Conditions for the safe storage of early glass

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Ancient glass corrodes through interaction with water in the soil or the atmosphere, losing alkalis and leaving a hydrated silica-rich weathered layer on the surface. Corrosion rates depend upon the composition of the glass and the ambient environmental conditions. When taken into storage, glass may continue to corrode unless the RH is low. The preferred RH value is typically cited as around 40%, but this does not take into account the specific composition of the glass and is based upon very old research. Many glasses continue to corrode at this RH level. Unfortunately further lowering of the RH can cause the surface layer to dehydrate and flake off the surface, leading to a disfiguring phenomenon known as "crizzling". While the pull and push of further hydration versus delamination can sometimes be balanced to an acceptable level in store, the issue is complicated when it comes to displaying glass objects alongside other environmentally sensitive materials such as organics and metals, which require a different set of conditions. Furthermore, we have no data on the lowest level of RH which is tolerated before crizzling will occur. English Heritage has large numbers of glass artefacts in its collections and some of these are in a state of advancing deterioration. The aim of the present project is to provide modern guidelines for storage of English Heritage glass informed by glass composition, rate of hydration and tendency to delaminate. We anticipate that these guidelines will be of value to custodians of ancient glass in other institutions, in the UK and internationally.