The Raku Process
Thursday, July 17, 2008 at 2:37PM The film below is an example of a raku firing using sawdust in the end. Once it is completed you can view other short examples of raku firings from all over the world.
About Raku Firing
The term raku is derived from the site where clay was dug in Kyoto in the late 16th century and is found in the Kanji character meaning "enjoyment" or "ease." For 15 generations it has been the title and seal used by a lineage of potters whose work formed the central tradition in Japan. This lineage believes that 'raku' refers to the potters who use the technique, not the technique itself.
The use of a reduction chamber at the end of the raku firing was introduced by the American potter Paul Soldner in the 1960s to compensate for the difference in atmosphere between wood-fired Japanese raku kilns and gas-fired American kilns. Typically, pieces removed from the hot kiln are placed in masses of combustible material (e.g., straw, sawdust, or newspaper) to provide a reducing atmosphere for the glaze and to stain the exposed body surface with carbon.
Western raku potters rarely use lead as a glaze ingredient, due to its serious level of toxicity. Japanese potters substitute a non-lead frit. Although almost any low-fire glaze can be used, potters often use specially formulated glaze recipes that "crackle" or craze (present a cracked appearance), because the crazing lines take on a dark color from the carbon.
Because temperature changes are rapid during the raku process, clay bodies used for raku ware must be able to cope with significant thermal stress. The usual way to deal with this is to incorporate a high percentage of quartz, grog, or Kyanite into the body before the pot is formed. Each is used to add strength to the clay body and to reduce thermal expansion. When used at high additions, quartz can increase the risk of dunting or shivering therefore, Kyanite is often the preferred material because it contributes both mechanical strength and, in amounts up to 20%, it significantly reduces thermal expansion. Although any clay body can be used, most porcelains and white stoneware clay bodies are unsuitable for the western raku process unless some material is added to deal with thermal shock.
Aesthetic considerations include clay color and fired surface texture, as well as the clay's chemical interaction with raku glazes.
