The traditional Japanese overglaze ceramic, akae porcelain, typically known for Kakiemon-style wares, has been prized around the world for over 400 years. The red color of akae porcelain provides an attractive contrast to white porcelain. In recent years, because the use of leaded frit in making akae has been restricted by law owing to the effects of lead on human health and the environment, lead-free glass frit mixtures for overglaze ceramic wares have been developed in various porcelain production areas throughout Japan. However, current lead-free frit for red overglaze enamels produces a color that is duller than that of conventional leaded frit. Therefore, the development of red pigments that are suitable for use with lead-free frit is strongly desired. In this study, we used a hydrothermal process to prepare nanosized iron oxide and oxyhydroxide powders, which showed good particle dispersion in molten lead-free frit. The precursor paste was prepared by reacting iron nitrate with ammonium hydrogen carbonate and heated in a sealed vessel at 80–150 °C. The sample powder prepared at 90 °C was almost monophasic α-Fe2O3 (hematite) and exhibited the brightestred color (highest L*, a*, and b* values). This powder featured primary particles of ∼15nm in diameter that aggregated to form secondary spherical particlesof ∼100 nm in diameter. This powder was mixed with lead-freefrit and applied as a red paint to a porcelain test piece. The resultant akae showed a bright red color with high L*, a*, and b* values that exceededthe values of akae prepared using conventional hematitepowder. Microscopic analysis revealed that the hematite particleswere well dispersed in the resultant glass layer, indicating thatthe hematite powders have good dispersibility in molten frit.
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