Production of Temperature.
In the fire the great miracle takes place and the dry clay, most friable and perishable of materials, becomes one of the most durable. This is accomplished by the softening of the feldspar grains which cement the whole together and thus form a dense mass. To produce the temperature at which this phenomenon takes place two things are necessary, fuel and draught, the former supplying the carbon, the latter the oxygen. The liberation of the carbon in the fuel and its union with the oxygen of the air develop combustion, during which heat is generated. Combustion can be complete or incomplete. It is complete when there is an excess of air and the carbon can combine with two molecules of oxygen to form carbon dioxide (CO₂). This condition is called oxidation. It is incomplete when there is not enough air and the carbon can get only one molecule of oxygen, forming carbon monoxide (CO). This condition is called reduction. Carbon monoxide, being very hungry for oxygen, will try to extract it from whatever source it can. If ferric oxide (Fe₂O₃) is present in the clay—which is the case in red clay—the carbon monoxide will take one molecule of oxygen from it and convert it into ferrous oxide (CO + Fe₂O₃ = CO₂ + 2FeO). The important feature in this process is that ferric oxide is red and will make the clay burn red; but ferrous oxide (FeO) is black and will give the clay a blackish color. If no ferric oxide is present in the clay, that is, if the clay is not a red clay, then reduction has no effect on the color of the clay and can be freely used. In Europe potters regularly burn under reducing conditions, while in America the general practice is to burn under oxidizing conditions. When the draught in the kiln is faulty, partial reduction will often occur, and many faults in the burning, especially in the glazes, are attributable to this cause.
Fig. 39. Open kiln
We shall see presently how important is a knowledge of these chemical changes during the process of firing when we come to consider the defects on Greek vases caused by injuries in the firing (cf. [pp. 44 ff.]).
Types of kilns.
Fig. 40. Muffle kiln with biscuit ware
There are two chief types of kiln construction in use today: (1) the open kiln, in which the flame passes through the kiln chamber (fig. [39]); (2) the muffle kiln, in which the flame passes around the chamber and not through it (fig. [40]). In the open kiln the ware either comes in direct contact with the flame, or is stacked in saggers, i.e., boxes made of fire clay fitting one on top of the other (fig. [41]). The muffle kiln is, so to speak, one large sagger, and the ware is stacked on shelves. The draught in the kiln can be either an up draught or a down draught; in either case the air supply, as well as the fuel supply, must be under control, so that the combustion shall be as desired. The draught can be regulated by means of dampers, the fuel by attention to the burners. An arched top is an advantage, for it imparts greater strength—an important item considering the strain to which the kiln is subjected—and it facilitates the circulation of the heat. Coal, wood, gas, or kerosene oil can be used for fuel. Of these, oil and gas are now the most popular; coal and wood are rapidly coming into disuse on account of the greater labor they entail.