Characterisation of complex alteration layers in medieval glasses

•New findings on long term atmospheric corrosion of medieval stained glasses.•Use of model and ancient stained glass altered in real atmospheric conditions.•Multiscale study (SEM + TEM) of short and long terms alteration layers.•Refining of atmospheric corrosion mechanisms of stained glass.

Silica–potash–lime stained-glasses from medieval age in Northern Europe are found in a poor conservation state. Their mechanisms of atmospheric corrosion are still not fully understood and need deeper investigation. A multi-scale analysis of K–Ca rich silicate glasses (ancient and model glass), showed modified layers characterised by a multilayer sequence consisting in the repetition of nm-thick laminae with different compositions. Crystalline phases (sulphates, carbonates and phosphates) are observed. The formation of these sequences is due chiefly to interdiffusion, followed by local dissolution phenomena. Precipitation of secondary phases is enhanced by the presence of fractures, which facilitate fluid circulation.