The identification of the chemical nature of varnish is essential for art restorers in order to choose a suitable solvent during its removal. Until today, such identification has been performed using chemical analysis after sampling. An innovative technique is presented here, using ultraviolet (UV) fluorescence spectroscopy. The method is nondestructive, workable in situ, and leads to results in real time. It is based on the comparison between the emission spectrum of an unknown varnish with those of fresh, artificially aged, or old reference resins and varnishes, for different monochromatic excitation wavelengths. The resin and the nature of the varnish as spirit, oil, or mixed can thus be identified. Various examples are presented on home-made samples applied on fluorescent backgrounds and on real works of art.
Mathieu Thoury, Mady Elias, Jean Marc Frigerio, and Carlos Barthou, "Nondestructive Varnish Identification by Ultraviolet Fluorescence Spectroscopy," Appl. Spectrosc. 61, 1275-1282 (2007)