Glazing is the process of coating the piece with a thin layer of material that during firing forms a glass coating. Compositions are varied but are usually a mixture of minerals that fuse at temperatures lowers than the body itself. This is important for functional earthenware vessels, which would otherwise be unsuitable for holding liquids due to porosity.
Glaze may be applied by dusting it over the clay, spraying, dipping, trailing or brushing on a thin slurry of glaze and water. Brushing tends not to give very even covering, but can be effective with a second coating of a coloured glaze as a decorative technique. With all glazed items, a small part of the item (usually on the base of the piece) must be left unglazed, or else it will stick to the kiln during firing.
Some clays and glazes are oxygen-sensitive, most notably those containing iron and copper, and will change colour depending on the presence of oxygen during the firing. Kilns can either be “oxidized” by opening a port to allow oxygen into the interior or “reduced” by closing off the kiln from outside air to attain colors as desired.
A number of various firing techniques can be used in addition to normal glaze-firing. Most of these involve heating the kiln to a high temperature and then delivering an amount of dry chemical into the kiln’s interior. Sulphur is commonly used, as are various salts or ashes. Such substances will stick to pieces within the kiln and melt onto their surfaces, often resulting in a mottled texture which has a distinctive “orange peel” feel. Colors generally depend on what chemical is added to the kiln. These techniques can have very unusual and frequently unexpected results whether used on an unglazed piece or in combination with normal glazing.
Wood firing is another type of firing which involves using wood, rather than gas or electricity as in most modern kilns, to heat the kiln’s interior. An example of a wood fired kiln is the Chinese Anagama, also adopted and used by Korean and Japanese potters. Wood firing is frequently time-consuming, as the kiln must be stoked for days, but the pieces which emerge often have characteristic patches of orange color on the clay itself, known as “blushing”.
The Western adaptation of Raku firing, a traditional Japanese technique, has enjoyed a deal of popularity due to its relative ease. The kiln is heated to a low temperature, usually no higher than cone 06, and then ware is pulled out of the kiln while still hot (using tongs, of course) and smothered in ashes, paper, or woodchips. This can be done in an enclosed container, which allows the supply of oxygen to be cut off and reduction to take place. The finished products of this process are not suitable for functional use, as the clay remains porous and may have some toxic chemicals held within it as a result of burning the surrounding woodchips or paper used to smother it. However, because of the low temperature, it is an extremely quick and easy technique to do, and the clay has a distinctive black color.
