An innovative analytical approach to the study of pigments and minerals through the use of Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) for cultural heritage applications

A study on a collection of powdered commercial pigments and massive minerals was carried out by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), a spectroscopic technique that is reacently appearing in the panorama of mineralogical and geological studies. DRIFTS is a powerful infrared technique generally applied for the characterization of both organic and inorganic materials, without the need of subjecting samples to a time-consuming preparation or to invasive treatments. Moreover, this non-destructive and noninvasive technique requires the use of a small and lightweight instrumentation which is well suited also for in situ analyses. The collection of studied samples includes commercially available natural and synthetic pigments of different colors and brands, and minerals widely used as pigmenting powders since ancient times, such as malachite, lapis lazuli, azurite, cinnabar, etc. Minerals were selected among the specimens belonging to the Museum of Mineralogy, Petrography and Volcanology of the University of Catania. The DRIFT spectra of the commercially available pigments, analyzed in powdered form after mixing with KBr, were compared with those obtained from the corresponding mineralogical phases in order to highlight similarities and differences in terms of position and intensity of the signals. It is well known that infrared spectroscopy, when operating in diffuse reflectance can generate spectral discrepancies, due to the different surface appearance (powder/massive; matt/shiny) despite the same chemical and mineralogical composition. The limited number of databases (Manfredi et al., 2017; Miliani et al., 2012) of DRIFT pigments and dyes spectra and the few specific information regarding the attribution of each feature to the corresponding individual vibrational modes for the identification of each species, made the research very compelling and challenging. Thus, the necessity of building up a database of reference spectra to be used in future studies became evident. This is essential in the optics of an increasing use of DRIFTS for the study of archaeological materials, frescoes, museum artifacts, ancient documents and so on, considering its potentialities in terms of versatility and speed and easiness of spectra acquisition.