Raku is frequently associated with Zen Buddhism, and the Japanese Tea Ceremony. It was developed in Japan in the 16th century. The word Raku means "joy" or "happiness".
The process of Raku firing and naked raku firing differs from other firing methods because the forms are removed from the kiln at their maximum temperature.My forms are heated to 1000-1050 degrees C, the kiln is opened and each molten glazed form is removed with a pair tongs. The extremely hot forms are placed into containers of sawdust which produces thick black smoke. The carbon is absorbed into the porous clay body, blackening the clay and accentuating the crackle pattern of the glaze.
Thermal shock of this rapid cooling is stressful on the forms. It is achieved by using an open clay body. The porosity of the clay body acts like a shock absorber, preventing the body from immediately fracturing when the form is removed from the kiln. Raku glazes are often fractured, which are referred to as crazing. These crackle glazes are enhanced by the post firing smoking of raku forms that embeds carbon into the crackles of the glaze.
When the forms have cooled, they are removed from the smoking chamber and doused with water. The soot covered forms are scrubbed clean to expose the crazed surface and unusual patterns created by this firing process. Because of the porous aspect of the clay body and crazing of the glazes, raku pots are not watertight.
Their function is in their beauty.
In naked raku, I take a bisque fired form and put a slip over its surface. This slip is used as a barrier between the form and the glaze so that they will separate from the form after firing. The fracturing action of the slip/glaze layer when penetratedby smoke leaves a soft crackle finish stained into the form's surface which is striking yet subtle in comparison to crackle raku glaze.
I usually paint 1 to 2 coats of the slip, although sometimes I pour on, or dip if the form is small enough for a single coat. The glaze is painted or poured over the slip in a thin single coat. I make sure no glaze is in direct contact with the form it should only be over the slip. Any glaze which touches the surface will stick, so I make sure to wipe off any that does not have a slip barrier between it and the surface. Once I have fired the forms and reduced them in a metal bin for at least an hour, I remove them and scrub gently with a sponge under running water to get any remaining traces of slip off the form.
The bisque fired pieces are painted/dipped with the chemical ferric chloride and wrapped in aluminium foil with a combination of various combustibles. These combustibles such as seaweed, poppy seeds, sawdust, sugar or salt all cause different effects. During the firing process the combustibles and ferric react to one another producing a rainbow of colour on the piece. The foil saggars trap the fumes and smoke allowing them to penetrate the clay body, it is impossible to repeat the same surface finish. Varing the firing tempurature and times also creates a variety of colour differences.