Corrosion of archaeological model glasses after 32 years of burial at Ballidon

The groundwater corrosion of three archaeological model glass compositions, a replica Roman glass (B1), a medieval glass (B2) and a 17th century glass (B3), recovered from the Ballidon burial site, were investigated by field emission scanning electron microscopy. Given the equivalent burial conditions for all three glasses, the different corrosion mechanisms of these glasses are dependent principally upon composition, and in particular the dominant alkali and network former content. Composition B1 was found to the most durable glass and showed progressive Na leaching coupled with dissolution of the silicate matrix in the outermost 5μm after 32 years of burial. The B2 and B3 compositions were found to be less durable and corroded via congruent dissolution of the silicate matrix and in situ condensation, resulting in the formation of a discrete 5-15μm thick layer after only 1 year of burial. The formation of a protective Ca-rich surface layer was observed for B2, hindering successive inward growth of the dissolution reaction and favouring localised pitting of the extant layer. The protective surface phenomenon was not observed for B3, and a large increase in layer thickness was observed after nine years of burial. (author)