Experimental Archaeological Study on Phase Separation and Coloring Mechanism of Sky Blue Jun Porcelain Glaze during the Song and Yuan Dynasties

Experimental archaeological methods are used to reproduce the process of Jun porcelain sky blue glaze in the Song and Yuan dynasties with feldspar, quartz, kaolin, calcite, talc, bone ash, iron oxide and other raw materials, including ultra depth of field microscope, transmission electron microscope, scanning electron microscope and colorimeter, in order to comprehensively analyze the fired samples. The effects of SiO₂/Al₂O₃ molar ratio and bone ashes content on appearance and microstructure of Jun porcelain glaze were explored, so as to further clarify the phase separation and color formation mechanism during the Song and Yuan dynasties. When bone ashes are absent and the Fe₂O₃ content is constant, as the molar ratio of SiO₂/Al₂O₃ gradually is increased, a densely distributed silicon rich micro phases was formed in the glaze, resulting in strong diffuse scattering. The glaze color was changed from transparent green to milky white, but sky blue is not formed. Therefore, the SiO₂/Al₂O₃ molar ratio was the main reason for the formation of milky white Jun glaze. As the content of bone ashes is gradually increased, the tiny particles in the glaze gradually grow into spherical and connected worm type ones, with sizes of 60–140 nm, enhancing the structural color. The glaze color varied from green to sky blue, similar to the Song and Yuan Dynasty sky blue Jun porcelain glaze. Therefore, the presence of bone ashes is the main reason for the formation of Jun glaze sky blue.