Experiments in accelerated ageing chambers to understand the sulphation process

This research focused on the process of black crusts formation, an important degradation phenomenon that affects much of the artistic heritage exposed outdoors. On this regard, heavy metals and carbon particles present in polluted environment seem to be involved as catalysts in the sulphation process. In this context, the present research work involved the exposure of Carrara marble samples treated with metal cation solutions in climatic chambers. In the “corrosion chamber” proper amounts of sulphur dioxide and humidity were set up; in the second chamber, xenon arc-UV radiation was supplied in order to simulate the sunlight irradiation naturally falling on monumental stone during the day. Properly treated marble samples were settled inside the chambers for assessing the catalytic action of metal cations commonly present in atmospheric particulate matter (PM 2.5). The first experimentation campaign involved a single exposure in accelerated aging chambers (4 weeks with high concentration of SO2) which highlighted the complexity of sulphation process [1]. Although some conclusive considerations could be drawn, no univocal evidence on the role of the catalysts came out from this initial study. Therefore, we decided to carry out a second experimentation campaign (3 weeks, with a SO2 concentration twenty-fold lower than the first one). The aim was to better discriminate the catalytic role of individual metals. Herein we report the novel results from the second exposure campaign. This research work is part of a broader interdepartmental project of the University of Milan, entitled SciCult.