The long term corrosion of glasses: Analytical results after 32 years of burial at Ballidon

Groundwater corrosion on three archaeological model glass compositions, a replica Roman glass (B1), a medieval glass (B2) and a 17th century glass (B3) from the Ballidon glass burial experiment has been characterised by optical and scanning ion microscopy (SIM), infrared reflection spectroscopy (IRRS) and field emission scanning electron microscopy (FE-SEM). A multi-technique approach was adopted to gather as much information as possible on the weathered sub-surface areas of the glasses. The Ballidon experiment, now into its 36th year, is one of the longest running tests of its kind. The samples studied chart up to 32 years of natural soil burial in controlled conditions (i.e. known burial time, composition and alkaline soil pH). As the three glass compositions are replicas of ancient counterparts, the findings are relevant to both the scientific and archaeological communities. Given the equivalent burial conditions for all three glasses, the different corrosion mechanisms are dependent principally upon composition, and in particular the dominant alkali and network former content. B1, the most durable glass, shows progressive Na leaching that is coupled with dissolution of the silicate matrix in the outermost 2-5 μm after 32 years of burial. This is not accompanied by the formation of a corrosion crust. Less durable B2 and B3 compositions, however, have corroded via congruent dissolution of the silicate matrix and in situ condensation, forming a discrete 5-15 μm thick crust after only one year of burial. The formation of a protective Ca-rich surface layer is observed for B2, hindering successive inward growth of the dissolution reaction and favouring localised pitting of the extant crust. The protective surface phenomenon is not observed for B3, which shows a large increase in crust thickness after nine years of burial. The 32 year sample for B3 has not yet been analysed for a more complete comparison.