Multi-technique analysis of the lead-(potash) glass beads from the Nanhai I shipwreck: Composition, colorants, forming and provenance

To gain deeper insights into the manufacturing techniques and raw material sources of the glass beads collected from the Nanhai I shipwreck dating from Southern Song Dynasty, a comprehensive analysis was conducted on the beads using digital microscopy, X-ray micro-computed tomography (Micro-CT), micro X-ray fluorescence spectrometry (μ-XRF), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), Scanning Electron Microscope- Energy Dispersive X-ray Spectrometer (SEM-EDS), multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and micro-Raman spectroscopy. The results indicate that these beads are typical Chinese potash-lead and lead silicate glasses, manufactured primarily using the coiling technique. The opaque yellow coloration was obtained with the use of lead stannate (lead-tin yellow type II) as both a colorant and opacifier. The orange interior of black glass beads was colored by the combined use of Cu2O and CuO, while the surface of black beads exhibit signs of weathering from the presence of such corrosion products as Pb2CO3Cl2, PbS, and 2PbCO3·Pb(OH)2. The technological lineage of these lead-(potash) beads is discussed below from the perspectives of their compositional system and the nature of the opacifiers that were used, linking them to Han Dynasty potash-lead glass from Lingnan. Based on the results of trace element and lead isotope analysis, the provenance of the raw material used to these glass beads was also investigated.