Ash glaze

Ash glazes are ceramic glazes made from the ash of various kinds of wood or straw. They have historically been important in East Asia, especially Chinese pottery, Korean pottery, and Japanese pottery. Many traditionalist East Asian potteries still use ash glazing, and it has seen a large revival in studio pottery in the West and East. Some potters like to achieve random effects by setting up the kiln so that ash created during firing falls onto the pots; this is called "natural" or "naturally occurring" ash glaze. Otherwise the ash is mixed with water, and often clay, and applied as a paste.

Ash glazing began around 1500 BC, in China during the Shang Dynasty, initially by accident as ash from the burnt wood in the kiln landed on pots. Around 1000 BC, the Chinese apparently realized that the ash covering the pieces was causing the glaze so they started adding the ash as a glaze before the pot went into the kiln. Ash glaze was the first glaze used in East Asia, and contained only ash, clay, and water.

The ceramic flux in ash glazes is calcium oxide, commonly known as quicklime, and ash glazes are part of the lime glaze family, not all of which use ash. In some ash glazes extra lime was added to the ash, which may have been the case with Chinese Yue ware. A relatively high temperature of around 1170 °C is required, high enough to make the body into stoneware or (above about 1200 °C and with the right materials) porcelain.

The glaze has glasslike and pooling (buildup of glaze) characteristics which puts emphasis on the surface texture of the piece being glazed. When the glaze is mostly made up of wood ash, the final result is mostly dark brown to green. The pots with these glazes resemble the earth in color and texture. As the ash percentage decreases, the artist has more control on the color and the final glaze color, using wood, differs from light to dark shades of brown or green, if no other coloring agents are added. Rice-straw ash glaze produces an opaque creamy-white glaze; it is high in silica. If the ash is very thick, there may be sufficient phosphorus to give an "opalescent blue"; rice-husk ash is good for this.

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Wikipedia