The production of a kiln, from this point of view, is given by the relation between the number c of calories theoretically necessary for firing products at a stated temperature, and the number C of calories produced by the combustion of the coal employed.

To determine the ratio c/C we must find e from the composition of the paste and the temperature T at which it is fired.

The factors of this calculation are -

1. The heat necessary to evaporate the weight e of the hygrometric water;

2. The heat absorbed by the departure of the weight E of the water of combination;

3. The heat available for raising 1 k. of clay from the ordinary temperature to T°, supposing .2 to be the specific heat of the clay, between 15° and 1400° C, which is a sufficiently near approximation.

It is supposed that the quantities of heat given out or absorbed by the chemical transformation of the silicates balance one another, and that the absorption of heat due to the decomposition of the carbonates, when there are any, may be neglected.

The number of calories theoretically necessary to fire a given weight of clay being thus calculated, we must add to this number that due to losses of heat, the principal of which are: (i) loss due to the heating of the inner walls of the kiln; (2) loss due to the radiation of the outer walls; (3) loss due to the hot gases passing through the channels to the chimney; (4) in gas heating, loss due to gasification.

All these losses are very difficult to estimate in practice, therefore it is much simpler to weigh the quantity of coal employed in firing a given weight of the products, and determine the calorific power of this coal. Thus we have the number of calories practically necessary for the firing. Comparing the number so found with the number theoretically necessary, we shall have the loss of heat due to the various causes enumerated above, and consequently the thermic production of the kiln.

This production is far from being constant; it depends, above all, upon the good management of the fire and the stacking, two important factors in the firing of pottery.