RAKU is actually a firing process rather than a specific style or clay. It was developed in Japan in the 16th Century. There, the thick raku cups, showing tong marks, crackles, and glaze runs, are highly sought after for the tea ceremony.

The piece is first bisque fired to cone 06 (1830 F.) Then it is glazed with various high "flux" (easily flowing) glazes, often containing copper. When the glaze is dry the piece is again fired to 1600-1800 degrees. At this temperature it is pulled out of the hot kiln with a pair of tongs or Kevlar gloves, and placed into a flammable agent (sawdust, straw, leaves, paper, etc.) in a container to smoke in a "reduced" (oxygen starved) atmosphere. Sometimes it is plunged into water after reducing for a while. This is what creates the beautiful play of crackle patterns and colors on the surface.

The clay used must be quite porous, usually a stoneware containing up to 30% sand or "grog" (ground previously fired clay) in order to tolerate the shock of the firing process. The ware is usually made a bit thicker to increase its strength. Because raku ware is so porous, it is not food safe and should NOT be exposed to liquids for long periods. The glaze could eventually flake or discolor, or germs may grow in the crackles. A plastic liner should be used inside flower vases or jardinieres.