Non-invasive identification of organic materials in wall paintings by fiber optic reflectance infrared spectroscopy: A statistical multivariate approach
INSTM Operative Unit of Perugia c/o Dipartimento di Chimica, Università di Perugia, 06123 Perugia, Italy.Analytical and Bioanalytical Chemistry (Impact Factor: 3.44). 09/2009; 395(7):2097-106. DOI: 10.1007/s00216-009-3108-y
The aim of this study is to develop a method for the non-invasive and in situ identification of organic binders in wall paintings by fiber optic mid-FTIR reflectance spectroscopy. The non-invasive point analysis methodology was set-up working on a wide set of wall painting replicas of known composition and using statistical multivariate methods, in particular principal component analysis (PCA), for the interpretation, understanding, and management of data acquired with reflectance mid-FTIR spectroscopy. Results show that PCA can be helpful in managing and preliminary sorting of the large amount of spectra typically collected during non-invasive measurement campaigns and highlight further avenues for research. The developed PCA model was finally applied to the case of a Renaissance wall painting by Perugino assessing it predictability as compared to the interpretation of the single spectrum.
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- "However the planar, smooth, and shiny varnish surface of the musical instruments together with the low absorption index of the organic molecules (particularly in the region above 1000 cm −1 ) , guarantee that the specular reflection is predominant; therefore, the KKT can be applied with accurate results. Nevertheless, many works point out the use of specific features in the mid and near-infrared reflection mode spectra in pseudo-absorbance for the characterization of natural and synthetic resins, oils, and proteins  . "
ABSTRACT: The study and characterization of the materials of historic musical instruments, especially violins, represent an important aim for scientists and conservators. Particularly, the varnishes have been often investigated, even to correlate their chemical characteristics with the sound produced by instruments. However, in the last years, the attention has moved to the study of those materials (inlays, pigments, binders, fillers, glues) used by important ancient violin makers, as Antonio Stradivari, to decorate the musical instruments in order to confer a higher aesthetic value. Because of the importance of the ancient violins, the analytical investigations have to be performed more and more with non-invasive methodologies and this could be a strong shortcoming for the complexity of the materials to study. Therefore, the scientific research has improving new methodological approaches that could provide several results without touching the violins. The “Hellier” violin (1679), actually held in Museo del Violino of Cremona (Italy), is one of the most important violin made by Antonio Stradivari and represents a perfect example for the study of both varnishes and decorations. This contribution focuses on the materials’ characterization through different totally non-invasive diagnostic techniques: UV-induced visible fluorescence imaging, optical microscopy, reflection FTIR spectroscopy, and XRF spectroscopy. In particular, reflection infrared spectroscopy has been applied for the characterization of several organic substances historically used to make the musical instruments, with the aim of achieving a correct interpretation of the violin varnish spectra. The varnishes, the black purflings, the black fillers and the white decorations of the inlays were investigated. The preliminary results suggest the presence of (i) supposed resinous drying oil varnish, (ii) metal-based ink used to dye the black filler of the inlays and the black strips of the purflings, (iii) bone or ivory as material for the white decorative elements of the inlays.
- "Regarding organic compounds, the interest has been mainly focused on natural binding media and varnishes    (with the exception of ref  in which the authors studied mid-FTIR spectra of alkyd mock-ups  by a portable FTIR equipped with a chalcogenide fiber optic sampling probe generally limited by a low signal-to-noise-ratio in the fingerprint region. "
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- "In the field of cultural heritage, non-invasive in-situ NIR spectroscopy investigations have been profitably applied for the identification of paint materials      and for the evaluation of the long-term stability of historical papers . Recently, Dooley et al. proposed the use of an advanced NIR reflectance imaging system for the identification of organic substances in paintings, exploiting the vibrational overtones and combination bands of fundamental absorptions, which are less affected by potential pigment interferences . "
ABSTRACT: The present research was aimed at exploiting and evaluating the potentialities of FT-NIR microscopy, as a complementary approach to analysis in the MIR region, for the chemical characterisation of paint cross sections. Even if FT-NIR technique is still underutilised in the field of cultural heritage investigations, the integrated use of information recorded in the NIR and in the MIR regions proved to be extremely useful in the molecular investigation of organic and inorganic substances. In fact, combination and overtone bands present in the NIR region, even if weaker and less selective than those in the MIR region, are not distorted by reflection phenomena. Furthermore, NIR spectra can be efficiently used as a spectral fingerprint for the stratigraphic characterisation of paint cross sections. The proposed analytical protocol was applied on two historical samples, presenting different stratigraphic structures. Suitable chemometric methods were applied for the elaboration of multivariate chemical maps recorded in the range 700–7500 cm− 1. In particular, a comprehensive and efficient procedure based on an interactive brushing approach, which combines the structural information of the score scatter plots with the spatial information of the PC score maps, was used. Interestingly, NIR data allowed a thorough characterisation of paint structures, providing information for the identification of components and suggesting the differentiation among different types of proteins. Moreover, NIR spectra permitted to achieve an efficient distinction of different classes of natural resins, demonstrating that, even working at a microscopic level, the NIR region may support the identification of different terpenoid materials. Multivariate analysis performed on MIR data did not provide satisfactory results, probably due to the distortion of the spectra and overlapping of bands. Nevertheless, MIR outcomes were investigated to support the interpretation of NIR spectra and in attempt to define an integrated protocol for the characterisation of complex paint mixtures.
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