A new methodological approach for the chemical characterization of black crusts on building stones: a case study from the Catania city centre (Sicily, Italy)

Mineralogical, petrographic and chemical analyses were carried out on black crusts covering the stone surface of monuments and buildings of the historical city centre of Catania, one of the most beautiful Baroque places in eastern Sicily. Black crusts were studied through the careful and synergic employment of traditional techniques, including polarizing optical microscopy (POM), scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM-EDS), and infrared spectroscopic techniques (FT-IR), in combination with an innovative technique, i.e. laser ablation inductively coupled mass spectrometry (LA-ICP-MS). The validity of such an integrated approach to study black crusts has been recently demonstrated by the authors. The main goal of this study was to develop and test the reliability of the LA-ICP-MS method on black crusts in order to evaluate the degree of chemical contamination of examined stones and to determine the role of the different sources of pollution in Catania, both anthropogenic (domestic heating, industrial combustion, vehicular traffic) and natural (emissions by Mt Etna) in the formation of crusts. Results obtained demonstrated that this innovative approach has a double potential in the study of black crusts, since it allows the analysis of alteration and degradation processes induced by migration of specific chemical elements from the crust to substrate, and, at the same time, it represents a reliable indicator of the environmental pollution.