Effect of dispersion and aggregation of wet-synthesized Al-substituted hematite particles on color tone of powders and red overglaze enamels

Abstract

The Japanese traditional red overglaze ceramic ware, akae, a composite in which hematite particles are dispersed in a glass-frit layer, has become extremely popular with porcelain lovers all over the world. Recently, development of hematite powders dedicated to akae is strongly desired by various porcelain producers. Inspired by Fukiya bengala, a high-quality old red pigment produced in Japan in the early 18th century, we produced Al-substituted hematite powder through wet-process in a large scale. In this study, we investigated the effect of dispersion and aggregation of hematite particles on the color tone of the resultant powder, and developed a technique to produce high-quality akae using the hematite powder. The prepared powder is primarily composed of hematite nanoparticles ³10 nm in diameter that contains 5 mol % of Al 3+ in the structure, and these primary particles are aggregated to form secondary particles 3070 nm in diameter. The powder showed dark brown color regardless of its nanometric character because the secondary particles form large aggregated particles with high volume fraction of hematite, leading to prevent transmission of visible light. Applying ultrasonic and silica-coating treatments that realize low volume fraction of hematite, we obtained the hematite powder exhibiting yellowish red color. In addition, high-quality akae test pieces having highly dispersive hematite particles in a glass layer were successfully prepared using ballmilling technique and subsequent combination process with the submicron-sized lead-free frit.

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

“…The akae layer in common products is several to 10 micrometers thick . In this study, akae layers over 100 μm thick were prepared to evaluate their color tone, removing reflections from the substrates including the glaze layer . The glaze layer is transparent due to vitrification of the glaze under heat treatment at 1250°C, and the substrate has an opaque white color due to the existence of quartz and mullite fine crystals and fine bubbles, which have different refractive index, in a glass matrix.…”

“…Although T‐hematite, which is a raw material of red paints, is composed of primary particles below 10 nm in diameter, and these particles aggregate to form secondary round particles of 30‐70 nm in diameter, hematite particles in the akae layer have a larger diameter than T‐hematite. Measured particle sizes from TEM images of D20 (Figure E) and R20 (Figure G) were 50‐110 and 65‐130 nm, respectively; that is, the particle size of hematite is slightly larger in R20.…”

“…Kaolin (10 wt%), a hydrated aluminum silicate, was added to the frit to prevent abrasion. The red pigment used in this study is 5 mol% aluminum‐substituted α‐Fe 2 O 3 (AF‐111, Terada Chemical Fabrik) prepared by a wet process . This is referred to as T‐hematite hereafter.…”

“…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 …”

“…By focusing on a red pigment mainly comprising α‐Fe 2 O 3 (hematite), microtubule materials composed of silicate‐coated hematite nanoparticles, composite materials of alumina/hematite nanoparticles having high thermal stability with respect to color, and well‐dispersed hematite nanoparticles that are suitable for akae have been synthesized. 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 …”