Kilns for High Firing
The design, construction, and operation of pottery kilns is a fascinating subject, and the potter who must depend on others to fire his ware is missing half of the fun of making pottery. He is also missing the chance to complete his work in the process of firing, which is just as creative and requires just as much knowledge, skill, and experience as other phases of the craft.
In acquiring a new kiln for making stoneware or porcelain, a decision must first be made as to the kind of fuel to be used. Natural gas is the ideal fuel, but since it is not available everywhere, and since gas-burning kilns require flues or chimneys which may be impossible to provide, an electric kiln may be the only possibility. Next it must be decided whether to build a kiln permanently in place, or to buy a ready-made portable kiln. Electric kilns are commonly portable, even fairly large ones. Portable kilns for firing with gas or oil are heavy, bulky, and expensive; but if one is working in rented space or does not wish to invest in a permanent kiln, the portable kiln may be the best type to get. Portable kilns made by established manufacturers usually work well. But stationary kilns which are built in place, if properly designed and constructed, are not only cheaper than portable ones but work better and last much longer.
Kilns are actually very simple contrivances--in one form or another they are all insulated refractory boxes which retain the heat put into them either by the combustion of some fuel or by the radiant heat of the electrical heating element. The reason pottery cannot be fired in an open bonfire is not so much the lack of sufficient heat from the fire as the fact that the heat from an open fire is rapidly dissipated by radiation and by convection away from the fire, and while the coals or embers of the fire may be red hot, continuous burning of quantities of fuel is required to keep them that way.
The first pottery was probably fired by simply placing the pots on the ground and building a fire around them. The fire was started slowly at first and then built up into a mass of red-hot embers surrounding the pots. At the height of the fire, wet grass, manure, or a mixture of grass and mud was thrown over the fire; this retained the heat, allowing it to penetrate the pottery and bring it to red heat throughout. Some primitive peoples still fire their ware this way. The trouble with this system is that even the pots which survive the open fire, and perhaps a third of them crack, are only very lightly fired and are therefore sure to be soft and porous. A great improvement over the open fire was the use of a pit for firing. The pots were put in a shallow pit or trench and were protected by layers of broken pots. The fire was then kindled around the pots, and continued until the whole pit was filled with red-hot embers. The earth around the pit served as insulation to retain the heat. When the