Glazes are applied prior to being fired a second time using the Raku firing process, hence the name Raku Pottery. Japanese potters developed this technique in the 16th century for ceremonial tea settings. American potters expanded the technique in the 1960's, by including the post fire reduction phase.
The metallic glazes have a small percentage of copper carbonate, which leaves a thin layer of copper on the pottery during the firing process. Pottery is placed in a gas kiln and heated to 1800 degrees (red hot) and then removed with tongs and placed into a pit lined with pine needles. As the needles start to burn, a lid is used to smother the fire, which provides for reduction (lack of air), which takes the oxygen molecule out of the glaze and leaves only the copper. As the piece cools, air leaks under the lid and reoxidizes the glaze, which provides the various colors, from blue to green and yellow. The entire firing process takes about two hours. Careful inspection shows where the pine needles burn against the glaze and produce images of themselves in the glaze. Although the clay is white, the unglazed areas turn black as the smoke penetrates the clay body.
The colors on each piece are different, due the many variables in the firing process. The main variable is heat, which results in the many different colors on each piece, depending on how fast it cools. Rapidly cooled areas will keep more of the copper, while slower cooling areas (around the base) will have more time to develop a greater range of the colors.
White or colored crackle glazes are fired the same way. When the piece is placed in the pit, the glaze cracks and shrinks from the clay and allows the smoke to penetrate and make black lines in the glaze. After cooling, the cracks shrink and leave the black lines behind.
Since the temperature change is so rapid from kiln to pit, the pottery undergoes very large thermal stresses. About ten percent of the pottery will crack and is rejected to the pot pile. Pieces that survive the firing are relatively tough. Since the firing temperatures are so low, the clay does not vitrify (turn glasslike) and remains porous and should not be used to hold water or placed outdoors.
Gay Liebert
July 25, 2007.