Non-destructive reflectance spectroscopy in the discrimination of two authigenetic minerals: gypsum and weddellite

In the present communication, non destructive fiber optics reflectance spectroscopy (FORS) was used in the near infrared region (NIR) to detect the probable presence of gypsum and weddellite in the limestone (calcirudites, packstones and wackestones) which constitutes the coating of the supportive brickwork structure of the Parma Baptistery (Italy). All the spectra were collected in situ two years after the restoration work. Around 1940 nm, gypsum spectrum presents a water absorption band due to the combination of stretching and bending modes, while weddellite shows an absorption band that is probably due to the combination of the oxalate ion C=O stretching with water O-H stretching. These bands can be vused for discriminating the two orthochemical products, since the band shape is more asymmetric and broader in weddellite than in gypsum. Principal Component Analysis (PCA) has been applied to the NIR spectra in order to describe the spectral variance by means of a small number of uncorrelated variables. Moreover the DISPEC prograsm was utilized in the evaluation of the asymmetry as well as of the area of the band for the two compounds. By means of the FORS technique, it is possible to monitor progressive authigenesis in the same small area of the surface during the post-restoration phase without the need for sampling operations.