Raku is a form of Japanese pottery characterized by low firing temperature and the removal of pieces from the kiln while glowing hot. It is considered the traditional method for creating bowls for the Japanese tea ceremony which are made from earthenware clay, each with a unique shape and style. In the traditional process, the pot is removed from the hot kiln (around 1850 degrees F) and put directly into water or allowed to cool in the open air. Raku techniques have been adopted and modified by contemporary potters worldwide.
The western tradition of raku uses the addition of a reduction chamber once the pieces are removed from the kiln. Typically, pieces are removed from the hot kiln and placed in a can filled combustible material (leaves, sawdust, newspaper, etc.) to provide a reducing atmosphere for the glaze and the stain the exposed clay body surface with carbon.
The use of a reduction chamber was introduced by the American potter Paul Soldner in the 1960's to compensate for the difference in atmosphere between Japanese wood-fired raku kilns and gas-fired American kilns. Once the pieces are placed in the reduction chamber, the combustible material ignites and then a lid covers the can to remove the oxygen, extinguishing the flames. It is during this phase where all of the magic happens and the brilliant colors form as the flames remove oxygen from the glaze and change its chemical composition.
Although almost any low-fire glaze can be used in a raku firing, potters often use specially formulas that have high copper contents (for a rainbow of colors) or glazes that crackle and trap carbon in the glaze as they cool. Crackle glazes have black/grey lines in the surface to give a cracked appearance although the piece is not broken.
The glaze firing times for raku are short, and hour or two as opposed for 16+ hours for high temperature firings. This is due to several factors: raku glazes mature at much lower temperatures, kiln temperatures can be raised rapidly and the kiln is loaded and unloaded while hot and can be kept warm between firings. Because of the rapid temperature changes, the clay used must be able to withstand significant thermal stress. The usual way to combat thermal shot is to incorporate a high percentage of quartz, grog or kyanite into the clay before the piece is formed. These are used to increase strength and to reduce thermal expansion, which decreases the frequency of a piece cracking.
The process is known for its unpredictability, particularly when the reduction is forced and pieces may crack or even explode due to thermal shock. When all of the parts of the parts of the process come together, the result is a brilliant surface with a completely unique pattern that can never be repeated.