments, there is one empty, one being discharged, one being charged, and nine full of bricks and with doors closed. Of these nine, three are fired and are cooling, one is "en grand feu," namely, the one which receives the fuel by chimneys or heating wells arranged in the mass of bricks and corresponding to the holes pierced in the roof. In these chimneys have been placed bricks "en chicane," which, by scattering the coal throughout the height of the mass, prevent it from accumulating at the bottom of the wells.

212. Horizontal Section of the Hoffmann Kiln, showing how it works.

The air necessary for combustion passes over the products which are already baked, hastens their cooling while gaining heat itself, and arrives over the coal at a high degree of temperature. In this way there is no loss of heat; the gases of combustion, mixed with the excess of air, continue their way after having baked the products, and heat the bricks in the compartment next to that "en grand feu" sufficiently to make them red-hot: this is the place where it is "petit feu." The gases, partially cooled, meet colder and colder products, to which they still yield a certain quantity of heat, and this produces "en-fumage," that is to say, the removal of the water always contained in raw bricks, however dry they may be. Then they reach the register R and penetrate into the channel B, which brings them to the central chimney C.

Cooling and heating are therefore progressive, which offers great advantages in the firing of pottery, and the heat of the fuel is as completely utilised as possible; the only important loss is the heat given out by the cooling products, a small part of which only is carried off to warm the air necessary for combustion. We have seen that this waste heat has been used for warming the drying-rooms, which are often placed over the kiln.

When the "grand feu" compartment is fired, and this takes from twelve to twenty-four hours according to its size, they pass on to the next; at the same time the draught is stopped, for the compartment which was being charged has been filled while that "en grand feu" was firing. The door of the compartment is closed and a new register applied against the brick; when it is placed in position, the first is taken out through the door (formerly it used to be removed through the roof, which had an opening for the purpose), or is burned when it is made of paper, which is most frequently the case. The open trap is then lowered and the next one opened.

The motion continues in this way, the fire being moved one or two compartments every day; on the twelfth day it is in the same place again.

The circular form adopted by the inventors presents certain disadvantages: it is costly on account of the considerable volume of masonry and sand that it requires, and of its delicate construedon. Therefore the shape was first modified into an oval, and finally it was made rectangular. These modifications have been praised in turn by Hamel, Simon, etc., who have given their names to the kilns thus changed by them. One of the best known is the kiln devised by the last-named, and we shall give a description of it with the help of Figs. 213, 214, 215, 216, and 217.