Unraveling the origin of aged varnishes for the proper restoration of old paintings using spectroscopic and spectrometric techniques

This study deals with the investigation of old varnishes from six paintings belonging to the collection of the “Teloglion Foundation of Arts”, Aristotle University of Thessaloniki in Greece employing three complementary analytical techniques. The purpose of this research was to identify the origin of the protective layers of the varnishes with the ultimate goal of their safe removal during their treatment. To identify the components of the varnishes, sampling was carried out in both a mechanical and chemical way from selected areas of all the works of art. The samples were firstly studied with Fourier-Transform Infrared Spectroscopy (FT-IR), secondly with Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC–MS), and finally with Liquid Chromatography–Orbitrap High-Resolution Mass Spectrometry (LC–Orbitrap HRMS). The application of these methods led not only to the verification of the compounds of the varnishes, but also to the presence of specific fatty acids included in the drying oils, as well as diterpene and triterpene resins. FT-IR spectra were very similar to those corresponding to natural resins, while Py-GC–MS evidenced the fatty acids profile in drying oils as well as the presence of dammarane skeleton among others. The main findings from LC–Orbitrap MS/MS included drying and essential oils, mastic, and dammar. Based on the obtained results, a solubility study of varnishes was also performed. The appropriate mixture of solvents for the coating varnishes removal was investigated with the aid of the Teas solubility triangle and the most suitable solvent mixture for the proper restoration was proposed. A conservation effort for one painting was carried out successfully, confirming the applicability of the method.