Laboratory study of the sulfation of carbonate stones through SWIR hyperspectral investigation

Stone-built Cultural Heritage is subjected to decay in urban environment over the centuries, due to surface interaction and reaction with natural atmospheric agents and, particularly in the last centuries, air pollutants. The Short wave Infrared (SWIR) characterisation of stone surface through portable instruments is attracting increasing interest in the field of Cultural Heritage. In this study, SWIR hyperspectral investigation of carbonate rocks, undergoing acid attack under laboratory conditions was performed with the aim of providing useful quantitative information on the degree of sulfation of the surfaces of carbonate stone. Six marble and six travertine specimens were attacked by aqueous solutions of H2SO4 at variable acid concentrations leading to the formation of gypsum. The reacted surfaces of stones were then investigated by a portable SWIR spectroradiometer. The resulting spectra were thus modelled through a full profile approach, in order to obtain a reliable and efficient fitting procedure. Thus, the SWIR characterisation of sulfated carbonate surfaces seems to be a promising, ready-to-use technique for monitoring the conservation state of carbonate stone monuments (e.g. facades, statues). This method could provide valuable support both for restoration practices and for continuous monitoring of stone alteration over time, when assessing the best strategy of intervention and conservation against sulfation processes of historical buildings.