The provenance of cobalt in glass from the Warring States to the Qin and Han periods excavated in China and the remelting of soda-lime glass in lead-barium glass

Abstract

To investigate cobalt provenance in early Chinese glass, this study statistically analyses published compositional data for cobalt-blue and comparative glass samples, focusing on finds from China dating from the Warring States to the Han period and incorporating selected comparative datasets from other regions and periods. The results reveal distinct cobalt-related compositional patterns among glass systems. Cobalt-blue potash glass is characterised by high MnO contents and MnO/CoO ratios, whereas cobalt-blue lead-barium glass shows consistently low manganese and nickel contents, apart from two Ni-rich outliers, resembling the low-Mn, low-Ni pattern of some Hellenistic Mediterranean natron glasses. Plant-ash cobalt-blue glass displays a distinct pattern, characterised particularly by more pronounced zinc enrichment. The Na 2 O/CaO, MgO/CaO, and K 2 O/CaO ratios of cobalt-blue lead-barium glass are also broadly compatible with a predominantly natron-glass contribution, although with some additional flux input. These results indicate that cobalt-blue potash and lead-barium glasses reflect distinct cobalt-colourant traditions. The compositional and flux-ratio similarities between cobalt-blue lead-barium glass and some Hellenistic Mediterranean natron glasses further suggest that cobalt-blue soda-lime glass may sometimes have been remelted into locally prepared lead-barium batches, contributing both glass and cobalt colourant.