Principal component analysis of near-infrared spectra of alteration products in calcareous samples: An application to works of art

Works of art made of calcareous stone are usually affected by decay processes when they are exposed to the wet and dry deposition of atmospheric pollutants in urban and industrialized areas. For many years now in our laboratory, we have been developing totally non-destructive and non-invasive spectroscopic techniques in the visible and near-infrared regions. Principal component analysis (PCA) of diffuse reflectance near-infrared (NIR) spectra has been used as a suitable methodology for discriminating areas involved in the degradation process of calcareous stones. NIR spectra of standard mixtures, containing gypsum (CaSO4. 2H2O), calcium sulfite hemihydrate (CaSO3· 1/2H2O) and calcium carbonate (CaC03), and of calcite tablets, differently treated in order to simulate the formation of gypsum and calcium sulfite hemihydrate on real calcareous surface, have been recorded. For all data sets submitted to PCA, a good discrimination between the two reaction products, i.e. calcium sulfite hemihydrate and gypsum, in the alteration process was obtained. In an actual case, we tried to detect and to distinguish the alteration products, such as gypsum and calcium oxalate dihydrate, CaC2O4 · 2H2O (weddellite), in the limestone (calcirudites, packstones and wackestones) which constitutes the coating of the supportive brickwork structure of the Parma Baptistery (Italy). Preliminary results are reported with regard to the application of PCA to all the in situ spectra collected on the calcareous surface of the monument.