Study of the growth mechanism of ε-Fe2O3 crystals in Chinese sauce glaze replications

The ferric oxide ε-Fe2O3 has recently been at the center of interest of scientists to understand its physical properties and find efficient methods of synthesis. In parallel to those studies, ε-Fe2O3 was discovered in several ancient brown and black glazed ware produced in different Chinese kilns and time periods. The ubiquity of such phase in ancient glaze in spite of its low stability could be a very interesting indicator to understand both the traditional manufacturing of ancient Chinese glaze and the growth of ε-Fe2O3 in a glassy matrix. This study is based on a set of glazed samples taken out from the kiln at different stages of the firing. Analyses show that ε-Fe2O3 crystals begin to grow at high-temperature at the beginning of the cooling stage. The growth continues through iron ion pumping from the glaze. The iron depletion of the glaze causes the dissolution of iron rich particles, which were formed during the heating and ion diffusion from deeper in the glaze. Al-substitution in ε-Fe2O3 was observed, with a higher Al concentration in more developed crystals. Ti- and Mg-substitutions were also highlighted. A set of equation is proposed to model the formation process.