Raku dates from 16th century Japan where the process was originally used to create teabowls for the tea ceremony. The glazed pieces were fired in a kiln to around 1000°C, then removed while red hot and either left to cool or plunged into water to snap cool them.
This traditional raku technique was modified by 'western' potters in the mid 20th century with the inclusion of an additional step, sometimes referred to as post-firing reduction, western raku, or smoking.
When the red hot pieces are removed from the kiln they are placed into a bin containing combustible materials, such as wood shavings or shredded newspaper. The materials ignite and the bin is then sealed with a lid to create a highly oxygen-starved reduction atmosphere which interacts with the glaze constituents (such as copper). The high-carbon smoke levels in the bin at this time also penetrate any unglazed clay on the artwork, colouring it black.
Glazes such as crackle glazes make use of the carbon effect by allowing the smoke to penetrate through the crazed glaze and blacken the claybody beneath, highlighting the network of crazing or crackles.
There is a fascinating sense of involvement and immediacy with raku firings; interaction, smoke, flame etc make for an exciting, spectacular event. Firings are typically completed in 45 minutes or so, compared to many hours for a 'normal' pottery firing.
The massive and rapid temperature changes that are imposed on raku work put incredibly high stresses on the claybody. Combined with the relatively low firing temperatures reached in the raku process (1000°C vs 1300°C for stoneware), the finished artworks are inclined to be somewhat more fragile and porous.
Raku works are therefore intended for decorative art, rather than functional pottery. They will not hold water unless a sealant has been applied to the inside of the piece.