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A kiln's purpose is to use heat to cause a number of chemical changes in objects made of clay. The most fundamental of these changes, dehydration, transforms dry fragile clay into a permanent material that is much stronger and will not dissolve in water. This chemical change begins to occur at about 1300o F and is the minimum temperature a ceramic kiln must reach in order to accomplish anything. Higher temperatures will vitrify the clay (making it very dense and much stronger), and melt glazes. Kiln Design and Construction Kilns, like ovens, are essentially heat containers. Kilns differ from ovens in that they are intended for relatively high temperatures and consequently employ more elaborate strategies in material and design. The ceramic technology has been discovered by almost every human culture that has advanced beyond hunting/gathering. The initial understandings of pot - making and firing invariably occur during a culture's earliest most primitive beginnings. It is possible to raise the temperature of a ceramic object to Dehydration temperatures by simply covering it with a sufficient amount of firewood for a bonfire. If the red hot coals produced by the fire are sufficient to bury the objects, there will be, just barely, enough temperature to dehydrate the clay. The clay will be porous and fragile but very useful to a culture that has few alternative ways of making containers.
While the bonfire will fire clay, it is very inefficient. Very little of the energy released by the fuel is transferred to the pots; most of the heat is simply exhausted into the air. Because of the high temperatures involved, ceramics always involve the use of large amounts of energy. The history of kiln design has primarily been a search for more efficient ways to trap the heat produced by a fuel. The easiest improvement over the simple bonfire is a shallow pit. The pit walls act as containers for the fuel- - keeping it close to the pots. The pit walls also act as a reflector helping to confine radiant energy produced by the fuel. A true Kiln is a heat container with a roof as well as walls and a floor. A sealed chamber with an exhaust port on top and a fire box at the bottom allows the potter to control combustion. Fuel can be burned slowly over a longer period of time and all of the fumes circulate through the pots before exhausting into the air.
As diagramed here, a simple version of a controlled combustion kiln using broken shards of old pots for a roof and the necks of broken bottles for a chimney are commonly used by third world cultures today. Although this type of kiln has to be substantially rebuilt every time it is fired it is vastly more efficient in the use of fuel than the open bonfire. Cultures familiar with arch structures are able to build kilns with a permanent roof that are loaded from the side through a simple door that is bricked up during firing. This type of kiln allows larger size and greatly simplified operation. Arched roof structures for kilns were used by the Romans and are still common today. An interesting variation on the arched roof, called a "Climbing Kiln" was developed by the Koreans and Chinese over a thousand years ago. This was essentially a series of kilns connected one behind another up the side of a hill. These kilns held a very large quantity of pots and were usually fired only a few times a year. Their advantage was in the more efficient use of fuel. With this type of kiln the still hot exhaust from the first kiln was not emptied into the air but passed through each succeeding chamber until all of the heat had been extracted from the gasses. Modern commercial ceramic factories use a variation on this concept called a tunnel kiln. These modern kilns are often 500 ft. long and fire continuously.
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