Effects of environmental vibration on ancient stained-glass windows

Ancient stained-glass windows are delicate pieces of art that need to be carefully maintained in order to be preserved over time. Several studies address the problem of stained-glass windows degradation, however, the effect of ambient vibrations on the conservation state of these artworks is not thoroughly studied and understood. In the present work, a wide and general approach was adopted to better address the problem. Two monitoring campaigns have been carried out on the stained-glass windows of the Mi- lan Cathedral, to understand the characteristics of the typical daily excitation and that of shorter duration events, like concerts taking place in the square in front of the cathedral. Starting from the gathered data, experimental tests have been performed on a facsimile window panel. The direct exposure to concert-like sound pressure waves has been reproduced with a laboratory test rig and the evolution of the window panel dynamics has been tracked through a series of measurements. Lead cames, which form the internal metallic structure of a window panel, proved to be the most critical element of a stained-glass window, rather than the glass pieces themselves. In fact, when sub- jected to a prolonged direct exposure to the pulsating actions produced by concerts or similar events, severe metal fatigue phenomena were observed. On the other hand, glass pieces remained intact during the tests. This suggests that their possible detachment is not due to their direct failure when subjected to a direct exposure to the pressure waves, rather to an improper or weakened fastening of the lead cames. The mechanical damaging processes of stained-glass windows appear to be more related to metal fatigue and subsequent glass chattering, rather than to an expected intrinsic fragility of the artwork. Nevertheless, since the windows of the façade of the Milan Cathedral are protected by an additional structure comprising glass windows, they are not directly exposed to the sound pressure waves, as a filter is introduced between the source and the window panes. This strongly mitigates the vibration levels reaching the window panels. Such an arrangement appears to be of importance for the conservation of the stained-glass windows from a mechanical point of view.