Raku refers to a firing process rather than a type of clay. Although special clay bodies are available that are optimized for the raku firing process, it is possible to use normal earthenware or stoneware bodies if care is taken. Raku firing was developed by Chojiro in Japan in about 1580. The ruler Hideyoshi was so impressed with this pottery that, in 1598, he gave Chojiro a gold seal on which was engraved the ideograph for the word raku which is loosely interpreted as 'enjoyment'. Raku thereby became Chojiro's family title.
What is practiced today in the United States is a western variation of raku process developed by Paul Soldner and others in the 1960s. Today the western raku process consists of glaze firing a pot in a kiln that can be readily opened. It is normally a gas-fired kiln; although the process can be carried out in an electric kiln in a less efficient way. The pot is carefully watched through peep-holes in the kiln and when the glaze has melted to the point is has a glossy appearance, the kiln is opened. The pot is picked up with tongs and plunged into a container of organic material--usually shredded newspaper, leaves or sawdust-- and the container is sealed for 10-20 minutes. Lots of smoke and fire and excitement surround this process. In the sealed container the metal oxides in the glaze and the clay are fully or partially reduced and beautiful colors and patterns of color result. On opening the sealed container (usually a garbage can or something equivalent) the pot, which is still very hot, is removed and plunged into water and lots of steam and hissing noises ensue. While raku firing needs to be supervised by someone who is skilled, it is great fun and, when demonstrated, never fails to attract a crowd.
The pottery obtained by the western raku process, is always quite porous and the glaze is often purposely cracked and/or pitted. These pots are, therefore, only for decorative purposes and are not for food or drink. In Japan, raku is used to make teacups for their tea ceremonies so they are processed in a way that makes them suitable for that use.