Raku ware (楽焼 raku-yaki) is a type of Japanese Pottery that is traditionally used in the Japanese Tea Ceremony, most often in the form of tea bowls. It is traditionally characterised by being hand shaped rather than thrown; fairly porous vessels, which result from low firing temperatures; lead glazes; and the removal of pieces from the kiln while still glowing hot. In the traditional Japanese process, the fired raku piece is removed from the hot kiln and is allowed to cool in the open air. The familiar technique of placing the ware in a container filled with combustible material introduced by Paul Soldner, is not a traditional Raku practice. Raku techniques have been modified by contemporary potters worldwide.
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.
Raku is a unique form of pottery making; what makes it unique is the range of designs that can be created by simply altering certain variables. These variables—which include wax resist, glazes, temperature, and timing—ultimately determine the outcome when firing a piece of clay. Wax resist, which is painted over the bare untainted clay, results in the suspension of wax in water before the raku glaze goes on. This is done so that the glaze does not cover the area where the wax resist was applied, thus creating a design. When in the kiln, the wax melts off and the carbon, that results from oxygen reduction, replaces the wax. This is the result of the combustion reaction. Raku glazes contain alumina, which has a very high melting point. Therefore, carbon will not replace the glaze as it does the melted wax. Raku glazes also contain metallic compounds such as copper, iron, and cobalt; which produce different colors. After the glaze has reached a certain temperature, the metal in the glaze reacts taking on a specific color. For example, cobalt produces dark-blue, and copper produces green but can also produce a red when the oxygen in the glaze is completely gone. Any unglazed areas turn black due to the carbon given off from the reduction of oxygen. Next, the clay is moved from the kiln to a container, usually a trashcan, which contains combustible organic materials such as leaves, sawdust, or paper. Once the lid of the container is closed, the reduction oxidation (redox) process begins. The temperature change from the kiln to the container is where the magic of raku occurs. The change in temperature and in the redox sometimes cause cracking or crazing. Crazing is a consistent cracking in the glaze of a piece, as is seen on the white crackle glaze. This either enhances or detracts from the design. The timing of removal and placement in water directly affects the shades of each color. Introducing the water to the raku at the right time is incredibly important; if the raku does not cool enough before placement in water, it can crack, break, or even explode.
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 miner als. 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 luster 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 color.
Horse hair raku
Is a method of decorating pottery through the application of horsehair and other dry carbonaceous material to the heated ware. The burning carbonaceous material creates smoke patterns and carbon trails on the surface of the heated ware that remain as decoration after the ware cools. Although preparation is similar to pit fired pottery and other primitive firing techniques, Horsehair raku it is generally considered an alternative form of Western style Raku ware, because it uses Western-style Raku kilns, firing techniques and tools.
Horsehair raku usually utilizes burnishing and/or Terra sigillata techniques to prepare the unglazed surface before biscuit firing. The bisque ware is heated in a kiln, then removed while still extremely hot. The decorating is performed when the ware is between 480 and 700°C; lower temperatures do not effectively combust the horsehair and other materials, while higher temperatures cause the carbon makings to burn off leaving no lasting decorative effect. Strands of horsehair laid across hot ware leave a wandering linear smoke design on the surface. Other materials which can be used to create surface effects include sugar sprinkled on the hot surface to leave spotted smoke marks and feathers applied to the hot surface to give dark, feather shaped silhouettes. Additionally, leaves can be used to attain a leaf shaped pattern on biscuit surface. Success with horsehair has been achieved at temperatures up to 1000°C.