Applications of Raman spectroscopy in art and archaeology

The 10th edition of the International Congress on the Application of Raman Spectroscopy in Art and Archaeology (RAA2019) was held in Potsdam (Germany) from 3 to 7 September 2019, with eight keynote lectures, 35 oral presentations and 18 Poster Presentations. The number of active participants was 68 delegates from 20 countries among the 236 authors that presented at least one work. The importance of the application of Raman spectroscopy in art and archaeology is illustrated by an increasing number of research papers published each year and by the scientific conferences and sessions dedicated year by year to this research area. In fact, Raman spectroscopy is considered now as the most adequate instrumental technique, working alone or coupled with other non-invasive instrumental techniques, to identify and characterize the material components of the objects of art and archaeological remains. The biennial International Congress on the Application of Raman Spectroscopy in Art and Archaeology (RAA) is probably the most important event dedicated to this topic. The RAA conferences promote Raman spectroscopy and play an important role in the increasing field of its application in art and archaeology. These prominent international events have a long tradition. Previously, they were held in London (2001),[1] Ghent (2003),[2] Paris (2005),[3] Modena (2007),[4] Bilbao (2009),[5] Parma (2011),[6] Ljubljana (2013),[7] Wrocław (2015)[8] and Evora.[9] The 10th edition of the International Congress on the Application of Raman Spectroscopy in Art and Archaeology (RAA2019) was held in Potsdam (Germany) from 3 to 7 September 2019. Our congress shown once again the ongoing European and worldwide interest in this field of Raman applications, bringing together researchers from diverse areas who represented dedicated work on the use of Raman spectroscopy techniques in the fields of art history, history, archaeology, palaeontology, palaeoenvironment, conservation and restoration, museology, degradation of cultural heritage, archaeometry, chemometrics and many other fields. As in the past years, developments of new instrumentation, in particular non-invasive methods, have received unbroken great attention. Besides ancient materials, such as pigments, dyestuffs, wood, glass, metals and others, more and more modern materials and their previously unknown deterioration processes have come into the focus of our studies, like the modern colours, inks, plastics and building materials. These studies were presented along eight keynote lectures, 35 oral presentations and 18 poster presentations. The number of active participants was 68 delegates from 20 countries among the 236 authors that presented at least one work to the Congress. The book of abstract and the details of the Conference were published by the University of Potsdam and can be downloaded from its Institutional Repository.[10] The high quality of the contributions is reflected in the selected 20 manuscripts covered in this special issue, ordered in topics suggested to participants when this RAA2019 edition was announced: characterization of materials, conservation issues affecting cultural heritage, Raman spectroscopy of organic-based materials and Raman applications in Archaeology and Forensics with authenticity research. The most interesting aspects of such manuscripts are highlighted below. The in situ application of micro-Raman (μ-Raman) spectroscopy to the analysis of two historic painted objects: a 15th-century illuminated manuscript and a late 16th-century portrait miniature, belonging to the Fitzwilliam Museum collection, revealed the unexpected presence of calomel (Hg2Cl2). The presence of sharp and well-defined peaks in the Raman spectra as well as stylistic considerations on the most ‘appropriate’ colour to appear in these areas confirm the deliberate choice of calomel as a white pigment to depict fine, intricate details in both objects, as reported by Crippa et al.[11] This is the first ever verified presence of calomel within the painting palette of Western European works of art, predating its documented use in South America. Authors suggest using non-invasive analyses of museum objects to decipher in the near future if calomel will no longer be considered an unusual pigment but rather will take its place as an integral part of the palette used by artists in England and beyond during the late Renaissance and the early modern period. The Kunstgewerbemuseum, Staatliche Museen zu Berlin-Stiftung Preußischer Kulturbesitz (SMB-SPK) holds a collection of medieval champlevé enamelled objects from the main production centres in Limoges and the Rhenish/Mosan region. Röhrs et al.[12] characterized enamels of 6 of such objects by Raman spectroscopy (using a 532-nm laser) in combination with additional element analytical methods such as μ-X-ray fluorescence (μ-XRF) analysis and environmental scanning electron microscopy with energy-dispersive X-ray analysis. Wavenumber parameters of the SiO bending and SiO stretching regions, δmax and υmax, and the polymerization index Ip derived from Raman spectra analyses give information on compositional differences in the glass matrix. Thus, it is possible to differentiate glass types of enamels and to correlate them with known principal glass families. Most of enamel data points fall into the proximity of soda lime glass. But other glass types like ‘high PbO’ and ‘lead arsenate in PbO’ were detected as well. The different glass types could be used to classify the objects according to production time and place. The authors furthermore discuss the advantages of a higher spatial resolution of Raman spectroscopy compared to the μ-XRF method in order to detect heterogeneities in enamels that could be related to technological differences between workshops. Corradini et al.[13] have reported the spectroscopic characterization (Raman, visible and near infrared reflectance, and FTIR in the ATR mode spectroscopies together with high-resolution microphotography) of 27 commercial pigments currently used for retouching purposes. Repeatability was checked by measuring five spots for each pigment using μ-Raman spectroscopy. Apart from the spectroscopic characteristics of the different pigments, some drawbacks are highlighted like the problems arising when dealing with Madder and other lakes for which very small spectroscopic signal are obtained together with the inorganic mortar. In addition, the problems related to obtaining good Raman spectra on natural earths, blue and green colours were evidenced when using the 785-nm excitation laser, due to the high background found for most of the pigments. To overcome these problems, authors suggest using a multianalytical approach and mix all the spectra to guarantee the unambiguous identification of the pigments used in past retouching interventions. A set of decorative tile panels, created by the Portuguese artist Maria Keil (1914–2012) in the middle of past century for the first 18 station of the Lisbon metro, was studied by Mortari et al.,[14] using a non-invasive multianalytical methodology. This methodology combines μ-Raman spectroscopy and μ-XRF spectrometry to characterize elemental and molecular composition of the glazed materials (glass matrix and network modifiers such as fluxes, opacifiers and colouring agents). The μ-XRF quantitative evaluation of the glassy matrix shown that all glazes had a lead silicate glass base with important amounts of potassium; it is known that Pb and K induce the reflectance of the glazes; thus, it can be assumed its intentional use in the tiles. To identify the opacifier, Raman and XRF were used, discovering a zirconium-based opacifier instead of the classical tin-based one. The number of colour shades was as much as 16, but only six pigments (minerals) were identified by μ-Raman namely, Naples yellow (Pb2Sb2O7), lead–tin yellow type II (Pb(Sn,Si)O3), Pb–Sn–Sb triple oxide, cobalt blue (CoAl2O4), chromium oxide (Cr2O3) and malayite sphene (CaO.SnO2.SiO2), all of them identified by μ-Raman. These set of pigments were used by themselves or in mixtures (binary, ternary, …) for obtaining the desirable colour shades intended by the artist. Raman spectroscopy has been used by Gao and Heide[15] to study the degree of metamictization of zircon and their influence in the colour of gemstones. The full width at half maximum (FWHM) value of the ν3(SiO4) band around 1000 cm−1 can classify the structural state of zircon as crystallized (FWHM less than 5 cm−1), intermediate (the FWHM value ranges from 5 to 15 cm−1) and metamict (FWHM higher than 15 cm−1). The study was focused on the zircon samples from Sri Lanka in the gemological collection of Abraham Gottlob Werner. These samples shown red to green, according to the Munsell colour system and the CIEL*C*h* colour system, being their density from 4.07 to 4.87 g/cm3 values. The comparison of colour and metamictization degree shown that crystallized zircon displays a more red tone with yellow, while the green colour in zircon occurs when metamictization increases. The density of zircon is also related to the metamictization degree as crystallized zircon is more dense than the metamictized one. As metamictization has a significant influence on the gemological properties of zircon, this work has shown how Raman spectroscopy is a convenient, efficient and non-destructive technique to estimate such metamictization degree of zircon. Raman microscopic in point-by-point and imaging measurements were used to analyse the detailed chemical composition and structure of three old papers dating from the 15th and 19th centuries. Raman mapping measurements were performed on the surface and along the cross-section of the papers with a lateral resolution of 1 μm. The data treatment on the raw spectra of the Raman images was performed using chemometric methods, mainly implemented in-house based on MATLAB software. The resulting Raman images visualized the detailed chemical structure of the papers: the different types of cellulose fibre, the filler pigments, the sizing agents, the colour pigments and also the non-intentionally added trace particles, such as minerals or products of biological activity that give information about the conservation problems of the paper items. The Raman imaging analysis concluded on the presence of particles of weddellite (CaC2O4.2H2O) systematically covering the fibres of the 15th paper, confirming the biological attack on this old paper. The study conducted by Pigorsch[16] demonstrates the great advantages of Raman imaging for chemical paper analysis, including both molecular composition and structure, to enhance considerably the understanding of former papermaking practices as well as the source of unexpected minerals that are related to degradation/alteration problems. Moreover, the extension of such works on secure items on paper support will help combating the forgery and the fraud of documents and artworks on paper. Author suggests performing further studies in this direction. Ongoing degradation caused by external agents (rainfall and atmospheric pollution) is a serious problem in the Archaeological Park of Pompeii (Italy). Prieto-Taboada et al.[17] studied the special case of a blue colour changing into greyish green hues in mural paintings in Ariadne's house using in situ and laboratory spectroscopic methods. Analyses were performed with a mobile Raman system (785-nm excitation), a laboratory-based confocal Raman system (785-nm excitation), an ATR-FTIR spectrometer and an energy-dispersive XRF spectrometer. Two painting layers were detected. The upper layer consists of pure Egyptian blue (CaCuSi4O10). The lower layer was created by a mixture of the green pigment celadonite (K[(Al,Fe3+),(Fe2+,Mg)](AlSi3,Si4)O10(OH)2), called Verona earth or creta viridis, with small amounts of Egyptian blue, dolomite, calcite, aragonite and quartz. The authors concluded that the green celadonite is not an alteration product but part of the underlaying and older, green-based paint layer, which becomes now visible as a result of the evident loss of the rain affected upper blue layer. Furthermore, it is suggested that the original two pigment layers in which the expensive Egyptian blue overlies the inexpensive celadonite layer reflects an improvement of the socio-economic status of the ancient owners of the residence. Several sulphate salts were identified in efflorescences that are due to the long-term exposure of the mural paintings to atmospheric acid gases. In addition, green pthalocyanine and wax—results of modern restoration works—were detected. The conservation state of mural paintings of two among the oldest Cappadocia churches, the so-called ‘proto-Byzantine paintings’ was undertaken by Sbroscia et al.[18] Those materials were never extensively investigated before although its knowledge should be of prime importance to understand the evolution of both materials and execution techniques during the ages in this extraordinary region of central Turkey. Authors selected the church of St. John the Baptist at Çavuşin, whose architectural structure is dated back to fifth to sixth and the 5 to in The painting materials were investigated with a multianalytical approach by using cross-section by μ-Raman with XRF and The paintings in the church of St. John the Baptist to different based on the characteristics of the the pigments, mainly based on and were but molecular like and materials were also mainly on the being a of chemical and biological degradation The 5 at shown a execution the pigments were on a layer, alteration as decipher from lead based such as lead and the presence of to the red and yellow pigments, suggest a derived from the areas used to the pigments of the a building material in the are with degradation processes of this material in cultural et using Raman spectroscopy as a for to on measurements on building of and to their conservation state surface two Raman and 785-nm both original materials and their degradation products in efflorescences were studied in two in the Bilbao and the of the University of the of were the by and the and the by the of the differences in and the original materials, the in degradation products sulphate and that the of and atmospheric in acid were detected in the This is probably due to in of as into the region. μ-Raman with an excitation at was used by et to a study of an century the Raman characterization of different samples was performed to have an adequate for comparison purposes. Authors important in the cm−1 from surface measurements to into the of the cm−1 the band at cm−1 from to the surface while the band at as degradation The extension of surface degradation of the can be using the between the of the band at cm−1 that of band at Authors that method to estimate the of the Moreover, they the of using confocal measurements instead of surface to adequate of the that when the is in good environmental the degradation of could from to being much higher for The work of et shown the of a for in and paint if the were The and were selected for this Several using different were namely, of the in the and with the acid the case of in a was using or with The of a cross-section of the from a work of art, to the of the of the pigment in one of the paint layer of the that such could be the base for the study of cultural As the analyses by of are authors studied also the of non-invasive FTIR spectroscopy analyses for the identification of the as a to the new Furthermore, this study revealed to identification of pigment in the paint layers based on the of the Portuguese objects between the and the were studied by et their red and objects were studied using non-destructive confocal μ-Raman spectroscopy and 785-nm ATR-FTIR spectroscopy and energy-dispersive XRF inorganic red pigments were identified by μ-Raman lead and In addition, three red pigments based on were of the pigments was in some on the analyses the authors an in situ multianalytical for the study of red pigments in This study into and information for further conservation The work conducted by et the of μ-Raman and reflectance spectroscopy to the of blue and The method has been using different blue to types of samples were created and in different to analysis. 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This work the of to study the heritage, being Raman spectroscopy the instrumental technique for a in this cultural area. are interesting archaeological objects as they as for for for or and lime from information about ancient and is but or is a when are and of products such as or other archaeological are et a method based on spectroscopy. were from the of and one in the ancient of near two in the ancient near and one near the ancient of in the of Raman spectroscopy laser, cm−1) and FTIR spectroscopy cm−1) were used to identify minerals of to analysis, the of the archaeological was checked using to possible with the minerals of on these the authors a in a to their The medieval of was in in the and it is now in the Archaeological Museum of including from the et studied of these alteration using the non-destructive analytical techniques μ-Raman spectroscopy and 532-nm laser) and energy-dispersive XRF A high-resolution XRF of a cross-section revealed very different element and which were the result of different alteration Raman analyses revealed that during degradation and on the environmental processes and the conservation state of the information can be used to a conservation In as had an importance in and These were the area. to is based on methods such as and These applications are or in of is not et laboratory-based μ-Raman spectroscopy to the of and used in The non-destructive method is based on spectroscopic parameters from Raman spectra of components in the from six different in and known to have been in were of the obtained Raman parameters that could be used for in situ analysis using Raman The 20 works in this special are of the applications of Raman spectroscopy from samples to and the state of the art in its application to art and archaeology. the of field analysis, to chemical chemometric analysis to and the presence of unexpected materials together with the original have been some of the approach presented in this papers dealing with the use of non-destructive as well as instrumental techniques to support the Raman information a to problems. of them have in Raman spectroscopy as the of the manuscripts in this special The of the the RAA2019 Congress to a research among from different fields has been shown and to such in the works to be presented in the in are to the participants and that in the In like to our main as well as the of of Raman spectroscopic techniques and the and the the support of the for and