Measurement of Sorption Isotherms to Guide Mixed Display of Archaeological Iron, Bone, and Glass

This study examines the preservation challenges of archaeological iron, bone, and glass within shared environments focusing on material-specific degradation mechanisms. The relative humidity (RH) requirements for these materials can vary significantly. Iron presents distinct stability groups at specific RH thresholds, albeit levels below 30% RH are recommended for sensitive artefacts. Little is known about the moisture response of bone, and the heterogeneity of the material poses additional challenges for its examination. Glass can undergo deterioration at both high and low RHs due to the threat of aqueous attack and transformation of the pristine glass or the crizzling and delamination of the already transformed glass. Experiments employing dynamic vapour sorption (DVS), acoustic emission (AE), and oxygen depletion analyses provided insights into the moisture response behaviours of these materials. It was found that the deleterious akageneite formation increases dramatically at RHs > 30% in archaeological iron, reinforcing the current guidelines. Bone exhibits significant hygroscopicity as well as isotherm steepening below 25% RH on desorption, suggesting this threshold could be advisable to avoid structural damage. In glass, there is evidence of significant mass fluctuations between c. 60 and 95% RH, as well as isotherm steepening around 30% RH on desorption, thus providing more empirical evidence to published storage recommendations. This work sheds more light on the risk assessment for mixed-material showcases and underscores the necessity of nuanced RH guidelines that consider the material-specific degradation mechanisms.