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A 16th century black Raku-style chawan, used for thick tea (Tokyo National Museum)

Rakuyaki (樂焼き) or Raku (樂) is a form of Japanese pottery characterized by low firing temperatures (resulting in a fairly porous body), lead glazes, and the removal of pieces from the kiln while still glowing hot. In the traditional Japanese firing process, the pot is removed from the hot kiln and allowed to cool in the open air. Raku is considered the traditional method for creating bowls for the Japanese tea ceremony. Raku tea bowls are hand-made from earthenware, each with a unique shape and style.

The term raku is derived from the site from which clay was dug in Kyoto in the late sixteenth century. For fifteen generations, it has been the title and seal used by a lineage of potters whose work formed the central tradition in Japan. In the 16th century, the first of these potters, Chojirō (長次郎), is said to have come under the patronage of the Japanese tea master Sen-No-Rikyu. According to myth, in 1598, the ruler Hideyoshi, after chojiro's death in 1592, bestowed upon his adopted son, jokei, a golden seal with the written symbol "raku". Both the name and the ceramic style have been passed down through the family to the present. Raku was also made by numerous workshops in and around Kyoto; by amateur potter tea practitioners; and by professional and amateur potters around Japan after the publication of a manual in the 18th century.

Raku ware marked an important point in the historical development of Japanese ceramics, as it is the first ware to use a seal mark and the first to focus on close collaboration between potter and patron. Other famous Japanese clay artists of this period include Donyu (1574-1656), Hon'ami Kōetsu (1556-1637) and Ogata Kenzan (1663-1743).

A vase glazed and fired using the Western Raku technique, showing the soot, crackle glazing, and random oxidation typical of this pottery technique.

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 body surface with carbon.

Western raku potters rarely use lead as a glaze ingredient, due to its serious level of toxicity. Japanese potters also 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.

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, bodies 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 quartz, Grog (prefired clay that has been finely ground), or Kyanite into the body before the pot is formed. Each of these materials is used to add strength to the clay body and to reduce thermal expansion. When used for over 10% of the clay body, quartz can cause 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.

• Pitelka, Morgan. Handmade Culture: Raku Potters, Patrons, and Tea Practitioners in Japan. University of Hawaii Press, 2005. ISBN 0-8248-2970-0.
• Hamer, Frank and Janet. The Potter's Dictionary of Materials and Techniques. A & C Black Publishers, Limited, London, England, Third Edition 1991. ISBN 0-8122-3112-0.

• Peterson, Susan. The Craft and Art of Clay. The Overlook Press, Woodstock, NY, Second Edition 1996. ISBN 0-87951-634-8.

• Chawan

• Raku museum Kyoto
• e-yakimono.net
• Raku Glaze Recipes