Applications of Raman Spectroscopy in Art and Archaeology

Abstract

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 11^th 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 20^th-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 even using portable instruments. Identified materials included haematite, calcite, graphite, bister, sepia, iron gall ink and crystal violet dye. Understanding the chemical composition aids conservation and is a reference for future art history and preservation studies. In this sense, non-invasive and non-destructive analytical techniques facilitated the systematic analysis of drawing materials, providing insights into diverse substances over centuries, in really short time periods of analysis.

I. Costantini et al. [19], employed Raman spectroscopy and other auxiliary analytical techniques in order to study building materials, mostly carbonated, belonging to Santa Maria Cathedral and the Medieval Wall of Vitoria-Gasteiz (Spain). This diagnostic research is aiming towards the development of the best approach for conservation/consolidation treatments on the stones. The studies of the lithology and the secondary compounds originated by environmental impacts on the two monuments were carried out using laboratory instruments (μ-Raman and micro-energy-dispersive X-ray spectroscopy, X-ray diffraction and ion chromatography) on selected samples provided by the restorers. The systematic presence of black crusts in the stones of the Santa Maria Cathedral was related to the growth of microcrystalline structures of secondary compounds and biological patinas and the deposition of atmospheric particles from traffic and house heating systems. The main materials characterized were carbon (C), and iron compounds such as haematite (α-Fe₂O₃), goethite (α-FeOOH), magnetite (Fe₃O₄) and lepidocrocite (γ-FeO(OH)). The detection of lead compounds (lead-rich hydroxyapatite (Ca,Pb)₁₀(PO₄,CO₃)₆(OH,F,Cl)_2.56 1.5H₂O) suggested in the same way the impact of the urban environment on the degradation and blackening of stone materials. The identification of sulphates, mainly gypsum (CaSO₄·2H₂O) and, to a lesser extent, epsomite (MgSO₄·7H₂O), anhydrite (CaSO₄) and bloedite (Na₂Mg(SO₄)₂·4H₂O), could be caused by the sulphation of carbonated compounds in the stones as a result of an acid attack of atmospheric pollutants. Degradation by microorganisms was more evident on the results on the secondary products of the Medieval Wall compared with the Cathedral. Microorganism degradation is probably connected to the large garden surrounding the fortification, where the grass is in direct contact to the lower part of the structure. Markers of biological activity, such as carotenoid pigments and calcium oxalate weddellite (СаС₂О₄·2Н₂О), together with other soluble oxalates were identified. Via ion chromatography the authors were able to characterize ammonium nitrate. The presence of this compound causes a chemical degradation of carbonate stone materials over time, because the acidic nature of the ammonium ion. In both cases, the presence of nitrate compounds, nitratine and potassium nitrate, was attributed to both natural factors (ammonium nitrate is coming from the decomposition of plant and animal excretions) and anthropogenic contamination.

The cist tomb (4^th century bc) discovered in 1987 at the area of Lakkoma Chalcidice (Macedonia, Greece) and featured a wooden funeral couch with intricate ornaments, such as clay “eyes,” coloured plaster and clay gorgoneions. S. Vivdenko et al. [20] applied several analytical techniques for the study of the red pigments used on various decorative features, as well as the purple dye from the deceased's attire collected from the site at Lakkoma Chalcidice. Identification of the chromophoric minerals in the colours of the samples was carried out with the examination of their thin sections using polarizing light microscope and with portable X-ray fluorescence during the cleaning at the excavation. A portable Raman spectrometer was used for the in situ analysis of the inorganic colourants of the decorative materials, whereas a laboratory Raman spectrometer was used for the characterization of inorganic and organic colourants of the grain samples and the dye. High-pressure liquid chromatography with diode array detector was applied for the identification of the colouring components of the purple organic dye from the deceased's clothes. The samples examined (0.5- to 1.5-mm diameter) were collected during the initial cleaning of the surface. Vermilion (HgS) and red ochre (haematite, α-Fe₂O₃) served as the main pigments of these decorative features. The pigments were applied in various combinations to produce a range of red hues, from deep to bright red. Vermilion was also used in every element of the couch. The stucco on the decoration was gypsum (CaSO₄·2H₂O), whereas the preparation layers over clay and stucco elements consisted of calcite (CaCO₃) and lead white (2PbCO₃·Pb (OH)₂), respectively. The cloth covering the deceased was dyed with the most expensive organic dye of antiquity, the Tyrian (shellfish) purple.

Yangshao culture, as the most important Neolithic culture in China, is famous for its finely decorated pottery. Y. Wang et al. [21], by using micro-Raman spectroscopy combined with optical microscopy, X-ray fluorescence and scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy, studied black and red decorations of Yangshao potteries excavated in Shanxi province, one of the core distribution areas. The colour of the red decor is derived from haematite (α-Fe₂O₃) and maghemite (γ-Fe₂O₃). The more complex black decorations on the surface could be divided into two types according to the Mn/Fe ratios and crystalline types: (1) low MnO₂ /Fe₂O₃ ratio and jacobsite (FeMn₂O₄) and magnetite (Fe₃O₄) and (2) high MnO₂/Fe₂O₃ ratio and hausmannite (Mn₃O₄). The former colour is brighter than the latter. Moreover, this study illustrates the potential of micro-Raman microscopy in analysing the nature of colourant crystals in Yangshao painted potteries, thus better deciphering the technical details involved in the potteries of Yangshao culture. The micro-Raman spectroscopy study on the black and red decorations of Neolithic Yangshao potteries was not submitted or presented at the RAA2023 conference, but it is included in the RAA2023 special issue because of its relevance to the conference topics.

Investigating Roman gems in jewellery from known archaeological contexts may provide valuable information for the trading routes of gems in antiquity, the techniques of their manufacture and decoration, the popularity of certain gems or their relation to a specific type of jewellery. Nineteen green-coloured gems, mounted in 14 Roman jewellery pieces, dated between the first and fourth century ce, belonging in the collections of the Archaeological Museum of Thessaloniki, Greece, were studied by M. Nikopoulou et al. [22]. The authors used nondestructive techniques, for example, optical microscopy as well as Raman spectroscopy and visible–near-infrared spectroscopy applied as mobile approaches. The chemical characterization of the samples was accomplished by the use of microenergy-dispersive X-ray fluorescence. Most samples were found during rescue excavations in western and eastern Roman cemeteries of Thessaloniki, whereas one was found in the city of Edessa. The 19 samples were identified as follows: seventeen samples as natural emeralds with characteristics similar to emeralds from Egypt, one as a natural chrome chalcedony and one as an artificial glass. The geographic origin of the artificial glass is unknown, whereas that of chrome chalcedony is under discussion with Turkey being the most possible source.

Colourimetry, micro-Raman spectroscopy, scanning electron microscopy, optical microscopy and powder X-ray diffraction were combined in a multianalytical protocol by A. Privitera et al. [23] in order to investigate Imperial (I-II century ad) Roman cooking ware samples. The highly distinctive production of pots coming from two different archaeological sites, ‘Villa della Piscina’ at Centocelle district and the so-called ‘Minerva Medica Temple’ at Esquilino district (Rome, Italy), was studied. In order to compare technological and provenance aspects, the colour, the chemical, mineralogical and petrographic distinctive properties of the investigated samples were characterized. Classification of ceramic fragment by colourimetry, integrated by compositional studies with the main contributor being micro-Raman spectroscopy, allows the discrimination between superior and inferior quality cooking ware and evaluate the compatibility of the investigated samples with some pottery realized in known manufacturing sites in Lazio. Compositional and petrographic characterization allow assignment of samples from ‘Villa della Piscina’ indicating productions in the surroundings of Rome. For the ceramic fragments from the so-called ‘Minerva Medica Temple’, an importation from outsider workshops is hypothesized. This work confirmed the advantages of applying multianalytical investigations in order to obtain thorough results in the analysis of ancient ceramics. The importance of micro-Raman spectroscopy is underlined especially in cases were other techniques fail to produce (reliable) results.

The multidisciplinary and noninvasive analysis of baroque amber artworks is part of a project that deals with the most valuable objects from the Museum of Gdansk (Poland). Among these objects, the analysis of the two most interesting ones, namely, a Baroque Gdansk wardrobe-shaped amber cabinet (made by Johann Georg Zernebach, Gdansk, 1724) and a crucifix (Gdansk, 17^th century), is discussed by A. Rygula et al. [24]. Macroanalytical (ultraviolet, infrared and X-radiography) and microanalytical (X-ray fluorescence and Raman spectroscopy) methods were applied to reveal traces of old conservation treatments and uncover the techniques of the 17^th and 18^th century amber masters. Ultraviolet photography reveals the differences between the amber plates that are not so easily detected under visible light. X-ray fluorescence detects elements atypical for amber objects, suggesting previous conservation treatments. Areas of previous conservation treatments were detected by the use of confocal Raman spectroscopy, especially Raman depth profiling. Single-point scans were collected from the surface (0 μm) to a depth of −200 μm, with a step of 50 μm. The results enabled the identification of three different types of zones: (1) areas where the amber was preserved without any protective layer, (2) areas where the amber was covered with a thin layer of a protective substance and (3) areas where amber elements were compensated for loss and completed by using a binding agent and filler. This study allowed the development of a comprehensive methodology for the analysis of amber objects, with a special focus in optimizing the capabilities of confocal Raman microscopy. The authors stressed that, with the application of macroanalytical and microanalytical approaches, a comprehensive and broad overview of such complex artworks was achieved.

The 14 works included in this special issue, together with their references, are excellent examples of current state of the art on innovative applications of Raman spectroscopy, from prehistoric samples to present-day artefacts. The advantages of field analysis, the multianalytical approach, the use of chemical modelling to interpret environmental impacts and/or the use of chemometric analysis to explain and interpret the presence of unexpected materials together with the original ones have been some examples of the innovative approaches presented in this conference. Papers dealing with the use of multianalytical approaches have demonstrated that Raman spectroscopy is used more frequently as the core and a first choice technique, complemented with other instrumental techniques, to solve complex heritage problems. Noninvasive and nondestructive analysis is one of the topics discussed, with Raman spectroscopy being an excellent candidate for this type of studies. Raman spectroscopy can be applied with mobile and benchtop Raman spectroscopy systems producing undoubtedly great and important results.

The contribution of the people attending the RAA2023 conference to a collaborative research among scientists from different fields (restorers, chemists, geologists, biologist, environmentalists, architects, etc.) has been again clearly shown. We hope multidisciplinary approaches will increase such cooperation in the future, and this will reflect in the works to be presented in the forthcoming RAA2025 Conference in Pisa, Italy.