Firing

Firing produces irreversible changes in the body. It is only after firing that the article can be called pottery. In lower-fired pottery the changes include sintering, the fusing together of coarser particles in the body at their points of contact with each other. In the case of porcelain, where different materials and higher firing-temperatures are used the physical, chemical and mineralogical properties of the constituents in the body are greatly altered. In all cases the object of firing is to permanently harden the wares and the firing regime must be appropriate to the materials used to make them. As a rough guide, earthenwares are normally fired at temperatures in the range of about 1000 to 1200 degrees Celsius; stonewares at between about 1100 to 1300 degrees Celsius; and porcelains at between about 1200 to 1400 degrees Celsius. However, the way that ceramics mature in the kiln is influenced not only by the peak temperature achieved, but also by the duration of the period of firing. Thus, the maximum temperature within a kiln is often held constant for a period of time to soak the wares, to produce the maturity required in the body of the wares.

The atmosphere within a kiln during firing can affect the appearance of the finished wares. An oxidising atmosphere, produced by allowing air to enter the kiln, can cause the oxidation of clays and glazes. A reducing atmosphere, produced by limiting the flow of air into the kiln, can strip oxygen from the surface of clays and glazes. This can affect the appearance of the wares being fired and, for example, some glazes containing iron fire brown in an oxidising atmosphere, but green in a reducing atmosphere. The atmosphere within a kiln can be adjusted to produce complex effects in glaze.

Kilns may be heated by burning wood, coal and gas, or by electricity. When used as fuels, coal and wood can introduce smoke, soot and ash into the kiln which can affect the appearance of unprotected wares. For this reason wares fired in wood- or coal-fired kilns are often placed in the kiln in saggars; lidded ceramic boxes, to protect them. Modern kilns powered by gas or electricity are cleaner and more easily controlled than older wood- or coal-fired kilns and often allow shorter firing times to be used. In a Western adaptation of traditional Japanese Raku ware firing, wares are removed from the kiln while hot and smothered in ashes, paper or woodchips, which produces a distinctive, carbonised, appearance. This technique is also used in Malaysia in creating traditional labu sayu