The use of a reduction chamber at the end of the raku firing was introduced by the American potter Paul Soldner in the 1960s, in order 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) in order to provide a reducing atmosphere for the glaze, and to stain the exposed clay surface with carbon.
Western raku potters rarely use lead as a glaze ingredient, due to its serious level of toxicity. 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.
Western Raku is typically made from a stoneware clay body, bisque fired at 900°C (1650°F) and glaze fired (the final firing) between 800-1000°C (1450-1800°F), which falls into the cone 06 firing temperature range. The process is known for its unpredictability, particularly when reduction is forced, and pieces may crack or even explode due to thermal shock. Pots may be returned to the kiln to re-oxidize if firing results do not meet the potter’s expectations, although each successive firing has a high chance of weakening the overall structural integrity of the pot. Pots that are exposed to thermal shock multiple times can break apart in the kiln, as they are removed from the kiln, or when they are in the reduction chamber.
The glaze firing times for raku ware are short, an hour or two as opposed to up to 16 hours for high-temperature cone 10 stoneware firings. This is due to several factors: raku glazes mature at a much lower temperature (under 1800°F, as opposed to almost 2300°F for high-fire stoneware), kiln temperatures can be raised rapidly, and the kiln is loaded and unloaded while hot and can be kept hot between firings.
Because temperature changes are rapid during the raku process, clays used for raku ware must be able to cope with significant thermal stress. The usual way of dealing with this is to incorporate a high percentage of sand or Grog (prefired clay that has been finely ground) into the clay before the pot is formed. Although any clay body can be used, most porcelains and white stoneware clay bodies are unsuitable for the Western raku process unless grog is added.