Study of sulphation of Candoglia marble by means of micro X-ray diffraction experiments

This research is aimed at assessing the particular kind of damage caused by the interaction between atmospheric pollution and the marble surface of a magnificent late gothic church, Milan Cathedral, restored in the early 1970s. On the basis of visual inspections and diagnostic investigations the facade appears to be seriously damaged by surface erosion and thick black crusts; biological crusts, micro-fractures and detachments are widely present. This paper reports on SR-mu XRD data collected at the Synchrotron Radiation Source (SRS), Daresbury Laboratory, and at the European Synchrotron Radiation Source (ESRF) from selected samples, covered with black crusts, originating from the Milan Cathedral facade. Mineralogical maps of the deterioration layers have been recorded by two spatial resolutions (5-100 mu m), from the bulk of the stone material to the outer layer of the black surface. These maps help in elucidating the mechanism of formation of the black crusts.