Raku is a traditional Japanese firing technique that originated in the 16th century in Japan.  Japanese raku was wood fired at a fairly low temperature, removed from the kiln while still glowing, and rapidly cooled.  This technique was very much associated with the Japanese Tea Ceremony.  For 15 generations the term "Raku" referred not only to this process, but to the family that developed and preserved this tradition.

 

Modern day raku has been modified somewhat by Western potters, but the basic process remains much the same.

The pottery is first bisque fired (at about 900 degrees).  The piece is then glazed and fired in a relatively cool kiln (800-1000 degrees).  The piece is removed from the kiln while still glowing and is usually buried in a combustible material (such as sawdust, newspaper, etc.) and allowed to cool.  This created an environment of "reduction" which helps to create some of the unique effects that we see in raku pottery.  The rapid heating and cooling of raku firing requires the addition of grog to the clay, to absorb some of the shock.  Raku pottery is somewhat porous and is not really suitable for holding water over a long period of time.

It is, however, quite desirable for its unique character.