In order to prevent this loss it is best to fire slow when you first begin to warm your kiln. You save money by not being in too big a hurry when you commence firing—until you get up a temperature in the kiln. When you begin to burn there is very little draft ; after it begins to warm and you get a heated atmosphere in the kiln, the more force has the draft.
After a kiln is " hot " to the top, the object then is to let the heat out of the arches as freely as possible, and hold it closely at the top. In this way the arches are not too hard, neither are there many pale brick under the platting. The heat can be driven rapidly out of the arches to the top by throwing air currents of a moderate degree of heat into them.
It is more economical and also better to color the brick with a lively heat instead of a dead heat. When a kiln has a tardy draft the heat remains too long in the arches, causing the " headers " to melt and giving several courses of pale red or salmon brick on the top bench, and also consuming extra time. When there is proper draft and moving heat, the heat is nearly uniform from bottom to top and color of brick the same, and a better, brighter color than those burned with a dead heat ; the latter are really baked instead of burned.
Practical brickmakers look at results rather than theories.
The hydrogen of the gases from coal and wood, and in the use of natural gas and artificial gas the oxygen necessary to supply combustion, are derived from the atmosphere. The hy drogen that is in the fuel has a greater affinity for the oxygen of the air than the carbon has ; and if the furnace or arch, or wherever the fuel is burned, has not a proper supply of air, a sufficiency of it, the hydrogen will take up the oxygen and leave the carbon to pass in the kiln unburned. Now, many manu
facturers of brick find in their kilns white brick, and gray brick, and striped brick, and laminated brick, for this very reason, and are not able to account for it. The carbon does not actually enter the body of the brick always. In the majority of cases, when the carbon is unburned in the furnace or in the arch, and passes into the kiln, it settles upon the brick, and when the brick becomes hot enough, it partially burns, or burns into an ash. If the kiln or furnace be not of proper construction to burn the carbon in the furnace, and it escapes out without be ing burned, and is consumed in the kiln, which can be shown is the case nine times in ten, you are burning your fuel in your kiln on your brick, because it settles there, and it burns there, and it is liable to discolor the brick. It will not discolor all clays, because there are certain substances in the clay that hold the carbon and make different sorts of color. It cannot help but be so, and the idea that this carbon entered the body of the brick and caused it to swell badly may no doubt be correct in many instances. For this reason we know that generally in almost all clays where the ground bituminous coal is mixed with the green clay, and the brick put in the kiln and burned, it swells. The reason is this : The ground coal is placed in the clay, and when it is heated sufficiently, the volatile matter is thrown off; it must get out, and it simply swells the brick in the effort to escape. It is a mistaken idea that the gas in the coal can be consumed by getting it hot enough. It will not burn until air gets to it, and hence the gas is generated before the air reaches it, escapes through the brick, and for that reason the soft coal cannot be used.