Raku is a ceramic technique which originated in sixteenth century Japan and Korea. Most types of ceramics are loaded into cold kilns and then slowly heated for periods of from 8 to 24 hours until the final temperature is reached; and then the kiln is allowed to cool slowly. By contrast, in the raku technique preheated pieces of bisque-fired pottery are placed in a hot raku kiln so that the pottery reaches its final temperature in less than half an hour. Instead of using measuring devices such as cones to monitor the firing, the maturity of the glaze is judged by eye, through the peepholes in the kiln. When firing is completed the still-hot wares (with still-molten glaze) are removed from the kiln immediately with tongs. In the modern version of the traditional Japanese raku technique, the wares are then placed in a container with sawdust or leaves and allowed to smoke. This post-firing reduction imparts interesting surfaces and effects to the pottery, such as metallic or crackled surfaces. The finished wares are washed when cool with abrasives to remove the soot and ash residue. The effect of the reduction process depends upon the type of the combustible material, the size of the particles (wood chips, shavings, or sawdust), and the dampness of the material used in the reduction chamber.
Raku clays optimally should contain a considerable amount of fireclay or other refractory material, which can withstand the sudden heating of the raku technique. The clay should be sufficiently open to contract and expand without its cracking. Most any low-fire glazes, up to cone 06 glazes, can be used to make raku ware. However, if a variety of glazes are used on the same piece, or on pieces in the same kiln load, then they should mature at the same final temperature. The unique bright metallic effects and crackle lines which are typical of the raku technique are achieved by fast firing to the maximum maturity (glaze melt) and then fast reduction after firing. It is important to remove the ware from the kiln to the reduction chamber and cover it before it has had much chance to cool. Crackle lines also depend upon thick applications of the glaze. Brighter colors can be achieved by spraying the parts of the ware with water, or holding it in the air for half a minute, before placing it in the reduction chamber; this oxidizes and cools the glaze.
Modern raku technique was popularized by Paul Soldner after the Second World War. Although traditional raku bowls were used in the Japanese tea ceremony, most raku ware is porous and fragile, making it unsuitable for actual use. Unless the raku is treated with a sealant such as acrylic or polyurethane, the pottery will sweat and break down eventually. Moreover, the low fire glazes used in raku ware are quite soft and easily chipped (and thus possibly ingested). Therefore, if raku ware is used for foods it should be limited to dry food.