Pumice Powder
A grey-green or black material which produces a grey colour in lead glazes and brown in glazes containing barium; in the presence of calcium the colour is tan and in high magnesium glazes a bright green. With small quantities of zinc, a steel blue develops while with larger quantities it is lavender blue, in stoneware glazes under reduction. Nickel is most often used to modify other colours (up to 5%).?