Non-invasive identification of surface materials on marble artifacts with fiber optic mid-FTIR reflectance spectroscopy

Research work using a compact and portable fiber optic mid-infrared reflectance spectrometer has been carried out to study surface materials on marble, first on laboratory reference models and then on historical objects. The laboratory research has shown that the complex optical reflectance phenomena of layered samples can be interpreted in terms of the nature of the compounds present (sulfates, oxalates, phosphates, resins, waxes and proteins were studied) even with a very low signal overlapping with the strong carbonate reflectance. After long experimentation in the laboratory to create a spectral database of known materials artificially deposited over the surface of marble samples, reflectance spectroscopy measurements have been carried out in case studies chosen to test the applicability of the method for the monitoring of surfaces of stone sculptures. The portable instrument and the spectral database were used for the in-situ non-invasive examination of the surface condition of two Italian marble works of art: the Deposizione dalla Croce by Benedetto Antelami in the Parma cathedral and the David by Michelangelo in the Galleria dell’Accademia in Firenze. The recorded spectra permitted the characterisation of a diffuse presence of calcium oxalate over the Deposizione della Croce and of residues of a lipidic and proteinaceous material probably applied in the past as protective. In the case of the David by Michelangelo, calcium oxalate, calcium sulphate and residues of a waxy material were found. The work clearly demonstrates that in situ non-contact mid-FTIR measurements can lead to relevant diagnostic information on precious artworks without the removal of any sample and with immediate results.