Morphological and structural study of crystals in black-to-brown glazes of Yaozhou ware (Song dynasty) using imaging and spectroscopic techniques

摘要

The Yaozhou kiln complex (Shaanxi, China) is a famous black-to-brown ceramic production center due to its significant volume of production, long manufacturing history (from Tang Dynasty to Yuan Dynasty) and variety of decorations. In this work, four representative types of black-to-brown ware from the Yaozhou kilns were selected to study the morphology, structure and distribution of the precipitated crystals in the glazes using a series of imaging and analytical techniques (optical microscopy, XRF, SEM-EDS, TEM-EDS, IIRS and UV-vis-IR). The results show that the rare metastable epsilon-Fe2O3 phase, already observed in typical sauce glaze, was also detected in other types of brown glaze decorations, such as light brown stripes on a darker background or irregular brown spots on a black background. Cross-section analyses also showed a three well-separated layered crystalline distribution in the glazes of lighter brown colors as it has been noted in glazes from Qilizhen kilns. Raman analyses revealed that the crystals contained in third layer are of varying nature: epsilon-Fe2O3 in the sauce glaze and magnetite in the light brown stripes. The peak shifts and line broadenings observed in the Raman spectra of epsilon-Fe2O3 crystals were also investigated. They are the result of Al, Ti and Mg substitutions, which were identified using TEM-EDS. Such ionic substitutions would stabilize the metastable epsilon-Fe2O3 crystals by increasing the cationic disorder. In addition, Mg-, Ca- and Fe-rich spinel crystals were observed for the first time in a black glaze of Yaozhou wares.