Characterization of glass beads from Nanhai I shipwreck and new evidence of lead tin yellow type II in China

Glass beads excavated from Nanhai I shipwreck were investigated with scanning electron microscopy with energy dispersive spectrometer (SEM–EDS), Raman spectroscopy, multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and X-ray diffraction (XRD). Dating back to the Southern Song Dynasty, these beads help to study Chinese maritime trades during the twelfth–thirteenth century. The bead samples are categorized into five groups by color, i.e., yellow, orange-red, blue, dark red and light red. All beads are of K2O-PbO-SiO2 and PbO-SiO2 glass systems and in form of coil bead, which further confirms the Chinese origin of them. Lead tin yellow type II was identified in the opaque yellow coil beads, and K2SnxPb(1-x)Si3O9 crystals were found in both yellow and orange-red samples. Through the preparation of glass with the same formula as the ancient yellow beads, lead tin yellow type II was probably synthesized beforehand and added as colorant because its raw materials tend to form K2SnxPb(1-x)Si3O9 crystals in the K2O-PbO-SiO2 glass during firing. As lead tin yellow type II is an atypical colorant in China, these beads from Nanhai I shipwreck may serve as the first clear evidence of lead tin yellow type II presenting in the Chinese glass system. The lead isotope ratios of the yellow beads suggest they were produced in Fujian Province, China. The introduction of the lead tin yellow coloring technique might have a close connection to the glass making in the Southeast Asia and these beads seemed to be made specifically for overseas markets.