A kiln is required to come to a high temperature, and so the design of the ovens normally focuses on insulation, and the ability to add fuel over a course of time. Care must be taken not to heat the kiln too much as clays are made of alumino and magnesium silicates which will degrade under high temperatures leaving oxides of aluminium, magnesium and silica, which can form glass at high temperatures.
Kilns have been made for as long as there has been pottery and items made of clay. The technology is thus very old. Early examples of kilns found in the United Kingdom, include those made for the making of roof-tiles during the Roman occupation. These kilns were built up the side of a slope, such that a fire could be lit at the bottom, and the heat would rise up into the kiln.
See also limekiln.