Non-destructive monitoring of molecular modifications in the restoration of works of art on paper. Application of theoretical and experimental optical spectroscopy

The discoloration of paper, due to the development of oxidized groups acting as chromophores in its chief component, cellulose, is responsible for severe visual degradation of works of art on paper. By adopting a diagnostic method based on in situ non-invasive optical reflectance spectroscopy and time-dependent density functional theory ab initio calculations, it is possible to describe and quantify the chromophores in cellulose fibers in a non-destructive way. In order to recover the absorption coefficient of cellulose fibers from reflectance measurements a specific approach based on the Kubelka-Munk theory was applied. The concentrations of carbonyl groups acting as chromophores were obtained by fitting the experimental optical absorption spectra to those simulated by using ab initio calculations. This method was applied for monitoring the restoration interventions of two great format engravings Le Nozze di Psiche and Gesù Cristo e l'adultera by Diana Scultori (1547-1612), as well as a contemporary artwork by Renato Guttuso, Bozzetto per Crocifissione (dated 1940). All artefacts were affected by chromatic deterioration due to a strong oxidation of the paper. Results quantified the decreasing of chromophores concentration after washing and reducing treatments evidencing the different behavior of the carbonyl groups as a function of the specific protocol performed.