Thermal Behavior of Iron-Rich Black-to-Brown Glazes Related to Ancient Jian and Yaozhou Ware

Black-to-brown glazes from the Song Dynasty constitute an interesting group in Chinese glazed ceramics, as they were manufactured across the empire in many different kiln complexes, each with its 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 behavior 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 in situ monitoring of the materials during firing by synchrotron X-ray powder diffraction and ex situ multitechnique 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 always scarcer in the calcium-rich material, even if favored under a more reducing atmosphere. In the other materials, hematite is favored by longer firings.