Microstructure and Formation Mechanism of Ocherous Goma on Traditional Japanese Bizen Stoneware

Traditionally, Japanese Bizen stoneware is produced by firing shaped green clay in a firewood kiln at approximately 1200 °C using red pine as fuel. In some cases, an ocher coloration known as goma appears on the finished product due to reactions between the clay and firewood ash. This work shows that goma results from the formation of augite [(Ca,Mg,Fe)2Si2O6], plagioclase [(Ca,Na) (Si,Al)4O8] and glassy phases. Bizen ceramicware exhibiting goma was fabricated by heating a mixture of Bizen clay with an artificial ash based on the analysis of red pine ash and produced using chemical reagents. An ocher coloration appeared on the sample surface after heating a 70:30 (on a mass basis) mixture of clay with this artificial ash at 1220 °C for 5 h followed by annealing at 1100 °C for 2 h in an electric furnace.