Chemometric tools to investigate complex synchrotron radiation FTIR micro-spectra: focus on historical bowed musical instruments

Abstract

<p class="Abstract">The investigation of the coating systems used on historical bowed string musical instruments is generally highly complex due to the coatings’ reduced thickness and multi-layered structure. Furthermore, sampling is rarely feasible, and non-invasive approaches do not always allow researchers to undertake a thorough characterisation. Thus, in the rare cases of availability, the opportunity must be taken to investigate the best micro-samples in detail using a suite of analytical spectroscopic techniques that allow for obtaining various informative spectra. Their subsequent interpretation should lead to the characterisation of the finishing layers, the preparation of which involves a careful selection of organic and inorganic compounds.</p>In the present work, synchrotron radiation and micro-Fourier-transform infrared spectroscopy were combined in terms of reflection geometry and chemometrics to investigate six cross-sectioned micro-samples detached from four bowed string instruments produced by Antonio Stradivari, Francesco Ruggeri, and Lorenzo Storioni. Various chemometric tools enabled us to perform a preliminary exploration of the entire collected infrared dataset, while a classification model based on partial least squares–discriminant analysis was used to discriminate the materials through the characteristic signals. High model specificity (> 0.9) was achieved in the prediction, providing the groundwork for the application of a fast and rigorous methodological approach.

Complementary mapping techniques to characterize the wood finish of musical instruments

“…The μFTIR maps (Fig. 8c) reveal the chemical feature of the layering: the spatial distribution of the varnish, highlighted through the vC=O band centered at 1710 cm −1 (whose characterization is also completed by the presence of vCH 2 and vCH 3 bands in the range 3000-2800 cm −1 with the related bending between 1450 and 1350 cm −1 and the signals related to vC-O at 1255 and 1185 cm −1 ) well describes the areas in correspondence of layer A and C and partially of layer B; whereas the distribution of the peaks at 1650 cm −1 (vC=O, amide I)-combined with the 1550 cm −1 (combination of vC-N and δN-H, amide II)-is mainly descriptive of the uppermost Layer D [59]. The spectra collected in the layer B and D areas (Fig.…”

“…10b, c), disclosed the composition of the layers. The spatial distribution of the intense vC=O band around 1710 cm −1 -completed by the identification of δsCH 2 and δasCH 3 at 1450 cm −1 , δsCH 3 around 1375 cm −1 , and vC-O at 1255 and 1185 cm −1 -can suggest the presence of resin-based varnish in layer C, D and E. Besides, the spatial distribution of the peak at 1650 cm −1 (vC=O, amide I) proper of the proteinaceous-complemented by the presence of the band at 1550 cm −1 as a combination of vC-N and δN-H, amide II-is mainly associated to the layer B, D and E areas [59]. In addition, numerous spectra in the area of the sizing and the uppermost layers (D and E) bear the signals of the inorganic fraction (see Fig.…”

“…In Fig. 4a, we report the abundance maps of selected FTIR bands, representative of each material: the peak at 1025 cm −1 related to the C-O stretching vibration of cellulose and hemicellulose for the wood (layer A), the band of vC=O around 1710 cm −1 for the oil-based varnish (layer B), the peak related to vC=O of the amide I at 1650 cm −1 for the protein-based material in the sizing (layer C) [59]. The spatial distribution of the peak at 1510 cm −1 attributed to vC-C aromatic ring is also reported for the embedding resin [19].…”

“…Upon the wood (A), the sizing layer (B) is well represented by the spatial distribution of the absorption bands at 1650 cm −1 (vC=O, amide I) and 1550 cm −1 (a combination of vC-N and δN-H, amide II) related to the proteinaceous, whereas the varnish layer (C) is well represented by the spatial distribution of the vC=O band around 1710 cm −1 ascribed to the natural resins. The characterization of the varnish is also completed by the presence of vCH 2 and vCH 3 bands due to the resins' contribution in the range 3000-2800 cm −1 with the related bending between 1450 cm −1 and 1375 cm −1 and the signals related to vC-O at 1255 and 1185 cm −1 [59]. The sizing layer represented by the blue area in Fig.…”

Surface and Interface Treatments on Wooden Artefacts: Potentialities and Limits of a Non-Invasive Multi-Technique Study

“…As for the IR dataset, a matrix of 72 × 490 was considered. Reflectance spectra are considered in the restricted interval 1400-400 cm −1 [55,56] in order to investigate the impact of the varnish film on the diagnostic bands produced by the mineral talc. First derivative (Savitzky-Golay) and mean centering spectral pre-treatments were applied.…”