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 . 2H(2)O, CaSO3 . 1/2H(2)O, and CaCO3 were recorded, For all data sets submitted to PCA, a good discrimination between the two reaction products, i,e., CaSO3 . 1/2H(2)O and CaSO4 . 2H(2)O, 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.