Archaeological glass altered under burial conditions: Evidence from turquoise glass beads excavated from Qinghai, China

Archaeological glasses present complex alteration phenomena after long-term burial in soil, providing crucial insights into glass alteration in aqueous environments. The glass beads as important decorations were dating back to mid-8th century, excavated from Qinghai province, China. However, these glass beads have been buried in soil for approximately 1300 years and resulting in a poor state. After observation of various glass beads, four pieces from the turquoise glass beads are selected to study the morphological structural features and chemistry compositional of the alteration layer. Several scientific techniques, including optical microscopy (OM), scanning electron microscopy equipped with energy dispersive X-ray spectrometry (SEM–EDS), and micro-Raman spectroscopy, are employed for analysis. The results show that pristine glasses are made of soda-lime silicate glass (NaOCaO-SiO₂), which are highly susceptible to weathering. The pristine glass transformed into a white alteration layer, which consists of laminated alteration layers and homogeneous dealkalized layer. The laminated sub-layers, with thicknesses ranging from 0.1 to 5 μm, are characterized by spherical silica particles and a minor Al-rich gel. The precipitation of amorphous silica with minor Al-rich gel forms a laminated structure with alternating light and dark features influenced by concentration differences. Inside the brittle glass, as the silica-rich alteration layers gradually replace the matrix along the network of cracks, both its appearance and overall strength are significantly altered.