Open kilns lose a considerable part of the heat formed by combustion of the coal or wood. Generally the upper part is badly fired; moreover, the wind and rain, in spite of a protecting roof, have an injurious effect on the progress of the fire. To avoid these disadvantages it has been found advisable to close the kilns by an arched roof which shelters the fire from the influence of the elements and throws back the heat on to the products before it is lost in the air.
These kilns are divided into -
1. Kilns with direct flame ((1) Rectangular; (2) Round).
In these, the furnaces are placed according to the greatest length of the kiln; above is a flooring pierced with a large number of holes which communicate with the oven. The upper vault is pierced with holes communicating with channels which lead to the chimneys, for the escape of the smoke and gas when they have produced their effect. A single chimney may serve for several kilns, and often the draught is produced without any chimney at all.
The method of filling these kilns depends upon the kind of bricks to be fired. For those which bake at a relatively low temperature, the bricks are arranged so as to spread the flame over as large a surface as possible; that is to say, they are separated about a finger's breadth from one another; the rows or clamps are crossed and inclined to one another, to ensure the solidity of the mass and at the same time divide the flame by forming "chicanes".
The firing begins with the "enfumage" or period of" petit feu," then when all trace of moisture has disappeared,the "grand feu" begins, which is the period of baking. The fuel used is wood, peat, coal or coke, according to locality. The coal should be of a kind giving a long flame; its use exposes the products more easily to sudden outbursts of heat than wood, the heat of which is always uniform; but cost should decide in these questions, unless delicate objects have to be baked, in which case wood should be used.
The longitudinal channels lead into a single transverse channel opening into the chimney. Registers are placed in each channel, so that the fire can be pushed at points where firing is progressing less quickly. Openings in the vault permit of the degree of contraction, and consequently of firing, being observed. In other kilns there is no pierced flooring, and the three furnaces are placed in front of the kiln, as in the case of open kilns firing "a la damme." This arrangement is defective; for there is danger of burning the products placed in front of the firebars, and very often those at the end of the kiln are not baked at all.
Fig. 204. Longitudinal Section.
Fig. 205. Transverse Section.
(Scale of 1 centimetre in the metre).
The furnaces in these are arranged round the products to be fired. Their number depends upon the size of the kiln. It is more easy to get a uniform firing with these than with the foregoing, for there are no corners into which, very often, the flame does not penetrate. Their round shape gives them a great resisting power which, in many cases, renders unnecessary any iron plating.