Fiber Optics Reflectance Spectroscopy in the Entire VIS-IR Range: a Powerful Tool for the Non-invasive Characterization
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Fiber Optics Reflectance Spectroscopy in the Entire VIS-IR Range: a Powerful Tool for the Non-invasive Characterization of Paintings M. Bacci1, R. Bellucci2, C. Cucci1, C. Frosinini2, M. Picollo1, S. Porcinai2 and B. Radicati1 1 Istituto di Fisica Applicata “Nello Carrara” – CNR – Florence, Italy 2 Opificio delle Pietre Dure, Florence, Italy ABSTRACT Fiber optics reflectance spectroscopy (FORS) in the entire 380 – 11,000 nm range has been used to non-invasively characterize Leonardo’s painting Madonna of the Yarnwinder. A preliminary analysis of the recorded spectra made it possible to obtain useful information concerning the pigments used, previous restoration work, the preparatory layer, and the binding medium. In particular, it was found that Thénard’s blue was used in a previous restoration instead of the original ultramarine. As regards the painting technique, typical spectral features of oil binding medium are present in the mid-IR spectra, while near IR spectra reveal the occurrence of clay in the preparatory layer. INTRODUCTION Spectroscopic techniques play a fundamental role in supplying information useful for knowledge of art objects and a diagnosis of their state of conservation. Until recently, these techniques required sampling of the object under examination. Such samplings, though limited, undoubtedly produce damage to the object. Moreover, sampling made it impossible to monitor future alterations in that area. Luckily, at present, developments in fiber optics and hyper spectral imaging technologies have overcome these difficulties by obtaining spectral information quite non-invasively. At this point, it can be noted that, although it is extremely useful, the single noninvasive technique is rarely, but in some fortunate cases, self-exhaustive. However, it is quite evident that non-invasiveness enables the gathering of a large amount of spectral data, which can be processed by means of statistical methods and compared subsequently to detect possible variations induced by aging. In any case, this method will drastically reduce the number of invasive samplings. By now, fiber optics reflectance spectroscopy (FORS), in particular in the visible range, is a well-accepted technique in the scientific and restoration communities for identification of pigments and monitoring of color alterations. Since the first application of FORS in the 1980s, improvements in detector technology and in the materials constituting fiber optics have made it possible to extend spectroscopic analysis up to the mid-infrared (mid-IR) region (ca. 900 cm-1). Consequently, the entire range of interest for electronic spectroscopy and most of the region involved in vibrational spectroscopy have been made accessible by means of completely non-invasive measurements. Here, as a case study, the results of an investigation by means of fiber optics on the Madonna of the Yarnwinder by Leonardo is reported. The whole 380 – 11,000 nm range was considered, using different instruments. Although elaboration of the data is still under way, a
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