An archeometric study of the bulk and surface weathering characterisitcs of Early Medieval (5th-7th century) glass from the Po valley, northern Italy

Compositional and structural characterisation was carried out on Early Medieval (5th–7th century) fragments of glass goblets excavated from the archaeological sites of Monte Barro, Brescia and Monselice (northern Italy) with the aim of identifying raw materials, glass-working techniques, and surface weathering characteristics. Optical analyses and X-ray spectrometry were used for bulk, and X-ray photoelectron spectroscopy and scanning and transmission electron microscopy for surfaces. The samples of each area were produced using siliceous–lime sands, with natron as flux. The differences observed in chemical composition allow to subdivide the samples from Monte Barro and Brescia from those of Monselice, the latter generally show higher silicon, calcium and aluminium and lower sodium contents than the others. By plotting reduced base glass compositions in soda–lime silica phase diagrams, melting temperatures varying from 900°C for Monte Barro and Brescia samples to 1000°C and more for those from Monselice were estimated. Differing Fe2O3, Sb2O5 and MnO2 contents are related to the different colours of the samples, Monselice samples being blue–green and Monte Barro and Brescia samples green and yellow–green. The chemical differences may be interpreted as related to different provenance and/or glass-working techniques. Surfaces are depleted in alkaline and alkaline-earth elements due to weathering process. Alteration lamellae show a nanostructure, similar to that of opal.