20 Years of on-site Raman Analysis of rare works of arts: Successes, Difficulties and Prospects
Résumé
The study, in particular of the state of conservation, and the authentication of works of art are issues in full development and Raman microspectroscopy in conjunction with X-ray fluorescence are the reference techniques due to their perfectly non-invasive character, and the availability of mobile instruments. Although mobile instruments have been available for more than 20 years, their use has only recently exploded with the tightening of formalities allowing the movement of valuable objects and changes in practices. Our team having been among the first to develop the Raman analysis of works of art, whether in the laboratory with the development of the MOLE in 1975 at LASIR [1,2] and on site from the 2000s [3, 4] at the same time of David Smith at MNHN, Paris [5], we will focus the presentation on fire arts objects, most of which are enameled (metal, glass or ceramic). These sophisticated objects were technological achievements at their time of production, made for the elite, traded very early on at long distance, and their value which can reach now several millions of € imposes on-site and non-invasive procedures. Except for the identification of gems and some minerals [6,7], their Raman analysis requires the use of high-performance due to the simultaneous presence of vitreous phases generating a signature of broad bands and crystalline phases giving narrow peaks but mainly at low wavenumbers.[4,5,8-10] With the development of particular procedures (use of sophisticated optics (very high magnification, zoom and telescope)[11-13], micrometric stages/mapping, subtraction of the continuous background, dating from proton corrosion[14], multivariate analysis [5,15]) and an understanding/modeling of the spectra [16], the parameters of processing (temperature) are deduced [5-8] and restorations/fakes/copies are identified. The combination with on-site pXRF analysis/mapping is highlighted [17-21] as well as the interest of measuring full spectral range in order to use information present in the 'fluorescence' background.[22] Difficulties (limitation in the choice of of exciting wavelength, control of the ambiant temperature and lighting, preservation of the artifact, limitedaccess time for measurement, etc.) and future developments (on-site mapping, multiscale analysis, etc.) will be discussed [23]. Figure 1 Examples of On-site Raman analysis using a mobile setup .
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