The waste is enormous and the quality of the bricks is poor. The waste is explained as follows: -
Let us suppose a mass of 500,000 bricks divided into 60 clamps of 8000 each. First 50,000 bricks will have to be used for building the furnace, which must be rebuilt every time. It is true that these bricks can be used again, but some will naturally have to be renewed. We will estimate these at . . 5,000 even admitting that the whole interior is well fired, which is not always the case; for we must account for sudden bursts of flame, welding together several bricks, and for too quick spread of fire, causing the bricks to be too little fired. The quality of the bricks is very mediocre, firstly on account of these irregularities in firing, and secondly because it is impossible to bake bricks well without causing accidents called "loupages," which consist of a dilatation of the brick if the fire has had only a violent passing effect, and of fusion if the action has been prolonged. The slag of the coal and sand become strongly attached to the bricks, and cannot be detached, even by energetic washing, and this is called "gresage." It prevents bricks fired by this method from being used for well-finished facings.
There are always two or three layers at the bottom not properly baked, say two of 8000 each....
At the top the same thing occurs to three or four layers of 8000 each, say
Finally, the facing bricks are badly fired, and we may estimate them at .
Which makes a total of .
This is very difficult to estimate, as it depends upon variable conditions, such as the quality and price of coal, the nature of the clay to be baked, the skill of the stoker, fate of the weather, etc.
Generally poor coal is used, giving a steady fire without sudden outbursts, and smokeless. Often any kind of coal is used, and passed through a sieve; the remaining slack is used for the firing at the foot. Approximately, 130 to 160 kilos of coal are burnt for every 1000 bricks kilned of the dimensions 0.22 x 0.11 x 0.06. We must not forget that the very perceptible waste (at least 1 5 per cent.) increases this quantity, and that the bricks receive a poor firing, not nearly as strong as that given by continuous kilns.