Impact of neocrystallisations on the SiO2–K2O–CaO glass degradation due to atmospheric dry depositions

The medieval SiO2–CaO–K2O stained glasses are particularly vulnerable to the impact of their environment. In the urban atmosphere, they will tend to rapidly deteriorate either by loss of matter or by addition of atmospheric material, that tend to form a crust on the surface of the glass. If the glass surface is protected from the rain run-off, this second phenomenon will be favoured. In the early stage of crust formation, it was shown that primary constituents were salts, called neocrystallisations. However, it is still not clear, if their presence induce further deterioration of the glass matrix.In order to answer this question both field exposure and laboratory experiments were carried out. Model glasses, chosen with compositions similar to those of ancient stained glasses, were exposed to the urban atmosphere in sheltered conditions. Samples exposed were analysed in order to identify the neocrystallisations and further tests were undertaken in order to observe their impact on the glass itself.The analyses of the weathering products on the glasses showed the presence of three principal minerals: syngenite, gypsum, and potassium carbonate. The observed mineral phases were then artificially deposited on model glass surfaces submitted to cycles of relative humidity. It was found that the presence of salts increased the glass degradation by extending the time of wetness of the glass surface and forming saline solutions in the case of deliquescent salts. Evidence of strong leaching could be observed on the glass surface and even more so when considering glass samples in contact with multiple salts. Finally, the impact of potassium carbonate (K2CO3) on the glass was dramatic, since it induced a loss of matter caused by the dissolution of the lattice in contact with the alkaline solution.

► Gypsum, syngenite, K2CO3 and other salts form during early stages of deterioration.
► Salts act as condensation nuclei, keep water on the glass, inducing more degradations.
► K2CO3 found on the glass (alkaline solution) leads to glass congruent dissolution.
► Synergic effect of salts induces more degradation than individual effect.