Rooted in the long tradition of Raman spectroscopy of cultural heritage materials, in this work we provide a personal perspective on recent applications and new frontiers in sampling modalities, data processing, and instrumentation.

International audienceVarious natural organic film-making media from several chemical families have been used as varnishes. An improvement in the identification of these materials by the combination of Raman and infrared spectroscopies is here presented. Fourteen unaged reference samples representative of resins, glues, gums and oils were analyzed. FT-Raman and IR spectra were all compared in order to discriminate the different classes of organic media by considering their spectroscopic features. It led to a flowchart for material discrimination based on simple, notable and specific criteria. It appears that the different families (gums, glues, oils and resins) and subfamilies (di/triterpenoids resins) are easily differentiated thanks to their specific structural composition. However, discerning between compounds from a same subgroup, for example diterpenoid resins, could be ambiguous because the spectra depend on the sample aspect, age and degradations

Natural organic substances are involved in many aspects of the cultural heritage field. Their presence in different forms (raw, heated, mixed), with various conservation states, constitutes a real challenge regarding their recognition and discrimination. Their characterization usually involves the use of separative techniques which imply destructive sampling and specific analytical preparations. Here we propose a non destructive approach using FT-Raman and infrared spectroscopies for the identification and differentiation of natural organic substances. Because of their related functional groups, they usually present similar vibrational signatures. Nevertheless the use of appropriate signal treatment and statistical analysis was successfully carried out to overcome this limitation, then proposing new objective discriminating methodology to identify these substances. Spectral decomposition calculations were performed on the CH stretching region of a large set of reference materials such as resins, oils, animal glues, and gums. Multivariate analyses (Principal Component Analyses) were then performed on the fitting parameters, and new discriminating criteria were established. A set of previously characterized archeological resins, with different surface aspects or alteration states, was analyzed using the same methodology. These testing samples validate the efficiency of our discriminating criteria established on the reference corpus. Moreover, we proved that some alteration or ageing of organic materials is not an issue to their recognition.

International audienceThe characterization of materials involved in painted works of art provides a better knowledge of artworks through the understanding of artistic and technological processes. In this study, some pieces selected from a large corpus of French Decorative Arts objects from 18 th century museum collections were analyzed. These materials are complex system made of a multilayered colored background, covered with varnish layers. Colored or gilded ornaments are applied, representing different characters, flowers or landscapes, then varnished again. The aim of this study was to improve the painting and varnishing techniques knowledge in the Decorative Arts field during the 18 th century. Vibrational spectroscopies, Raman, surface enhanced Raman spectroscopy (SERS) and infrared, were used to analyze some specific parts of these objects, especially organic materials such as lake pigments, binders and varnishes, and some of them were identified. For this purpose, a set of experimental setups and parameters were used according to the samples. The performed analyses were thus done using conventional FT-Raman at 1064 nm, SERS with a 458 nm excitation wavelength, infrared using a micro-ATR mode, and by defining for each technique and each analysed sample the best set of analytical parameters. The results obtained are all complementary and allowed us to completely identify and characterize multilayered paint systems as well as varnish compositions using specific data treatment methodology. This study is a description of the various possibilities that vibrational spectroscopies can provide when the right settings are employed, with a deeper look into the vibrational features using new data treatments

The compositions of ancient varnishes are mainly determined destructively by separation methods coupled to mass spectrometry. In this study, a methodology for non-invasive quantitative analyses of varnishes by vibrational spectroscopies is proposed. For that, experimental simplified varnishes of colophony and linseed oil were prepared according to 18th century traditional recipes with an increasing mass concentration ratio of colophony/linseed oil. FT-Raman and IR analyses using ATR and non-invasive reflectance modes were done on the "pure" materials and on the different mixtures. Then, a new approach involving spectral decomposition calculation was developed considering the mixture spectra as a linear combination of the pure materials ones, and giving a relative amount of each component. Specific spectral regions were treated and the obtained results show a good accuracy between the prepared and calculated amounts of the two compounds. We were thus able to detect and quantify from 10% to 50% of colophony in linseed oil using non-invasive techniques that can also be conducted in situ with portable instruments when it comes to museum varnished objects and artifacts.

Roman‐period mummy portraits are considered to be ancient antecedents of modern portraiture. However, the techniques and materials used in their manufacture are not thoroughly understood. Analytical study of the pigments as well as the binding materials helps to address questions on what aspects of the painting practices originate from Pharaonic and/or Graeco‐Roman traditions, and can aid in determining the provenance of the raw materials from potential locations across the ancient Mediterranean and European worlds. Here, one of the largest assemblages of mummy portraits to remain intact since their excavation from the site of Tebtunis in Egypt was examined using multiple analytical techniques to address how they were made. The archaeological evidence suggests that these portraits were products of a single workshop and, correspondingly, they are found to be made using similar techniques and materials: wax‐based and lead white–rich paint combined with a variety of iron‐based pigments (including hematite, goethite and jarosite), as well as Egyptian blue, minium, indigo and madder lake to create subtle variations and tones.

Natural resins are prone, as all natural organic products, to ageing and degradation. Characterizing this alteration provides a better knowledge about their chemistry, and when they are involved in cultural heritage artefacts, it helps to define relevant conservation protocols. In this study, a set of archaeological African copals that present specific surface degradations and colors were analyzed by FT-Raman and ATR-IR spectroscopies. A methodology based on the modifications of the vibrational features between the bulk and the resins surface was define. A relation between the evolution of the molecular structure of the copal samples and some vibrational bands could be established; and possible ageing reactions occurring at the surface of the samples could be proposed. This specific surface aspect is actually related to an alteration, and the surface states and the colors variability are linked to different alteration degrees. This could refer to whether different conservation contexts (presence of air, water, etc), possible preparation or different origins: for example the collection of fresh or partly fossilized resins.

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