Applications of Raman spectroscopy in art and archaeology

Abstract

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 century, and the Church nr. 5 (Süslü church, eighth to ninth century) in Güllü Dere. The painting materials were investigated with a multianalytical approach by using cross-section examination by μ-Raman spectroscopy, assisted with XRF and FTIR. The wall paintings in the church of St. John the Baptist present superimposed layers, attributed to different historical periods based on the characteristics of the mortar/ground layer; the pigments, mainly based on iron earths and ochres, were clearly identified, but key molecular phases like gypsum, anhydrite, oxalates and organic materials were also detected, mainly on the surface, being a clear indication of chemical and biological degradation processes. The Church nr. 5 at Güllü Dere shown a unique pictorial execution because the pigments were applied on a gypsum-based ground layer, exhibiting clear alteration patterns as decipher from lead based compounds, such as lead oxides and carbonates; the nonexpected presence of anatase, associated to the red and yellow pigments, suggest a clear tracer derived from the characteristic soils areas used to prepare the ochre pigments of the paintings. Reinforced concrete became a common building material in the 20th century. Currently, we are confronted with degradation processes of this material in modern, cultural heritage buildings. Ibarrondo et al.[19] proposed using portable Raman spectroscopy as a ‘new hammer for architects’ to perform on site measurements on building elements made of reinforced concrete and Portland-type mortars to diagnose their conservation state without taking surface samples. Using two portable Raman systems (532- and 785-nm lasers), both original materials and their degradation products in efflorescences were studied in two buildings in Biscay province (Punta Begoña Galleries nearby the Bilbao Commercial Harbor, Getxo [1918] and the Library of the University of the Basque Country UPV/EHU, Campus of Leioa [1968]). Both buildings were strongly affected, the Galleries by direct marine and industrial environment nearby the harbour and the library by the urban-industrial atmosphere of Bilbao. Despite the differences in locations and the original materials, the results show similarities in degradation products (salts, sulphate compounds and iron compounds) that mostly reflect the input of aerosol and atmospheric SO2 dissolved in infiltrating acid rainwater. However, organic compounds (metal carboxylates) were detected in the Galleries only. This difference is probably due to carboxylic acids in exhaust gases of ship engines transported as marine aerosols into the harbour region. Confocal μ-Raman spectroscopy, equipped with an excitation laser at 785 nm, was used by Rigula et al.[20] to perform a depth-profiling study of an 18th century gun-powder horn. Also, the Raman characterization of different reference Baltic amber samples was performed to have an adequate database for comparison purposes. Authors observed important difference in the range 1400–1700 cm−1 from surface measurements to bulk ones. Taken into account the intensity of the 1450 cm−1 (CH bending) band, the characteristic band at 1645 cm−1 (CC nonconjugated structures) decreases from bulk to the surface while the band at 1615 (CC aromatic rings) increases as degradation proceeds. The extension of surface degradation of the amber can be ascertained using the ration between the intensity of the band at 1645 cm−1 against that of band at 1450 cm−1. Authors proposed that method to estimate the thickness of the degradation. Moreover, they highlight the convenience of using confocal measurements instead of surface ones to obtain adequate values of the ratio. They observed that even when the amber object is stored in good environmental condition, the degradation of surficial amber could reach depths from 100 to 150 μm, being much higher for non-adequate storing. The work of Retko et al.[21] shown the potential of a photoreduced SERS substrate for detecting organic colourants in lipidic and proteinaceous paint layers, even if the several colourants were mixed. The organic dyes madder lake, cochineal lake and lac dye were selected for this study. Several SERS procedures, using different approaches, were tested, namely, direct application, soaking (incubation) of the sample in the substrate and hydrolysis with the hydrofluoric (HF) acid vapours. For the case of colourants in linseed oil, a pretreatment step was required either using soaking/incubating or hydrolysing with HF. The direct exposition of a cross-section of the sample, taken from a polychrome work of art, to the vapours of HF, allowed the SERS detection of the cochineal lake pigment in one of the paint layer of the cross-section, showing that such SERS substrates could be the base for the stratigraphic study of real cultural heritage samples. As the analyses by means of SERS are microinvasive, authors studied also the potential of non-invasive reflection FTIR spectroscopy analyses for the identification of the organic colourants as a complementary procedure to the new SERS approach. Furthermore, this study revealed to positive identification of madder lake pigment in the paint layers based on the characteristic bands of the hydrated alumina. Portuguese everyday plastic objects presumably produced between the 1950s and the 1970s were studied by Angelin et al.[22] identifying their red colourants. Polyethylene, polystyrene and polypropylene objects were studied using non-destructive optical microscopy, confocal μ-Raman spectroscopy (633- and 785-nm excitations), ATR-FTIR spectroscopy and energy-dispersive XRF microspectroscopy. Three inorganic red pigments were identified by μ-Raman spectroscopy; cadmium sulfoselenide (Cd(S,Se)), lead chromate/molybdate/sulphate (Pb(Cr,Mo,S)O4) and hematite (α-Fe2O3). In addition, three organic red pigments based on β-naphthol (2-naphthol) were identified. Fading of the organic pigments was proven in some objects. Based on the analyses the authors propose an in situ multianalytical protocol for the study of red pigments in plastics. This study provides insights into historical plastic formulations and gives information for further conservation works. The work conducted by Łydżba-Kopczyńska et al.[23] proposes the applicability of μ-Raman and VIS–NIR reflectance spectroscopy to determine the age of blue ballpoints inks containing phthalocyanine and crystal violet. The method has been developed using 74 different random blue pens, classified to various types of inks. Then, handwritten samples were created and stored in different light conditions prior to analysis. The first set of samples was stored for 7 years at room temperature in an envelope without direct exposure to the light while the second set of samples was aged by exposing them to natural light cycle for 85 days. The age, the obtained L,a,b values and the recorded Raman and VIS–NIR spectra were used for chemometric analyses, mainly partial least squares. The models obtain correlated age, ∆E, Raman and reflectance spectra with Relative Standard Errors of Prediction (%RSEP) 1.97–5.13 for Raman and 1.20–2.27 for VIS–NIR spectra, respectively. Thus, the proposed non-destructive method allowed to estimate the age of daylight exposed samples of blue ballpoints inks and can be extended to other inks with pigments sensible to sun ageing. The work conducted by Pinto et al.[24] on some archaeological glass pieces from the Pintia archaeological site (Padilla de Duero, Valladolid, Spain) gave information on the local production processes of the Vaccaei culture. Glass beds were highly appreciated goods in the Protohistory of the Iberian Peninsula, but the known specialized production of those glass pieces seemed to be noncompatible, in opinion of the archaeologists, with beds found in Pintia, suggesting local workshops with own production processes. To clarify this, 15 representative pieces, covering the period IV-I centuries of the Vaccaei culture, were selected among the 600 beds found in closed tombs during a campaign excavation performed in 2018, were examined by Raman spectroscopy assisted with ESEM/EDX. The pigments employed on these samples were identified by Raman spectroscopy, finding lead oxides, calcium antimonate, haematite and Naples yellow as the typical ones used in more than 400 years. Moreover, the thermal alterations found in most of the beds revealed a maximum temperature of 600°C, which confirmed previous estimations derived from the state of conservation of the bone remains about the cremation rituals in the Vaccaei culture. The combination of the two spectroscopic techniques provided valuable information about the fabrication of these analysed glass beds, identifying diverse features that could be related to different workshops. In particular, the characteristics of a sophisticated bifacial pendant were found to be compatible with their provenance from Carthage. μ-Raman spectroscopy analysis of a set of Southern Italy pottery materials, dated between 13th and 16th century, was performed by Caggiani et al.[25] A total of 27 samples taken from six archaeological sites of Campania and Sicily were studied non-invasively to obtain information on the nature of glass surface, the opacifying minerals and the pigments used to obtain the different colours. The oldest samples were medieval lead–tin-glazed ceramics (13th century), followed by transition-enameled wares (14th to 16th centuries) and then white enamels with blue figurations (16th century). The archaeological hypothesis states that these three classes resulted from the technological evolution in the field of glazed ceramic production and the scientific research was conducted to identify the characteristic materials used during these four centuries. To that, μ-Raman spectroscopy assisted with SEM/EDS was the two analytical non-invasive techniques used in this work. The opacifying minerals were identified as the tracer of the technological changes among the three periods of ceramic production. Authors suggested the use of a new parameter (equal to the ration between the main cassiterite Raman band area and the glass polymerization index) to group the different samples according to the three different production technologies. Crystallization of lead-potassium feldspars in the coating-body interface was confirmed by SEM/EDS and the nonreferred Raman bands in literature were assigned to different element–element moieties for these assemblages. The classical technique to study the minero-petrographic assemblages is X-Ray diffraction (XRD). But when dealing with fine-grained ceramics, this technique does not distinguish the different minerals. In the work presented by Odelli et al.,[26] on several archaeological fine-grained ceramics from the site of Volterra (Tuscany, Italy), μ-Raman spectroscopy is demonstrated to be the instrumental technique of choice, because it provides not only information about the raw materials but also on the firing temperatures attained during the manufacture process. The work had been conducted using not only μ-Raman spectroscopy but also optical microscopy, XRD and SEM/EDS microscopy. In conclusion, when fine-grained ceramics have to be studied and firing temperature information are relevant in archaeological interpretation, μ-Raman analysis on small spot areas, and on mineral grains acting as temperature indicators such as feldspars, is recommended. Since the last decades of past 20th century, the conservation of Industrial Heritage (IH) objects has been recognized in emerging studies related to its history. However, scientific-based investigations are still rather scarce. The work presented by Tissot et al.[27] on the paint coatings of three energy generators from the early 20th-century powerplant at Levada de Tomar, Portugal, is an example of how this scientific knowledge can be gained to prepare adequate intervention action to an industrial remain that, in this case, ceased its activity in 1996. Samples from the three generators installed in 1924, 1927 and 1944 were investigated using μ-Raman spectroscopy assisted with μ-XRF spectroscopies and scanning electron microscopy with energy-dispersive spectroscopy. This multianalytical approach was used to identify together with traditional pigments like Prussian blue, red iron oxide and carbon black, pigments used in industrial areas like copper phthalocyanine and toluidine red were identified as modern organic pigments. Complex paint systems of the oldest equipment (1924) were revealed as well as the recoating of the equipment that worked during a longer time (1944–1990). The identified pigments provided a chronological chromatic pallet of the use of the three energy generators. Moreover, important clues related to maintenance and subsequent corrosion events were ascertained. For example, powdery carbon black layers were detected, probably coming from incomplete hydrocarbon combustion, between metallic substrates and coating layers, suggesting the absence of paint coating removing as a maintenance practice after the powerplant shutdown. Also, magnetite was identified as a corrosion product of the iron alloy substrate, revealing that corrosion developed after the engine shutdown and not during the operation period. This work highlights the need of scientific-based approaches to study the industrial heritage, being Raman spectroscopy the key instrumental technique for a rapid advance in this emergent cultural area. Furnaces are interesting archaeological objects as they function as indicators for productive sites for (a) melting metals, (b) making ceramics for bricks or pottery and (c) preparing lime from carbonate rocks (calcara), thus offering information about ancient technologies and trading routes. Identifying metal smelting furnaces is typically easy, but identifying ceramic or calcara producing furnaces is a challenge, especially when kilns are poorly preserved and remnants of final products such as pottery, bricks or other archaeological indications are absent. Rossi et al.[28] developed a discrimination method based on vibrational spectroscopy. Samples were taken from the operating tops of four Roman and pre-Roman furnaces, one in the ancient sanctuary of Hera near Paestum, two in the ancient city Velia near Ascea, and one near the ancient settlement of Roscigno in the innermost area of Cilento. Optical microscopy, Raman spectroscopy (514-nm laser, range 200–2000 cm−1) and FTIR spectroscopy (range 400–4000 cm−1) were used to identify potential precursor minerals of final products. Prior to analysis, the geology of the archaeological sites was checked using geological maps to rule out possible interferences with the precursor minerals of interest. Based on these results, the authors proposed a procedure, summarized in a flow diagram, to assign furnaces their productive functions. The medieval shipwreck of Urbieta (Gernika, North Spain) was discovered in 1998 in the Urdaibai estuary. Following salvaging and preservation measures, it is now conserved in the Archaeological Museum of Bilbao, including iron nails extracted from the wreck. Estalayo et al.[29] studied four of these iron nails showing obvious alteration using the non-destructive analytical techniques μ-Raman spectroscopy (514- and 532-nm laser) and energy-dispersive XRF microspectrometry. A high-resolution XRF map (25-μm step width) of a nail cross-section revealed very different element distribution patterns (Fe, Cu, Mn, K, Ca, S, Si, Ti and Zn) which were the result of quite different alteration processes. Raman analyses revealed mineral phases that formed during degradation and thus shed light on the underwater environmental conditions, corrosion processes and the conservation state of the nails after excavation. Such specific information can be used to develop a precise conservation plan. In antiquity, black limestones (known as bigi morati) had an outstanding importance in sculpture and architecture. These precious stones were quarried throughout the Mediterranean area. Up to now, black limestone provenance is based on microdestructive methods such as minero-petrography and stable isotope analyses. These applications are restricted or impossible in cases of precious artefacts where sampling is not allowed. Raneri et al.[30] tested laboratory-based μ-Raman spectroscopy to establish the provenance of grey and black limestones used in antiquity. The applied non-destructive method is based on spectroscopic parameters extracted from Raman spectra of carbonaceous components in the rocks. Samples from six different quarries in Italy, Greece, Turkey, and Tunisia known to have been exploited in antiquity were analysed. Different plots of the obtained Raman parameters show convincing discrimination patterns that could be used for in situ analysis using portable Raman equipment. The 20 works included in this special issue are excellent examples of the innovative applications of Raman spectroscopy from prehistoric samples to present-day artefacts and represent mostly the state of the art in its application to art and archaeology. From the possibilities of field analysis, to chemical modelling and/or chemometric analysis to explain and interpret the presence of unexpected materials together with the original ones have been some examples of the innovative approach presented in this congress. Also, papers dealing with the use of complementary non-destructive as well as microdestructive instrumental techniques to support the Raman information continuing a clear growing applicability to solve complex heritage problems. All of them have in common Raman spectroscopy as the core of the manuscripts included in this special issue. The contribution of the people attending the RAA2019 Congress to a collaborative research among scientists from different fields (restorers, chemists, geologists, biologist, environmentalists, architects, etc.) has been clearly shown and we hope to increase such cooperation in the works to be presented in the forthcoming RAA2021 in Athens, Greece. We are extremely grateful to the participants and institutions that assisted in making the conference possible. In particular, we would like to thank our main sponsor DBU-Deutsche Bundesstiftung Umwelt, as well as the gratifying variety of exhibitors of Raman spectroscopic techniques and instrumentations, the ‘Sans souci’ sponsors, HORIBA Jobin Yvon GmbH, Renishaw GmbH, B&W Tek, Inc., Witec GmbH and the ‘Neues Palais’ sponsor Thermo Electron GmbH. J. M. Madariaga acknowledges the support of the UNESCO Chair for Cultural Landscapes and Heritage (UPV/EHU).