Chinese Blue pigment

We have studied a faience bead fragment containing Chinese blue pigment from the Late Warring States Period (Tomb M4, Majiayuan Cemetery, Zhangjiachuan, Tianshui City, Gansu Province, China) using Scanning Transmission Electron Microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDS). The TEM thin foils were prepared by Focused Ion Beam (FIB). Different crystalline impurities as BaSO4, PbSO4 or Pb3O4 have been observed close to the crystals of pigment (BaCuSi4O10). The presence of micron or sub-micron scale BaSO4, BaSO4-PbSO4 solid solution, Pb3O4 and amorphous SiO2 inclusions were first observed and reported, whose formation are closely related to raw materials selection and manufacturing process. Based on possible formation mechanisms to these inclusions from a chemical point of view, it is likely that witherite (BaCO3) had been used as source of barium, and galena (PbS) or anglesite (PbSO4) had been used as source of lead and sulfur. However, the possibility that sulfur might be also from Cu-containing minerals such as chalchocite (Cu2S) cannot be ruled out.