Compositions of early Islamic glass along the Iranian Silk Road

Abstract

The composition of archaeological glass reflects the geochemical nature of its raw materials. To determine the origins and distribution of early Islamic glasses from Iran, a set of 169 glass samples from five different sites was analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of 58 elements. The glasses were classified into six different plant ash glass groups, three of which were attributed to a Mesopotamian origin, while three further groups are presumed to represent regional Iranian productions. The ratios of MgO/CaO and K2O/P2O5 of the different groups reflect variations in the plant ash component. Minor elements Cr, Ti, Zr, La and Th and their ratios proved effective in distinguishing the base glass types. Mapping their frequency across the Iranian plateau revealed the relative movement of glass and likely source areas. The decline in the frequency of glass types with elevated Cr/La ratios east of the Zagros Mountain range confirms that elevated Cr/La ratios together with an augmentation of the magnesium levels are features of Mesopotamian glass production apparently inherited from the geochemical environment of the Euphrates and Tigris river valleys. Some exceptionally clean Mesopotamian glasses made from a quartz-rich silica with low levels of accessory minerals are consistent with ninth-century glass from Samarra, which was evidently traded widely along the Silk Road network. No evidence of local glass production was detected in Nishapur. The Iranian groups were produced from a quartz-rich silica source, high in thorium but with different zirconium contents resulting in different Th/Zr ratios. Aluminium concentrations tend to increase from west to east, with the highest values found among glass assemblages from Central Asia.