Principal Component Analysis of Near-Infrared Spectra of Alteration Products in Calcareous Samples

Principal component analysis (PCA) of diffuse reflectance near-infrared (NIR) spectra has been used as a suitable methodology for discriminating areas involved in the sulfation process of calcareous stones. NIR spectra of standard mixtures containing CaSO₄ · 2H₂O, CaSO₃ · 1/2H₂O, and CaCO₃ were recorded. For all data sets submitted to PCA, a good discrimination between the two reaction products, i.e., CaSO₃ · 1/2H₂O and CaSO₄ · 2H₂O, in the alteration process was obtained. The actual availability of fiber-optic spectrum analyzers working in the NIR region suggests that the proposed procedure can be used as a safe and nondestructive method for monitoring alteration processes in calcareous works of art.