FT-NIR Spectroscopy for the Non-Invasive Study of Binders and Multi-Layered Structures in Ancient Paintings: Artworks of the Lombard Renaissance as Case Studies

Abstract

This work deals with the identification of natural binders and the study of the complex stratigraphy in paintings using reflection FT-IR spectroscopy, a common diagnostic tool for cultural heritage materials thanks to its non-invasiveness. In particular, the potential of the near-infrared (NIR) spectral region, dominated by the absorption bands due to CH, CO, OH and NH functional groups, is successfully exploited to distinguish a lipid binder from a proteinaceous one, as well as the coexistence of the two media in laboratory-made model samples that simulate the complex multi-layered structure of a painting. The combination with multivariate analysis methods or with the calculation of indicative ratios between the intensity values of characteristic absorption bands is proposed to facilitate the interpretation of the spectral data. Furthermore, the greater penetration depth of NIR radiation is exploited to obtain information about the inner layers of the paintings, focusing in particular on the preparatory coatings of the supports. Finally, as proof of concept, FT-NIR analyses were also carried out on six paintings by artists working in Lombardy at the end of the 15th century, that exemplify different pictorial techniques.

UV-Excited Fluorescence as a Basis for the In-Situ Identification of Natural Binders in Historical Painting: A Critical Study on Model Samples

“…It can therefore be concluded, at least with reference to model samples, that the method based on spectrofluorimetry can allow a first hypothesis on the nature of the binders used in a painting but must necessarily be accompanied by more specific analytical techniques, even non-invasive, such as vibrational spectroscopies. As previously mentioned, NIR reflectance spectroscopy allows an in-situ analysis [9,10], with the possibility of distinguishing mixed binders or overlapping layers containing different binders, or even obtaining information on the binder when a surface layer of varnish is present on the painting, as demonstrated in references [10,32], on a part of the model samples examined in the present work and on ancient paintings. The greater specificity of the vibrational spectra is due to the fact that they show a greater number of narrower signals for a given substance in comparison with electronic spectra, both of absorption and emission, that have just a few broad bands.…”

“…The use of non-invasive spectroscopic techniques for the identification of binders has also been proposed, based in particular on the reflectance in the near IR (NIR) region [9,10]. NIR reflectance spectroscopy allows for the recognition of the use of a drying oil rather than an egg tempera or even mixed binders such as tempera grassa [10].…”

“…The use of non-invasive spectroscopic techniques for the identification of binders has also been proposed, based in particular on the reflectance in the near IR (NIR) region [9,10]. NIR reflectance spectroscopy allows for the recognition of the use of a drying oil rather than an egg tempera or even mixed binders such as tempera grassa [10]. However, in the literature, the measurement of the characteristic fluorescence emission of some binders, especially if aged, has also been suggested as a non-invasive method for their investigation [11,12].…”

“…Indeed, quinone-like structures can act as fluorophores with emission maxima between 380 and 520 nm [17], and fluorescence properties have been suggested for conjugated imines [11]. For proteinaceous binders, amino acid oxidation and the Maillard reaction between amino acids and sugars have been proposed as sources of fluorescent products [10,18], although egg white and yolk have been observed to contain higher levels of easily oxidizable aminoacids than animal glue [19]. In the case of egg yolk, the lipid component also contributes to the observed fluorescence in a similar way to that described for drying oils [11].…”

“…It can therefore be concluded, at least with reference to model samples, that the method based on spectrofluorimetry can allow a first hypothesis on the nature of the binders used in a painting but must necessarily be accompanied by more specific analytical techniques, even non-invasive, such as vibrational spectroscopies. As previously mentioned, NIR reflectance spectroscopy allows an in-situ analysis [9,10], with the possibility of distinguishing mixed binders or overlapping layers containing different binders, or even obtaining Figure 6. Score plot of the first two principal components of the emission spectra (λ exc = 365 nm) of pictorial layers with egg tempera and oil as binders, pure or in mixture and super-imposed layers (explained variance 90.2%).…”