Effects of rice straw on the color and microstructure of bizen, a traditional japanese stoneware, as a function of oxygen partial pressure

The effects of oxygen partial pressure during thermal treatment on the color and microstructure of Bizen, a traditional Japanese stoneware, were studied through model experiments using clay pellets covered lightly with rice straw as a coloring assistant. When heated in flowing nitrogen, the model pellet turned blackish owing to the formation of α-Fe particles coated with graphite. However, schreibersite (Fe₃P), which is also blackish, was formed specifically on the pellet surface in direct contact with the straw. The rice straw seems to have generated a strongly reducing atmosphere, strong enough for the metallization to α-Fe, and also to have provided phosphorus through contact. When oxygen content in the surrounding gas atmosphere was raised to N₂/O₂=99/1, the pellet surface turned yellowish brown because the main coloring material was Fe³⁺-containing mullite. At oxygen contents of N₂/O₂=98/2 or more, the formtion of hematite (α-Fe₂O₃) pushed the color to deep red.