This pottery technique originated in Japan. in the late 16th century. At this time, RAKU potters were producing wares expressively for the Japanese tea ceremony. RAKU, meaning "pleasure" or "enjoyment" was not introduced to the western world until as late as the first half of the last century.
RAKU is a process of taking pots, while they are still glowing red, (about 1800 F) from the kiln and placing them immediately into closed containers filled with combustible material, such as sawdust. dried leaves, newspaper, etc. While the pots are in these containers a reduction or carbonization process begins as soon as the hot pots ignite the combustible materials. Lids are then put on the containers to create a totally smoked filled atmosphere. The end result is that any unglazed areas on the pots will absorb and turn black from the smoke. During this extreme temperature change of cooling down, crazing or cracking occurs on many of the glazed areas of the pots.
The cracks may make the RAKU pots fragile, and the relatively low temperatures compared to high fired Stoneware and Porcelain, make the pots and the glaze soft, and are therefor not recommended for food or drinks because of the cracks, and the chance of lead poisoning from some types of the glazes.