Thermal Behaviour of iron-rich black-to-brown glazes related to ancient Jian and Yaozhou wares

Black-to-brown glazes from the Song Dynasty constitutes an interesting group in Chinese glazed ceramics, as they were manufactured all across the Empire in many different kilns, each with their specific firing conditions and raw materials. In spite of this diversity, all glazes systematically contain dendritic crystals of ε-Fe2O3, an uncommon Fe2O3 polymorph, which gives them their specific brown hue. In some cases, flower-like structures composed of mullite and hexagonal cordierite crystals are also present. This study focuses on the thermal behaviour of three different raw materials collected from the Shaanxi and Fujian provinces to understand the complex chemistry that leads to the growth of these different phases. It combines is situ monitoring of the materials during firing by synchrotron X-ray powder diffraction and ex-situ multi-technique analysis of samples fired under different conditions. Results show that indialite/cordierite grows during the heating stage of the firing when the glaze begins to melt and that the atmosphere of the firing controls the substitution of magnesium by iron. We also highlight that for a given raw material composition, hematite and ε-Fe2O3 growth can be adjusted through the control of the firing atmosphere and total duration: ε-Fe2O3 is favoured in calcium-rich materials fired under a less reducing atmosphere, while hematite is favoured in other materials subjected to longer firings.