A portable hyper spectral device for monitoring the chemical and mineralogical composition of historical buildings surfaces

The modern strategies for the preservation of outdoor cultural assets are addressed to the individuation of suitable parameters which allow a continuous and/or remote control of the conservation state of the objects in order to plan timely conservative interventions. Hyperspectral instruments, routinely used for remote sensing, might play a crucial role in the characterization of the components in exposed stone surfaces, and therefore contribute to understanding their conservation state, providing a new evaluation approach to Cultural heritage. Remote sensors, which discriminate materials on the basis of their different patterns of wavelength-specific absorption, are in fact successfully used for identifying minerals and rocks , as well as detecting soil properties including moisture, organic content and salinity .In this study gypsum is used as precursor symptom of damage, as it mainly is the reaction product between carbonate matrices (i.e. marble) and atmospheric pollution (SOx). A portable hyperspectral device (ASD-FieldSpec FP Pro spectroradioneter) operating in the 350-2500 nm spectral range, has been used for checking the validity of the technique for a qualitative and quantitative characterization of gypsum in a complex matrix as that found in the façade of Santa Maria Novella church (Florence – Italy), where “black crusts” are alternated to gray/beige or white zones. The strongest signals between 1450-1550 nm and at 1750 nm are used for identification and estimation of gypsum. Systematic reflecting measures on small areas (1-2 cm in diameter) of the façade, in particular a grid of spot analyses on the marble sepulchral arches (“Avelli”) located on the left side of the church façade, have been performed by FieldSpec device using the contact probe in order to create a gypsum distribution map, as well as calibration and reference points for Hyperspectral Imaging Systems (i.e. the Selex Galileo Multisensor Hyperspectral System, SIM-GA, which operates in the VNIR and SWIR regions, respectively between 400-1000 nm and 1000-2500 nm) . Small fragments have been also sampled from representative zones of the façade for FT-IR, XRD and SEM-EDS analysis, in order to validate the results obtained with the hyperspectral device. The results show that the hyperspectral technique is a valid tool for detecting the gypsum content in complex matrix; however the presence of black crust (carbon black) reduces the reflectance of the surface giving a lower estimation of gypsum than the real value.