Science in the art of the master Bizen potter

Bizen stoneware, with the characteristic reddish hidasuki or "fire-marked" pattern, is one of Japan's best known traditional ceramic works of art. The means of creating and controlling the various hues of the hidasuki pattern has remained a mystery to outsiders for about a thousand years; the methods were known only to master potters who served under generations of master potters before them. In this Account, we present the results of 30 years of study in which we investigated the microstructure and color-formation process in Bizen stoneware. We discovered that the hidasuki pattern results from the precipitation of corundum (alpha-Al(2)O(3)) and the subsequent epitaxial growth of hematite (alpha-Fe(2)O(3)) around it in a approximately 50-microm-thick liquid specifically formed in the ceramic surface. The epitaxial composites include hexagonal plate-like alpha-Fe(2)O(3)/alpha-Al(2)O(3)/alpha-Fe(2)O(3) sandwiched particles and also surprisingly beautiful flower-like crystals, centered by hexagonal corundum crystals and decorated by several hexagonal hematite petal crystals. Bizen stoneware is produced from a unique clay that can only be mined from the Bizen area of Okayama Prefecture, Japan. The clay has an unusually high Fe content compared with the traditional porcelain clay, as well as Si, Ca, Mg, and Na. Prior to firing, the Bizen works are wrapped in rice straw that was used originally as a separator to prevent adhesion. The hidasuki pattern only appears where the rice straw is in direct contact with the clay; the rice straw supplies potassium, which reduces the melting point of the ceramic surface, thereby converting the contact area into a site for these reactions to take place. The effect is almost accidental and is produced without the aid of any artificial glazing and enameling. An unexpected variety of substances, including metallic iron coated by graphite, Fe(3)P, and epsilon-Fe(2)O(3), were also found to appear at low oxygen partial pressures. Many of the techniques used by master potters are passed down through an apprenticeship system; an unfortunate consequence is that they are poorly documented. Moreover, the masters of these techniques are often unaware of the underlying chemical reactions that take place. Chemical studies of traditional processes can provide new inspiration to artists, allowing them to control the various factors and thus produce new works, and perhaps new functional materials. We studied the process of creating Bizen stoneware and then mimicked the color-producing process under controlled laboratory conditions, demonstrating the possibilities of the endeavor.