The characteristic reddish color pattern called hidasuki appears where the rice straw is in direct contact with the clay because the rice straw supplies potassium that reduces the melting point of the ceramic surface and thereby converts the contact area into a site for the interesting reactions to take place. The sticklike mullite (3(Al,Fe)₂O₃・2SiO₂) particles, 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-thick. The corundum precipitated as hexagonal platelike crystals, and on the edges of these crystals the hematite grew epitaxially.