Insights into long term glass corrosion mechanisms from the Ballidon experiment.

At the Ballidon experiment, one of the longest running glass durability studies, modern and simulant archaeological glasses were buried in mildly alkaline, under-saturated, conditions for 52 years. Glass surfaces were analysed to determine the extent and mechanisms of alteration. Alteration layer chemistry was complex and included Ca from the surrounding limestone sediment and P from porewater resulting in Ca, Pb and Fe-phosphate rich phases interspersed with Si and Al rich regions. There was evidence for ongoing evolution of the alteration layer structure due to continued fluid ingress. Lamellae in the silica-rich regions approximately numbering the years of burial and indicating a possible link between their formation and seasonal climate cycling. Comparison of field samples with laboratory dissolution tests highlighted the impact of surface finish on initial alteration rate and the limitations of using alteration layer thickness to estimate the amount of glass that has dissolved.