The Corner Palace in Venice: a case of study on stone damage in urban area

In order to evidence both the products of damage reactions and the components due to atmospheric deposition, and to identify the pollutant sources, specimens of damaged stone, consisting of surface black crusts with underlying unaltered material, were sampled on the blackened façade of the Corner Palace on the Canal Grande in Venice and submitted to a combination of physico-chemical analytical techniques. The samples were observed in thin section under optical microscope, and the damaged surface was investigated using a scanning electron microscope. X-ray diffractometric analyses, gravimetric and differential thermal analyses, and ion chromatographic analyses were performed in order to identify the main crystalline species, quantify gypsum and carbonate, and measure the anion concentrations, respectively. Finally, the carbon fractions were discriminated and measured following a chemical-thermal methodology specifically developed for damage layer specimens. The data obtained indicate gypsum as the main product of damage processes occurring due to wet and dry SO2 deposition. The burning of fossil fuels produces gases, smoke and particulate carbonaceous matter that deposit on building surfaces and react with the underlying calcareous materials forming the damage layer. Carbonaceous particles have a catalytic effect on CaSO4·2H2O formation and the ensuing surface blackening on building exteriors.