Corrosion behaviour of R2O–CaO–SiO2 glasses submitted to accelerated weathering

Four glasses in the R2O–CaO–SiO2 (R = Na, K) system, following similar compositions to historical glasses used for production of stained glass windows, were prepared with different CaO/R2O ratio. The samples were submitted to accelerated weathering test under different temperature and relative humidity conditions, as well as under controlled SO2-polluted atmosphere. The experimental set up was selected to simulate the main conditions responsible for the deep deterioration submitted by the historical stained glass windows. Such damage has been very important during the last century, as a consequence of highly polluted industrial environments. as the results concern, it has been shown that 400 cycles of variable temperature and humidity are capable to develop pits on the glass surface, its shape and intensity depending on the glass composition. When weathering cycles were carried out with 10 ppm SO2 under high humidity conditions, calcium carbonate crystals appeared upon the glass surface, whereas calcium sulphate crystallisation occurred for higher SO2 concentration. The mechanism of chemical damage has been proposed and discussed in terms of the reaction between the Ca2+-ions leached from the glass sample and the environmental SO2. The former whole reaction occurred through an intermediate stage: the calcium carbonate crystallisation. That salt was displaced by sulphate only when the SO2 concentration was high enough.