I began working in clay in 1981, and have worked in high-fired porcelain and stoneware since 1988.
All of my glazes will vary from piece to piece. The Copper Red, however, is far more unpredictable than the others. Although temperature can effect the color, the kiln atmosphere during the firing is the principal cause of color variation. Kiln atmosphere in a fuel burning kiln – I use propane – is controlled by limiting the amount of oxygen available. There are a number of ways of accomplishing this, but in my case I use a damper in the chimney, just as you would in a woodstove. Restricting the amount of oxygen causes a “Reduction” atmosphere, and this is what causes the copper to turn red. (Copper in its oxidized state is green.)