Opaque ancient K2O-PbO-SiO2 glass of the Southern Song Dynasty with fluorite dendrites and its fabrication

Here we first report a piece of K2O-PbO-SiO2 ancient glass opacified by fluorite dendrites with archaeological background. This piece of glass was excavated from Nanhai I shipwreck, a merchant ship once heading for Southeast Asia, but sinking near Yangjiang, Guangdong province. Analysis of scanning electron microscopy with energy dispersive spectroscopy, X-ray diffraction and Raman spectroscopy were used to identify the glass matrix and the fluorite dendritic crystals within it. The elemental feature is consistent with the prevailing K2O-PbO-SiO2 glass system during Tang and Song Dynasties. However, the presence of fluorite dendrites with almost no Na, Al and P elements strongly suggests an astonishing conclusion that the fluorite has already been used as an opacifying agent around 800 years ago. Moreover, through replication of glass samples with similar compositions to the ancient one, it is suggested that fluorite dendrites with a similar size can be obtained when fired at around 1000-1050 degrees C and cooled within the furnace. On the basis of different crystal growth outcomes of two cooling strategies and the presence of the large undisturbed dendrite, it is inferred that the original glass vessel was probably made though die-casting instead of blowing technique.