Study on Ru ware glaze of the Northern Song dynasty: One of the earliest crystalline-phase separated glazes in ancient China

Ru Guan kiln is the most famous imperial kiln in the Northern Song dynasty (960–1126 A.D.) of ancient China. In this study, green-glazed porcelain shards excavated from Qingliangsi Ru Guan kiln site in Henan province were adopted as test samples. Based on the physico-chemical foundation for crystallization and phase separation of Ru glaze, the correlation among composition, microstructure, firing technique and visual appearance has been investigated by means of energy-dispersive X-ray fluorescence, X-ray diffraction, high temperature microscope, field emission electron microscopy and transmission electron microscopy. For the first time, the study demonstrates that Ru glaze is typical of crystalline-phase separated glaze with a dual coloring mechanism.