Sol–gel silica coating for potash–lime–silica stained glass: Applicability and protective effect

The aim of thiswork is to test the applicability and the protective effect of a sol–gel silica coating on potash–lime–silica glasses of low durability. Such glasses were applied during the Romanesque and Gothic periods for manufacturing stained glass objects still in situ at historic buildings such as cathedrals or smaller churches north of the Alps. Their high potash content and low silica amount led to a low chemical stability and to degradation processes on the glass surfaces; a protection is hence necessary for the conservation of those art objects. Two synthetic glasses with compositions close to the historical stained glass were used for the studies. The sol–gel coating does not alter the optical characteristics of the surface and is compatible with the substrate since its composition and chemical–physical properties are similar to the glass it is applied on. Moreover, it can be easily applied and does not require any heating treatments after the application. It is therefore particularly suitable for the application on cultural heritage objects. The effect of the sol–gel application was monitored and leaching tests with different times and pH values were performed both on coated and uncoated parts. The protective effect was verified using light microscopy and ToF–SIMS analyses were performed to check the ion migration (e.g. K, Na, Ca and Mg) and the interface between bulk glass and coating. The coating protects the glass from the formation of cracks also after severe treatments that would have caused serious damages on the uncoated glass.