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Inspired by the ion diffusion at the interface between the red glaze and glaze layers, which was confirmed by the study of overglaze painting on porcelain test pieces, we developed frits with compositions in which a lime glaze was systematically added to a lead-free multicomponent alkali borosilicate glass frit. When the mixed powder of the frit and hematite was heat-treated, the excessive reaction with hematite was suppressed as the lime glaze content increased, due to the increase of the frit viscosity. The dissolution of hematite was suppressed in the sample with the same viscosity at a higher heat treatment temperature, i.e., with a higher lime glaze content, confirming that the composition of the frit strongly affects the dissolution of hematite. We attempted to improve the chroma of lime-glaze-added samples, and the highest chroma in this system was achieved by adjusting the amount of initial hematite and heat treatment temperature. • Frits with compositions in which a lime glaze was systematically added to an alkali borosilicate glass frit were developed. • Reaction between the lead-free frit and hematite was investigated. • Viscosity and chemical composition of the frit affected the reaction. • Red color was successfully controlled by adjusting initial experimental conditions. [ABSTRACT FROM AUTHOR]

• Crystal growth mechanism of hematite in high-temperature lead-free glass frit was investigated. • Hematite particles slightly grew due to sintering up to near softening point of the frit. • Hematite crystal growth occurs through Ostwald ripening above softening point. • Resultant hematite crystal was surrounded by low index crystal planes having low surface free energy. To elucidate the crystal growth process of hematite in high-temperature lead-free multicomponent alkali borosilicate glass, which is essentially important to control the color of red overglaze enamels, frit and hematite mixture is heat-treated and subjected to microscopic observations. Hematite particles slightly grew due to sintering at low temperature. Once the glass matrix formed near the softening point of frit, hematite dissolved into glass fluid. Hematite crystal growth concomitantly ensued with decrease in the number of hematite particles via Ostwald ripening as the temperature increased. The grown particles exhibited an anisotropic morphology with straight outlines reflecting crystal planes, the morphology of which is completely different from those grown by sintering and particles prior to heating. These results suggest that comprehensive understanding of frit and hematite from the perspectives of glass science and chemistry as well as powder technology are important to truly control the color of red overglaze enamels. [ABSTRACT FROM AUTHOR]

• Silica-coating treatment was adopted for hematite/alumina composite pigment. • The bright yellowish-red pigment showed excellent thermostability in color. • The pigment can be used as coloring material for red overglaze enamels on porcelain. Surface coating is a key technique used to enhance various properties of functional inorganic materials. We adopted a silica-coating treatment for preparing bright yellowish-red pigments using hematite/alumina composites, which have a unique porous disk-like structure. This treatment enhanced color, mechanical strength, and thermostability. The porous disk-like structure comprising Al-substituted hematite particles and alumina nanoparticles, which is fragile to mechanical impact and/or high-temperature heating, was a key factor to obtain high chroma bright-red color in the present system. Silica-coated samples having various Si molar ratios [ x =Si/(Fe + Al + Si)] were prepared, and mechanical-strength and thermostability tests were conducted. The mesopores generated by an assembly of hematite particles and alumina nanoparticles were gradually filled with amorphous silica, thus achieving significant enhancement in mechanical strength and thermostability in color. The composite pigment can be used as a colorant for red overglaze enamels for porcelain, and shows significant potential for various other industrial coloring applications. [ABSTRACT FROM AUTHOR]

For red overglaze enamel decoration, a type of colored ceramic coating for ceramics, a partial peeling might occur in mass-produced ceramic ware products. In this study, multinuclear core/shell hematite/glass composite particles, comprising a hematite core and a glass shell, where hematite particles are highly dispersed in a glass matrix, were developed to prepare a red coating material that could achieve a bright red-colored overglaze enamel without peeling. A peeling-free, bright yellowish red-colored overglaze enamel could be obtained when using the composite particles as the red paint. The former function (preventing peeling) can be derived from micrometric size of the shell glass particles and the latter function (high chroma and lightness) is caused by highly dispersive hematite particles in the shell glass particles. Our results suggest that the composite particles are suitable for mass-production due to their peeling-free nature, indicating a potential to be widely used as coloring for ceramic coating products. [ABSTRACT FROM AUTHOR]