Raku is a process where pots are taken from the kiln while they are still glowing red and placing them in combustible material such as sawdust, dried leaves, newspaper etc. This starves the pot of oxygen and creates a reduced atmosphere round the pots. This reduction of oxygen gives the glaze on the pots their huge variety of colour. Any areas not glazed are carbonised by the burning material turning the clay charcoal to black colour. During the extreme temperature change from the kiln to container of combustible material crazing of the glaze very often occurs enhancing the pots surface still further with the crazing being accentuated by the carbon deposits in the cracks. A final scrub of the pot when cool to clean the surface of any loose carbon reveals the finished piece.

Raku means "enjoyment", "comfort" or "ease" and is derived from Jurakudai, the name of a palace, in Kyoto, that was built by Toyotomi Hideyoshi (1537–1598), who was the leading warrior statesman of the time.

In the 16th century, Sen Rikyū, the Japanese tea master, was involved with the construction of the Jurakudai and had a tile-maker, named Chōjirō, produce hand-moulded tea bowls for use in the wabi-styled tea ceremony that was Rikyū's ideal. The resulting tea bowls made by Chōjirō were initially referred to as "ima-yaki" ("contemporary ware") and were also distinguished as Juraku-yaki, from the red clay (Juraku) that they employed. Hideyoshi presented Jokei, Chōjirō's son, with a seal that bore the Chinese character for raku. Raku then became the name of the family that produced the wares. Both the name and the ceramic style have been passed down through the family (sometimes by adoption) to the present 15th generation (Kichizaemon). The name and the style of ware has become influential in both Japanese culture and literature.

In Japan, there are "branch kilns" (wakigama), in the raku ware tradition, that have been founded by Raku-family members or potters who apprenticed at the head family's studio. One of the most well-known of these is Ōhi-yaki (Ōhi ware).

After the publication of a manual in the 18th century, raku ware was also made in numerous workshops by amateur potters and tea practitioners in Kyoto, and by professional and amateur potters around Japan.

Raku ware marked an important point in the historical development of Japanese ceramics, as it was the first ware to use a seal mark and the first to focus on close collaboration between potter and patron. Other famous Japanese clay artists of this period include Dōnyū (grandson of Chōjirō, also known as Nonkō; 1574–1656), Hon'ami Kōetsu (1556–1637) and Ogata Kenzan (1663–1743). Internet shows the original Raku family museum at http://www.raku-yaki.or.jp/e/. Here the complete lineage of the Raku family can be studied as well as the Raku literature written by the present Raku master, Raku Kichizaemon XV.

Oxidation Reduction

In the western style of raku firing, the aluminium container acts as a reduction tube, which is a container that allows the carbon dioxide to pass through a small hole. A reduction atmosphere is created by closing the container. A reduction atmosphere induces a reaction between oxygen and the clay minerals, which affects the color. It also affects the metal elements of the glaze. Reduction is a decrease in oxidation number. Closing the can reduces the oxygen content after the combustible materials such as sawdust catch fire and forces the reaction to pull oxygen from the glazes and the clay. For example, luster gets its color from deprivation of oxygen. The reduction agent is a substance from which electrons are being taken by another substance. The reaction uses oxygen from the atmosphere within the reduction tube, and, to continue, it receives the rest of the oxygen from the glazes. This leaves ions and iridescent lustre behind. This creates a metallic effect. Pieces with no glaze have nowhere to get the oxygen from, so they take it from clay minerals. This atmosphere will turn clay black, making a matte colour.

Reduction Firing.

Reduction firing is when the kiln atmosphere, which is full of combustible material, is heated up. (Reduction is incomplete combustion of fuel, caused by a shortage of oxygen, which produces carbon monoxide). Eventually, all of the available oxygen is used. This then draws oxygen from the glaze and the clay to allow the reaction to continue. Oxygen serves as the limiting reactant in this scenario because the reaction that creates fire needs a constant supply of it to continue; when the glaze and the clay come out hardened, this means that the oxygen was subtracted from the glaze and the clay to accommodate the lack of oxygen in the atmosphere. Consequently, the raku piece appears black or white, which depends upon the amount of oxygen that was lost from each area of the piece. The empty spaces that occur from the reduction of oxygen are filled in by carbon molecules in the atmosphere of the container, which makes the piece blacker in spots where more oxygen was retracted.

Western Raku Techniques

Warren Gilbertson is credited as being the first American to experiment with Raku having learned it while stationed in Japan prior to 1940. After reading about the technique in Bernard Leach's A Potters Book, Hal Reigger and a group of friends including Jean Griffith began to experiment with Raku in the mid 1940s in the Pacific Northwest. raku became popularized in America in the late 1950s with the help of Paul Soldner. Americans kept the general firing process, that is, heating the pottery quickly to high temperatures and cooling it quickly, but continued to form their own unique style of raku.

It is raku's unpredictable results and intense color that attract modern potters. These patterns and color result from the harsh cooling process and the amount of oxygen that is allowed to reach the pottery. Depending on what effect the artist wants, the pottery is either instantly cooled in water, cooled slowly in the open air, or placed in a barrel filled with combustible material, such as newspaper, covered, and allowed to smoke. Water immediately cools the pottery, stopping the chemical reactions of the glaze and fixing the colors. The combustible material results in smoke, which stains the unglazed portions of the pottery black. The amount of oxygen that is allowed during the firing and cooling process affects the resulting color of the glaze and the amount of crackle.

Unlike traditional Japanese raku, which is mainly hand built bowls of modest design, western raku tends to be vibrant in colour, and comes in many shapes and sizes. Western raku can be anything from an elegant vase, to an eccentric abstract sculpture. Although some do hand build, most western potters use throwing wheels while creating their raku piece. Western culture has even created a new sub branch of raku called horse hair raku. These pieces are often white with squiggly black lines and smoke-like smudges. These effects are created by placing horse hair, feathers, or even sugar on the pottery as it is removed from the kiln and still extremely hot.

Kilns and Firing

The first Japanese-style kiln in the west was built by Tsuronosuke Matsubayashi at Leach Pottery, St Ives in 1922.

A vase glazed and fired using the western raku technique, showing the soot, crackle glazing, and random reduction-oxidation typical of this pottery technique.

The type and the size of kilns that are used in raku are crucial in the outcome. One aspect that can affect the results is the use of electric versus gas kilns. Electric kilns allow easy temperature control. Gas kilns, which comprise brick or ceramic fibers, can be used in either oxidation or reduction firing and use propane or natural gas. Gas kilns also heat more quickly than electric kilns, but it is more difficult to maintain temperature control. There is a note-worthy difference when using an updrift kiln rather than a downdrift kiln. An updrift kiln has shelves that trap heat. This effect creates uneven temperatures throughout the kiln. Conversely, a downdrift kiln pulls air down a separate stack on the side and allows a more even temperature throughout and allows the work to be layered on shelves.

It is important for a kiln to have a door that is easily opened and closed, because, when the artwork in the kiln has reached the right temperature (over 1000 degrees Celsius), it must be quickly removed and put in a metal or tin container with combustible material, which reduces the pot and leaves certain colours and patterns.

The use of a reduction chamber at the end of the raku firing was introduced by the American potter Paul Soldner in the 1960s to compensate for the difference in atmosphere between wood-fired Japanese raku kilns and gas-fired American kilns. Typically, pieces removed from the hot kiln are placed in masses of combustible material (e.g., straw, sawdust, or newspaper) to provide a reducing atmosphere for the glaze and to stain the exposed body surface with carbon.

Western raku potters rarely use lead as a glaze ingredient, due to its serious level of toxicity, but may use other metals as glaze ingredients. Japanese potters substitute a non-lead frit. Although almost any low-fire glaze can be used, potters often use specially formulated glaze recipes that "crackle" or craze (present a cracked appearance), because the crazing lines take on a dark color from the carbon.

Western raku is typically made from a stoneware clay body, bisque fired at 900 °C (1,650 °F) and glaze fired (the final firing) between 800–1,000 °C (1,470–1,830 °F), which falls into the cone 06 firing temperature range. The process is known for its unpredictability, particularly when reduction is forced, and pieces may crack or even explode due to thermal shock. Pots may be returned to the kiln to re-oxidize if firing results do not meet the potter's expectations, although each successive firing has a high chance of weakening the overall structural integrity of the pot. Pots that are exposed to thermal shock multiple times can break apart in the kiln, as they are removed from the kiln, or when they are in the reduction chamber.

The glaze firing times for raku ware are short: an hour or two as opposed to up to 16 hours for high-temperature cone 10 stoneware firings. This is due to several factors: raku glazes mature at a much lower temperature (under 980 °C or 1,800 °F, as opposed to almost 1,260 °C or 2,300 °F for high-fire stoneware); kiln temperatures can be raised rapidly; and the kiln is loaded and unloaded while hot and can be kept hot between firings.

Because temperature changes are rapid during the raku process, clay bodies used for raku ware must be able to cope with significant thermal stress. The usual way to add strength to the clay body and to reduce thermal expansion is to incorporate a high percentage of quartz, grog, or kyanite into the body before the pot is formed. At high additions, quartz can increase the risk of dunting or shivering. Therefore, kyanite is often the preferred material, as it contributes both mechanical strength and, in amounts up to 20%, significantly reduces thermal expansion. Although any clay body can be used, white stoneware clay bodies are unsuitable for the western raku process unless some material is added to deal with thermal shock. Porcelain, however is often used but it must be thinly thrown.

Aesthetic considerations include clay color and fired surface texture, as well as the clay's chemical interaction with raku glazes.

Raku in the west has been abstracted and is now a more philosophical approach with the emphasis on the spontaneity of surface pattern creation rather than purely a firing technique. Consequently this has expanded its application from pots to sculptural ceramics.