Bright Yellowish-Red Pigment Based on Hematite/Alumina Composites with a Unique Porous Disk-like Structure.

Inspired by a bacteriogenic, iron-based oxide material and a traditional Japanese red pigment, a bright yellowish-red pigment was prepared by heating an Al-containing iron oxyhydroxide precursor. The obtained red pigment had a unique porous disk-like structure, comprising Al-substituted hematite particles and crystalline alumina nanoparticles. Although these disk-like structures loosely gathered to form an aggregate in powder, they can be easily dispersed into a single, disk-like structure by simple ultrasonic irradiation. The powder exhibited a bright yellowish-red color and high thermostability, making it attractive as a coloring material for various industrial products needing a bright-red color, high weather resistance, and durability. Quantitative color measurements revealed extremely high L*, a*, and b* values that are much greater than those of commercially available hematite. The thermostability test showed that even after exposure to high temperatures, the pigment retained the red color, indicating its high thermostability. The unique microstructure should be strongly related to the bright yellowish-red color and the high thermostability of the developed red pigment.

Controlling the Color of Lead-Free Red Overglaze Enamels and a Process for Preparing High-Quality Red Paints.

Akae porcelain, an artistic Japanese traditional overglaze ceramic typically known for Kakiemon-style ware, has fascinated porcelain lovers around the world for over 400 years because of the graceful red color displayed by akae that matches so well with white porcelain bodies. In this work, we clarified the factors that control the color of akae and those that are conventionally controlled by artisans based on empirical experience. Inspired by a recent particle-design method, we also developed a practical facile process to prepare red paints that yields high-quality akae. Various akae samples were prepared from a combination of lead-free alkali borosilicate glass frits with different particle sizes and hematite powders with differing dispersibilities. Polarized light microscopy, scanning electron microscopy, and transmission electron microscopy analyses indicate that considering only the dispersibility of hematite powders is not sufficient, but the frit-particle size must be controlled to obtain high-quality akae with a high reflectance value for ≥580 nm visible light. In addition, we developed a process for preparing high-quality red paints that uses a large-particle frit powder and a strongly aggregated-hematite powder, both of which are easily obtainable. The red paint composed of frit, hematite, and the solvent is mixed until the paint is drying. By adding more solvent and repeating this process three times, we obtained high-quality akae with a higher reflectance value than for the akae prepared from a frit with submicron-sized particles and weakly aggregated-hematite powder. On the basis of transmission electron microscopic observations, we consider the red paint to consist of a core/shell-like composite structure of frit and hematite, forming a three-dimensional network in the akae glass layer. The good dispersibility of these particles leads to high-quality akae.

Well-Dispersed α-Fe2O3 Particles for Lead-Free Red Overglaze Enamels through Hydrothermal Treatment.

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 brightest red color (highest L*, a*, and b* values). This powder featured primary particles of ∼15 nm in diameter that aggregated to form secondary spherical particles of ∼100 nm in diameter. This powder was mixed with lead-free frit 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 exceeded the values of akae prepared using conventional hematite powder. Microscopic analysis revealed that the hematite particles were well dispersed in the resultant glass layer, indicating that the hematite powders have good dispersibility in molten frit.