Alteration mechanisms of ancient glass objects exposed to the atmosphere

International audienceThe development of new efficient protective methods for ancient glass objects that decay under atmosphere could only be achieved through a better understanding of the alteration mechanisms on a very small scale. To this end, ageing experiments (temperature and humidity control) are conducted on relevant chemically unstable ancient glass composition replicas (alkali (Na,K)–rich silicate). Ageing temperature and glass composition effects on the chemical, morphological and structural evolution of the glass surface replica are evaluated by a combination of multi-tool analysis. Our results put in evidence the non-correlation between the weathering process (formation of a hydrated surface layer) and the departure of alkalis. The slow diffusion of alkalis may be related to the all-solid-state character of the altered layer: all molecular water is H-bound to the network or as Si-OH. These water molecules do not have the same solvation properties than liquid water. Moreover, we highlight a strong influence of the ageing conditions and of the glass composition on the atmospheric alteration mechanisms. In particular, these parameters affect the proportion of alkali elements that are retained into the hydrated layer, which would severely impact the glass chemical durability. Based on preliminary results, a rinse operation promoting the removal of a fraction of alkalis from the altered layer appears very interesting from a glass conservation perspective