Insights into long-term glass corrosion mechanisms from the Ballidon project: a 50-year-old glass burial experiment in an alkaline near surface environment.

Abstract Results are presented from the Ballidon experiment, one of the longest running glass durability experiments. Nine glasses of varying composition including simulant Roman and medieval glasses, as well as borosilicate, plate glass, soda-lime-silica, E-glass and a lead optical glass were buried in mildly alkaline and under-saturated conditions for 50 years. The relative durability of the nine glasses compared well with that predicted by the stirred reactor coupon analysis (SRCA) short-term durability test with alteration layers present on the least durable glasses and only localised (vermiform) attack observable on the most durable glasses. Alteration layers on the naturally altered Ballidon samples were analysed with particular attention to the influence of elements from the environment on their chemical composition and to evidence of post-formation evolution of the layers. It was evident that Ca (from the limestone sediment) and P (from the local groundwater) were sequestered in alteration layers and contributed to the formation of Ca, Pb and Fe-phosphate rich phases. Separation of the alteration layers into ‘bands’ or lamellae with different chemical environments occurred in poorly durable glasses, with Si and Al rich bands alternating with Ca, Fe and P rich bands. There was evidence that banding in alteration layers moved and evolved over time in response to water ingress, for example from cracks and fissures.