Microstructure and formation process of the characteristic reddish color pattern Hidasuki on Bizen stoneware: Reactions involving rice straw

The formation process of Hidasuki, a characteristic reddish coloring pattern on traditional Japanese unglazed stoneware called Bizen, was studied through model experiments. Pellets of the same type of clay used to form Bizen stoneware were heated to 1250 °C with and without contact with rice straw and then were cooled at different rates. A reddish color appeared for relatively slowly cooled samples when rice straw was present. Owing to the presence of potassium in the rice straw, the mullite (3(Al,Fe)₂O ₃·2SiO₂) the major phase formed in the absence of rice straw, was replaced by corundum (α-Al₂O₃), hematite (α-Fe₂O₃), and others in the surface region of about 50 μm in depth. The corundum precipitated as hexagonal platelike crystals, and on the edges of these crystals the hematite grew epitaxially. The growth continued so that the primary corundum crystals were wholly covered by hematite to form a specific single crystalline α-Fe₂O ₃/α-Al₂O₃/α-Fe₂O ₃ structure. The contribution of this unique microstructure to coloring is discussed. On the basis of these analytical results, the Hidasuki pattern was reproduced artificially in the form of written characters.