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

Abstract

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.

Interaction between lead‐free multicomponent alkali borosilicate glass frits and hematite in red overglaze enamels

“…Solid‐state sintering of hematite particles and dissolution‐precipitation process of hematite and frit could occur below and above the glass transition temperature of frits, respectively, in the heating of hematite with frits, resulting in larger hematite particle size in akae than in dry process because of the additional latter effect. Aggregation of hematite particles occurred in D10 and D20 (Figure B and C) because these gathered in the spaces between frit particles in red paints . The above observations clearly reveal that hematite particles grew during the production of akae and that the degree of particle growth in akae is higher than that in dry heat treatment.…”

“…D‐frit (4.0 g), T‐hematite (0.2‐0.8 g: 5‐20 wt% of the frit powder), and the green tea solution (2.5 g) were mixed using an automatic mixing alumina mortar (101F, ISHIKAWA KOJO) for 1 hour; the solvent was evaporated during mixing, and the dried powder was obtained. We call this mixing and drying process “precombination treatment.” This process can improve the color of akae . The same process was applied when using R‐frit.…”

“…A green tea solution was used as a solvent for red paints . Akae artisans empirically know that tannic acid in green tea works as a good dispersant, and this has been reported as such .…”

“…Although the overglaze enamels are depicted by red, green, yellow, blue, purple, and gold colors, they are known as “ akae ,” which means red overglaze enamels, because of difficulties in controlling the red color . We have conducted comprehensive research on lead‐free akae with the aim of controlling its color tone …”

“…We also converted a wet‐synthesized Al‐substituted hematite into a commercial reality, and the effects of dispersion and aggregation of the hematite particles on the color tone of powders and red overglaze enamels was clarified . In addition, the effects of the particle sizes of a lead‐free frit powder, particle sizes and dispersibility of hematite powder, and mixing conditions of lead‐free frit and hematite powders on the color tone of akae were investigated in detail, indicating that the homogeneous distribution of hematite particles in the resultant akae is important in obtaining a bright red color …”