Applications of Raman Spectroscopy in Art and Archaeology

Raman spectroscopy is considered as one of the most valued and important techniques in the art and archaeology analysis field. Its increasing importance when the technique is applied on cultural heritage objects is reflected on the number of peer reviewed papers published each year on a variety of journals regarding cultural heritage science, analytical chemistry and (vibrational) spectroscopy, among others. Moreover, the importance of the application Raman spectroscopy on works of art and archaeology together with its improvements and breakthroughs is underlined on dedicated international scientific conferences (and sessions) such as the International Conference on the Application of Raman Spectroscopy in Art and Archaeology (RAA). The RAA conferences is a great gathering of scientists working on Raman spectroscopy and its instrumental developments and an excellent opportunity to be informed on the latest advances of the technique on Cultural Heritage studies. The first edition of the RAA conference was realized in London in 2001 [1], followed by Ghent (2003) [2] and then Paris (2005) [3], Modena (2007) [4], Bilbao (2009) [5], Parma (2011) [6], Ljubljana (2013) [7], Wroclaw (2015) [8], Évora (2017) [9] and Potsdam (2019) [10]. The 11th edition of the RAA conference (RAA2023, 6 to 9 September 2023) was hosted by the National Gallery-Alexandros Soutsos Museum, located in Athens, Greece in collaboration with Ghent University, Ghent, Belgium. For the first time, the RAA2023 conference included a two-day Raman spectroscopy training school (4 to 5 September 2023) on selected topics addressed to students and early career researchers. As the previous editions, the scientific programme was focused on characterization of materials associated with cultural heritage research (natural and synthetic inorganic and organic materials including biological materials), degradation processes, conservation related themes and challenges, surface enhanced Raman spectroscopy (SERS), chemometrics, Raman spectroscopy related topics in paleontology, paleoenvironment and archaeology, the development and progresses of Raman techniques and the application, advantages and challenges of new Raman instrumentation. The aforementioned topics can be connected either to laboratory or in situ analysis, or both while Raman spectroscopy should act as a core technique in studies that include other analytical techniques. During the RAA2023, these type of studies were organized in 3 keynote and 1 plenary lectures, 30 oral presentations, 4 sponsor oral presentations and 37 poster presentations. The RAA2023 conference garnered 100 registries from 21 countries. There were 332 authors and co-authors in the studies of the conference's scientific programme. This special issue includes 14 selected manuscripts reflecting the philosophy of the conference and the high quality of its contributions, organized in topics suggested to the participants when the RAA2023 was announced. This editorial reviews the most interesting features of the 14 peer-reviewed manuscripts. Nitrogen-based compounds are widespread in the environment due to various sources of natural and anthropogenic origin that introduce them from the most reducing form (the acidic ammonium cation) to the most oxidized one (the nitrate anion). In addition, some environmental conditions, such as pH and redox potential, favour secondary reactions of nitrogenous compounds. In this work, J. Huidobro et al. [11], have compiled the sources and pathways that contribute to the formation of nitrogen-based compounds, especially nitrate salts, in various components and objects of Cultural Heritage. The harmful effects of nitrogen based compounds are due to their high solubility and mobility, which allow them to penetrate the structure of the materials. As a result of their crystallization/dissolution and hydration/dehydration cycles, the precipitation of nitrate salts in the pores causes internal fractures, leading to the subsequent deterioration and loss of the material. Moreover, the ammonium cation is an acid that can react irreversibly with alkaline compounds like carbonate, enhancing their harmful effects. Raman spectroscopy is the only non-destructive analytical technique that can identify all those nitrogen based compounds, and the manuscript covers different published works where Raman spectroscopy was used to identify unequivocally all those compounds. Finally, a data-base of ammonium, nitrite and nitrate compounds is given including mixed nitrate compounds. The difference in the maximum of the main vibration band allow us to clearly differentiate among the 17 referred compounds in the data-base, except for niter and nitrocalcite, In this case both compounds can be differentiate by their full width at half maximum (FWHM). This Raman properties of nitrogen based compounds are really important because they can easily be detected in the field using portable Raman spectrometers. Black and white porcelain plays a critical role in Chinese decorative porcelain history. It is famous for its decorative styles and techniques, which create a strong black and white colour contrast in its appearance. Representative black and white porcelains produced in Shanxi province, Jin and Yuan dynasties (1115–1368 ad), China, were analysed by M. Wang et al. [12] by Raman spectroscopy, X-ray fluorescence and SEM-EDS. The results showed that both Ca-rich (~5.33 wt%) and Ca-poor (~1.99 wt%) glazes were used leading to quite different microstructures. The pigment particles of Ca-rich glazes are characterized by small size (≤ 2 μm), wide distribution and tightly wrapped by anorthite. In contrast, they are larger (≥ 2 μm) and tightly cumulated together in Ca-poor glazes. Haematite is the major crystal in pigment, double substituted by Al and Ti. The Al/Fe ratios are similar, but Ti/Fe ratios are quite different: They do not exceed 0.03% in Ca-poor glazes, whereas they reach 0.12% in Ca-rich glazes, suggesting that the origin of the pigments must be different. Al-rich mineral such as kaolin was added to the pigment preparation in Ca-rich glazes. The colour of the pattern is mainly influenced by the size, quantity and concentration of brown Ti-doping haematite, as well as the thickness of the glaze layer and other crystals and Fe ions. Overall, authors suggest that black and white porcelain has a variety of production techniques, which they attribute to the craftsman adjusting the techniques according to the composition of raw materials. Another characterization work based on the use of Raman spectroscopy and complementary techniques (X-ray Fluorescence and micro-FT/IR spectroscopy) was developed by S. Valadas et al. [13] on the green colours of six mural paintings, executed in 1949 by Almada Negreiros to decorate the maritime station of Rocha do Conde de Óbidos, in Lisbon. These paintings are considered as the highest artistic achievement of the painter. On these paintings, the identification of the green samples is the most challenging work, as Almada Negreiros seems to have explored different hues by using simultaneously inorganic (emerald green, Scheele's green and viridian were detected) as well as organo-synthetic pigments that were rather unusual when employing a traditional mural painting technique (e.g., PG7 and PG8 were identified). Moreover, he seems to have admixed white (titanium white was clearly seen) or blue pigments (surprisingly ultramarine was used) to modify the hues. The different colourants found, as well as the use of mixtures of pigments, hints that Almada Negreiros was keen on experimenting and applying relatively novel painting materials. Identification of the green pigments by Raman spectroscopy is, however, not always straightforward, and it is demonstrated how changes in relative band intensities and band broadening can point to mixtures, where the Raman spectral features of some compounds can easily be overseen in the spectrum. That's way, the use of complementary techniques is really important to clearly identify the mixtures and their pigments. Chinese sauce glazed wares of Yaozhou kilns are famous for their high gloss and distinctive glaze colour palette varying from yellowish-brown to reddish-brown. In the work presented by T. Wang et al. [14], sauce glazes were successfully replicated using the traditional technology of Yaozhou kilns. To characterize the materials, to ascertain the nature and distribution of crystals and to clarify the method used to colour the glazes, micro-Raman spectroscopy, combined with optical microscopy, scanning electron microscopy, X-ray fluorescence and reflective spectroscopy, was systematically applied by authors. Authors found that the yellowish-brown glaze has dendritic ε-Fe2O3 crystals, whereas the reddish-brown colour is mainly derived from dendritic haematite crystals. Such dendritic structure could account for slightly colour variations of the glaze surface observed in different angles, which are also reported in Japanese Bizen ceramics. Principal Component Analysis (PCA) was further applied to study the Raman spectra of these iron oxides, indicating the structural disorders of these crystals introduced by ion substitutions. These substitutions could not only stabilize the crystals but also darken the crystals colour. Besides, high Mg was found in the raw materials probably added to benefit the growth of magnesioferrite crystals. Authors suggest that the relative low level of Fe2O3, high level of SiO2 and CaO may relate to the formation of ε-Fe2O3 crystals. Parchment plays a significant role as the substrate of many archival documents. The assessment of its preservation state is very important towards its conservation and defines the preventive measures that would lead to better-controlled storage and exhibition conditions in museums and libraries worldwide. E. Malea et al. [15] studied the artificial ageing of 48 new goat parchment samples. Five factors have been examined at two levels (low and high): (1) relative humidity, (2) NO2, (3) SO2, (4) exposure time and (5) the order of sequential exposure to NO2 and SO2. The temperature was kept constant at 25°C. Statistics was utilized in advance in the experimental design. In order to study the condition of collagen in parchment and the degradation at molecular level, the authors used Raman spectroscopy along with Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy. Chemometrics involving analysis of variance (ANOVA) was used for the investigation of the entire set of environmental factors along with detected changes in the spectra. According to the results, the onset of collagen's secondary structure decomposition was observed. Statistical elaboration of data reveals that the two analytical methods function in a complementary manner. Ancient textiles are now rapidly analysed using Surface-Enhanced Raman Spectroscopy (SERS) due to its ability to provide chemical information with single-molecule sensitivity, although it is not easy the compromise between attaining a high level of detection sensitivity and maintaining the damage of samples to a minimum extent. In the work by X.-H. Xi et al. [16] such a compromise has been achieved with a SERS approach combining both microextraction and detection functions. As dyes strongly bound on textiles can be extracted by ethanol (EtOH), authors used an alcohol–water droplet with iodide-modified Au nanoparticles (AuIMNPs) to directly drop on the textile As a result, the sample can be well preserved from being damaged. In particular, the volatility of EtOH allows the molecules to be captured in the hot spots through the capillary effect during droplet evaporation, resulting in a dramatic increase in Raman signal intensity. This highly sensitive strategy can be used to measure dyes in plant extracts and mock-up textiles. Furthermore, the capability of SERS to provide fingerprint information allows to distinguish different dyes in overdyeing textiles. This rapid, universal and negligibly invasive approach provides a powerful way to study coloured ancient textiles. Art Déco housewares made of hammer-beaten copper-based alloys, known as dinanderie, was an innovative method developed by the European Jean Dunand (1877–1942) artist. Nowadays, the nature of the constituting materials has not been well clarified; therefore, for the first time, three objects, namely, a bowl, a trinket bowl and a vase, have been investigated by a multi-modal work performed by A. Passaretti et al. [17]. Mobile or benchtop Raman, X-ray fluorescence and Fourier-transform infrared (FTIR) spectrometers were used to adapt the analyses to the shape and size of the artefacts. Elemental analysis verified that the vase consisted of brass Cu70Zn30, whereas the bowl and the trinket bowl were made by the so-called nickel silver alloy Cu70Zn19Ni11. Raman spectroscopy identified tenorite (CuO) as the pigment for the black finishing appearance, ascribing the patina to an intentional artist's willing and not to spontaneous tarnishing processes. Metal soaps of copper and zinc were documented as degradation products by FTIR spectroscopy. The drawings adorning the vase and the trinket bowl were identified as silver-based, contrary to what was hypothesized by conservators (i.e., tin based) due to conventional Dunand's inlaying technique. Besides single-point analysis, Raman imaging was performed in-situ, applying for the first time a Virsa™ Raman Analyser (Renishaw) in the field of cultural heritage. The fibre-optic-coupled instrument allowed complying constantly with the artefacts' geometry thanks to the modular probe and the motorized focus-tracking stand. The synergic combination of elemental and vibrational analyses resulted successful, providing new and unique information on artist's technique in view of possible restoration interventions. A collection of six drawings spanning from 1700 to 1899 from the National Museum in Krakow and a 20th-century birch bark artefact were analysed in the work presented by A. Klisińska-Kopacz et al. [18] aiming to expand knowledge across diverse periods and materials on drawings. Authors selected Raman spectroscopy, assisted by X-ray Fluorescence (XRF) as the most promising techniques to obtain the required information using portable materials included haematite, iron and crystal the chemical composition conservation and is a for art and preservation studies. In this and non-destructive analytical techniques the analysis of materials, providing diverse in really time periods of et al. Raman spectroscopy and other analytical techniques in order to study materials, to and the of This research is aiming towards the development of the approach for on the The studies of the and the secondary compounds by environmental on the two were using laboratory and X-ray spectroscopy, X-ray and ion on selected samples by the The of black in the of the was related to the growth of of secondary compounds and biological and the of particles from and The main materials characterized were and iron compounds such as haematite and The detection of lead compounds suggested in the way the of the environment on the degradation and of materials. The identification of mainly to a and could be by the of compounds in the as a result of an acid of by was on the results on the secondary products of the with the degradation is probably connected to the the where the is in to the of the of biological such as pigments and together with other were ion the authors were to characterize ammonium The of this causes a chemical degradation of materials time, because the acidic nature of the ammonium In both the of nitrate compounds, and was to both natural factors nitrate is from the decomposition of plant and and anthropogenic The in at the of and a with such as coloured and S. et al. applied analytical techniques for the study of the pigments used on various decorative as well as the from the from the at Identification of the in the colours of the samples was with the of their using and with portable X-ray fluorescence during the at the A portable Raman was used for the in situ analysis of the inorganic colourants of the decorative materials, whereas a laboratory Raman was used for the characterization of inorganic and organic colourants of the samples and the with was applied for the identification of the components of the organic from the The samples examined to were during the of the and as the main pigments of these decorative The pigments were applied in various to a of from to was also used in of the The on the was whereas the preparation and consisted of and lead white The the was with the most organic of the as the most important in China, is famous for its Wang et al. by using micro-Raman spectroscopy combined with optical microscopy, X-ray fluorescence and scanning electron with X-ray spectroscopy, studied black and of in Shanxi province, one of the core distribution The colour of the is derived from haematite and The black on the surface could be two according to the ratios and (1) low and and and (2) high and The colour is the Moreover, this study the of micro-Raman in the nature of crystals in the in the of The micro-Raman spectroscopy study on the black and of was not or presented at the RAA2023 but it is included in the RAA2023 special issue because of its to the conference in from known may provide information for the of in the techniques of their and the of or their to a type of in 14 between the first and in the of the Museum of were studied by M. et al. The authors used techniques, for optical as well as Raman spectroscopy and spectroscopy applied as The chemical characterization of the samples was by the use of X-ray samples were found during in and of whereas one was found in the of The samples were identified as samples as natural with to from one as a natural and one as an artificial The origin of the artificial is whereas that of is with being the most possible micro-Raman spectroscopy, scanning electron microscopy, optical and X-ray were combined in a by A. et al. in order to samples. The highly distinctive production of from two different at and the so-called at was In order to and the the and distinctive properties of the investigated samples were of by by studies with the main being micro-Raman spectroscopy, allows the between and quality and the of the investigated samples with some realized in known in and characterization allow of samples from indicating in the of For the from the so-called an from is This work the advantages of applying in order to obtain results in the analysis of ancient ceramics. The importance of micro-Raman spectroscopy is underlined especially in were other techniques to The and analysis of is of a that with the most objects from the Museum of these objects, the analysis of the two most interesting namely, a by and a is by A. et al. infrared and and (X-ray fluorescence and Raman spectroscopy) methods were applied to of conservation and the techniques of the and reveals the between the that are not easily detected X-ray fluorescence for objects, suggesting previous conservation of previous conservation were detected by the use of Raman spectroscopy, especially Raman were from the surface μm) to a of with a of The results the identification of three different of (1) where the was preserved (2) where the was with a layer of a and (3) where were for loss and by using a and This study allowed the development of a for the analysis of objects, with a special in the of Raman The authors with the application of and a and of such was The 14 works included in this special together with their are excellent of state of the art on innovative of Raman spectroscopy, from samples to artefacts. The advantages of field analysis, the the use of chemical to environmental the use of analysis to and the of materials together with the have been some of the innovative presented in this with the use of have demonstrated that Raman spectroscopy is used as the core and a first with other instrumental techniques, to heritage and analysis is one of the topics with Raman spectroscopy being an excellent for this type of studies. Raman spectroscopy can be applied with and benchtop Raman spectroscopy great and important The of the the RAA2023 conference to a research among scientists from different has been clearly increase such in the and this in the works to be presented in the Conference in are to the participants and that assisted in the RAA2023 conference In particular, would like to the National Gallery-Alexandros Soutsos Museum, Athens, Greece and Ghent University, Ghent, Belgium. The and thanks the the and both of them for also at the Raman spectroscopy training (and (and and (and The of and is would like to especially the of the of Raman Spectroscopy and the of Raman Spectroscopy for the RAA2023 conference through this special the of Ghent for