Studying and Utilizing Traditional Technologies: Microstructure and Formation Mechanism of ε-Fe₂O₃ on Traditional Japanese Bizen Stoneware

Traditional Japanese Bizen stoneware is produced by firing a specific type of green clay in a wood-fired kiln at approximately 1200 °C. During this process, single crystalline branched dendrite-like particles of Al-substituted ε-Fe₂O₃ (ε-Fe_1.7Al_0.3O₃) with widths and lengths of approximately 15 and 30 μm, respectively, are formed on the surface of the ceramic. Composite particles consisting of ε-Fe₂O₃ epitaxially connected to spinel structure compounds [comprising the Fe-substituted spinel (Mg,Fe)(Al,Fe)₂O₄ and γ-Fe₂O₃)] with lengths of approximately 3 μm are also generated. The present work clarified the crystallographic relationship between ε-Fe₂O₃ and the spinel structure compounds. In addition, brown-colored samples similar to Bizen pottery and with surface Al-substituted ε-Fe₂O₃ particles were prepared by heating clay with K₂CO₃ under a 10 vol % CO gas and 90 vol % Ar gas mixture using an electric furnace instead of a firewood kiln. Hence, a traditional method was adapted to achieve the industrial production of ε-Fe₂O₃ crystals.