Multispectral Imaging and Optical Spectroscopy of Two Letters of St. Francis de Sales

Multispectral imaging (MSI) in ultraviolet (UV), visible (Vis) and near-infrared (NIR) spectral regions is mainly used for in-situ qualitative identification of writing media and substrate alterations of artworks on paper. Reflectance spectroscopy in the UV–Vis-NIR, also known as Fiber Optics Reflectance Spectroscopy (FORS), is able to provide qualitative spectra of inks, dyes and pigments. Quantitative information is also accessible through FORS by means of a suitable sampling method and analysing the reflectance data by Kubelka–Munk theory. Punctual fluorescence spectroscopy provides qualitative information for the identification of materials. All these diagnostics methods are based on inexpensive techniques characterized by ease of use and portability for in-situ measurements and were employed for a diagnostics study of two letters written on paper by St. Francis de Sales (1567–1622) preserved at Palazzo Chigi in Ariccia (Rome, Italy). FORS measurements were interpreted by comparing the acquired spectra with Time Dependent-Density Functional Theory (TD-DFT) computational simulations of the chromophores’ optical spectra. Results showed that letters are affected by an intense degradation whose spatial extension was imaged by using MSI. FORS results and subsequent analysis by using the TD-DFT simulations provided the concentration of oxidative functions acting as chromophores. The presence of gelatin was detected throughout the whole document by MSI and fluorescence spectroscopy. In addition, starch glue was detected in specific regions of the letters and in the remedies, suggesting its use in a past restoration. MSI and FORS also allowed identifying iron-gall ink. Such clear insight into the letters current state of preservation represents invaluable information to conservators to plan restoration interventions.