Invasive and non-invasive analyses for knowledge and conservation of Roman wall paintings of the Villa of the Papyri in Herculaneum

Non-Destructive Prediction of the Mixed Mineral Pigment Content of Ancient Chinese Wall Paintings Based on Multiple Spectroscopic Techniques

This study first developed non-destructive and accurate methods to predict the relative contents of mixed mineral pigments in ancient Chinese wall paintings using multiple spectroscopic techniques. The colorimetry, attenuated total reflection Fourier transform infrared spectroscopy (ATR FT-IR), ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy, and Raman spectroscopy were employed. Analyses were conducted including color difference, spectral reflection, ATR FT-IR spectra, and Raman mapping for simulated samples (malachite-lazurite mixed with rabbit glue samples) before and after aging. Models were then established for predicting the relative pigment contents of samples using UV-Vis-NIR and ATR FT-IR spectral data with Beer-Lambert law, and mathematical methods comprising principal component analysis (PCA) and nonlinear curve fitting. In particular, PCA and empty modeling methods combined with non-negative partial least squares were developed to predict the relative pigment contents based on Raman mapping data. The results demonstrated that approaches comprising PCA, mathematical model, and empty modeling based on the spectral data were effective at predicting the relative pigment contents. The predicted results obtained using the mathematical model based on UV-Vis-NIR spectra had an error of about 2%, and the best prediction based on ATR FT-IR spectra had an error of <3.6% at 1041 cm-1. The errors for the predictions using PCA and empty modeling based on Raman mapping data were 0.01-9.30% and 0.28-7.15%, respectively. However, the predicted relative pigment contents obtained based on ATR FT-IR data combined with the Beer-Lambert law had higher errors. The findings of this study confirm the strong feasibility of using spectroscopic techniques for quantitatively analyzing mixed mineral pigments.

Objective interpretation of ultraviolet-induced luminescence for characterizing pictorial materials

Ultraviolet-induced Luminescence (UVL) is the property of some materials of emitting light once illuminated by a source of UV radiation. This feature is characteristic of some mediums and pigments, such as some red lakes, widely used for the realisation of works of art. On the one hand, UVL represents a like strike for a researcher in the cultural heritage field: in fact, UVL allows to characterise the state of conservation of the paintings and, in some cases, to recognize at glance some of the materials used by the artists. On the other hand, the contribution of UVL to the study of the artefacts is almost always limited to qualitative observation, while any speculation about the cause of the luminescence emission relies on the observer's expertise. The aim of this paper is to overcome this paradigm, moving a step toward a more quantitative interpretation of the luminescence signal. The obtained results concern the case study of pictorial materials by Giuseppe Pellizza da Volpedo (1868-1907, Volpedo, AL, Italy) including his iconic masterpiece Quarto Stato (1889-1901), but the method has general validity and can be applied whenever the appropriate experimental conditions occur. Once designed an appropriate set-up, the statistical comparison between the acquisitions performed on Quarto Stato, on a palette belonged to the master, on drafts made by the author himself and on a set of ad hoc prepared samples both with commercial contemporary pigments and prepared with the traditional recipe, shed some light on which materials have been employed by the artist, where they have been applied and support some intriguing speculations on the use of the industrial lakes in the Quarto Stato painting.

Review of recent advances on the use of mass spectrometry techniques for the study of organic materials in painted artworks

The study of painted artworks using scientific methods is fundamental for understanding the techniques used in their creation and their appropriate conservation. The ethical constraints involved in the handling of, and sampling from, these objects has steered recent developments in the field of Heritage science towards a range of new non-invasive/non-destructive spectroscopic techniques capable of providing important insights into their elemental or bulk chemical compositions. Due to the inherent complexities of heritage artefacts, however, their organic components are especially difficult to study in this way and their identification and degradation pathways are thus often best investigated using mass spectrometric (MS) techniques. The versatility, sensitivity and specificity of MS techniques are constantly increasing, with technological advances pushing the boundaries of their use in this field. The progress in the past ten years in the use of MS techniques for the analysis of paint media are described in the present review. While some historical context is included, the body of the review is structured around the five most widely used or emerging capabilities offered by MS. The first pertains to the use of spatially resolved MS to obtain chemical maps of components in cross-sections, which may yield information on both inorganic and organic materials, while the second area describes the development of novel sample preparation approaches for gas chromatography (GC)-MS to allow simultaneous analysis of a variety of components. The third focuses on thermally assisted analysis (either with direct MS or coupled with GC-MS), a powerful tool for studying macromolecules requiring zero (or minimal) sample pre-treatment. Subsequently, the use of soft ionisation techniques often combined with high-resolution MS for the study of peptides (proteomics) and other macromolecules (such as oligosaccharides and triglycerides) is outlined. The fifth area covers the advances in radiocarbon dating of painting components with accelerator MS (AMS). Lastly, future applications of other MS techniques to the study of paintings are mentioned; such as direct analysis in real time MS (DART-MS) and stable isotope ratio MS (IRMS). The latter, having proven its efficiency for the study of lipids in archaeological artefacts, is envisioned to become a valuable tool for this area, whereas DART-MS is already being utilised to study the surface composition of various museum objects. Rapid technological advances, resulting in increased sensitivity and selectivity of MS techniques, are opening up new approaches for paintings analysis, overcoming the fundamental hurdle of sample size available for destructive analysis. Importantly, while the last decade has seen proteomics applications come to the fore, this review aims to emphasise the wider potential of advanced MS techniques for the study of painting materials and their conservation.

Visualization and mapping of literature on the scientific analysis of wall paintings: a bibliometric analysis from 2011 to 2021

As non-renewable cultural heritage, wall paintings play an important role in society. To reveal the trends in the scientific analysis of mural paintings, 845 relevant research articles published from 2011 to 2021 were collected from the Web of Science database and analyzed. The VOSviewer software was adopted to map the network data of scientific publications, so that relationships among authors, countries, institutions can be displayed, and the co-occurrence of keywords and co-citation can be analyzed. The results revealed close and strong interconnections between the top authors, suggesting a considerable strong research link in this field. The cooperation between research institutions was relatively close. The most productive country of relevant publications was Italy. The leading journals for the scientific analysis of wall paintings were Journal of Raman Spectroscopy and Journal of Cultural Heritage. At present, the hotspots of scientific analysis and research on wall painting are revealing the composition, distribution, origin, and deterioration mechanism of pigments, alongside with evaluating the effects and mechanism of conservation materials and techniques. On the one hand, a possible development direction in this field is introducing more cutting-edge analysis and data processing methods. On the other hand, scientific analysis is increasingly adopted to guide the research and development of mural conservation materials.

A Multi-Analytical Investigation of Roman Frescoes from Rapoltu Mare (Romania)

(1) Background: Due to the precarious situation of many monuments or archeological sites, analytical investigations are necessary to obtain information about the used materials, as well as to identify the most appropriate solutions for their conservation/restoration. This paper addresses the characterization of mural painting fragments collected during the excavation in 2018 in Rapoltu Mare (La vie), Deva. (2) Methods: Specific analytical techniques were used, as follows: X-ray diffractometry (XRD), wavelength dispersive X-ray fluorescence (WDXRF), optical microscopy (OM), zoom microscopy and scanning electron microscopy (SEM), spectroscopic techniques (UV–Vis, FTIR, Raman), porosity and thermal analysis, all of which provide information about the structure, chemical composition, morphology and topography of pigments and their deterioration as well. (3) Results: Up to seven different pigments were identified: Egyptian blue, carbon, calcite, gypsum, hematite, goethite and green earth. Egyptian Blue is identified in all the other color areas, except the white area: in the green zone (as degradation product with beeswax) and in the red zone (in mixture with ochre) too. In addition, carbon and beeswax were highlighted as toner and binder for pigments, respectively. In the presence of the organic beeswax binding environment, the Egyptian blue pigment particles darkened or turned yellow significantly, changing the blue to a greenish color. It is also possible to identify wollastonite (CaSiO3) in the blue pigment, which indicates that the temperature used in the manufacturing of Egyptian blue was higher than 950 °C from thermal analysis. The presence of apatite, hematite and gypsum deposits in the Hunedoara region certifies that these pigments could have been of local origin, as demonstrated by the presence of analytically identified elements (Fe, P, S, Ca). (4) Conclusions: The analytical techniques used for such investigations have highlighted the main pigments used in Roman times for various Roman murals.

The combined use of SEM-EDX, Raman, ATR-FTIR and visible reflectance techniques for the characterisation of Roman wall painting pigments from Monte d'Oro area (Rome): an insight into red, yellow and pink shades

The aim of this work has been the identification of the painter's materials employed in the wall decoration of some destroyed buildings dating approximately between the first century B.C. and the first century A.D. This research originates from a previously started joined archaeological and analytical investigation concerning a varied group of findings that resulted from a rescue excavation performed by Soprintendenza Archeologica in the area of Monte d'Oro in Rome. The focus of this study progression has been directed to a numerous selection of monochrome red, pink and yellow-pigmented fragments. The analyses were performed by means of scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDX) combined with Raman and Fourier transform infrared (FTIR) spectroscopies; visible reflectance measurements have also been carried out and the relevance of this technique in such a kind of archaeological studies has been highlighted. Most attention has been given to the assessment of the performances of non-destructive techniques achieved by portable Raman, and visible reflectance instrumentation to test their diagnostic capabilities. In addition to the expected and well-known pigments such as cinnabar, red ochre, hematite for the reds and yellow ochre for the yellows, the study highlighted a diffuse use of mixed colours and in some cases the possible presence of overlapped painted layers and confirmed the presence of gildings. Among the mixtures of pigments, the most singular outcome concerns the pink fragments revealing the possible application of bone white, which seems to be rather uncommon as a pigment in Roman wall decorations.

Acquisition of High Spectral Resolution Diffuse Reflectance Image Cubes (350-2500 nm) from Archaeological Wall Paintings and Other Immovable Heritage Using a Field-Deployable Spatial Scanning Reflectance Spectrometry Hyperspectral System

There is growing interest in bringing non-invasive laboratory-based analytical imaging tools to field sites to study wall paintings in order to collect molecular information on the macroscale. Analytical imaging tools, such as reflectance imaging spectrometry, have provided a wealth of information about artist materials and working methods, as well as painting conditions. Currently, scientific analyses of wall paintings have been limited to point-measurement techniques such as reflectance spectroscopy (near-ultraviolet, visible, near-infrared, and mid-infrared), X-ray fluorescence, and Raman spectroscopy. Macroscale data collection methods have been limited to multispectral imaging in reflectance and luminescence modes, which lacks sufficient spectral bands to allow for the mapping and identification of artist materials of interest. The development of laboratory-based reflectance and elemental imaging spectrometers and scanning systems has sparked interest in developing truly portable versions, which can be brought to field sites to study wall paintings where there is insufficient space or electrical power for laboratory instruments. This paper presents the design and testing of a simple hyperspectral system consisting of a 2D spatial spot scanning spectrometer, which provides high spectral resolution diffuse reflectance spectra from 350 to 2500 nm with high signal to noise and moderate spatial resolution (few mm). This spectral range at high spectral resolution was found to provide robust chemical specificity sufficient to identify and map many artists' materials, as well as the byproducts of weathering and conservation coatings across the surface of ancient and Byzantine Cypriot wall paintings. Here, we present a detailed description of the hyperspectral system, its performance, and examples of its use to study wall paintings from Roman tombs in Cyprus. The spectral/spatial image processing workflow to make maps of pigments and constituent painting materials is also discussed. This type of configurable hyperspectral system and the imaging processing workflow offer a new tool for the field study of wall paintings and other immovable heritage.

Enriching the knowledge of Ostia Antica painted fragments: a multi-methodological approach

This paper presents the study of selected painted fragments from different contexts of Ostia Antica city, dating between 2nd century BCE and the end of the 1st century CE. The aim is to identify the raw materials used and to understand the execution techniques through a non-invasive protocol including techniques based either on multiband imaging (Visible-VIS, Ultraviolet induced Luminescence - UVL and Visible Induced Luminescence - VIL) and single spot analyses (Fiber Optic Reflectance Spectroscopy- FORS and portable X-Ray Fluorescence spectrometry - XRF). The most representative and interesting fragments were sampled for further studies with laboratory techniques such as optical microscopy (OM) and electron microscopy (SEM), Fourier Transform Infrared and micro-Raman Spectroscopies (FT-IR and μRaman). The extensive use of non-invasive techniques, even working on fragments, is proved to be the most robust and effective approach enabling the analysis of a high number of areas, dramatically increasing the statistical meaning of the collected data. The elaboration of such a huge number of data allows highlighting differences and similarities, thus achieving a more realistic overview of the materials composition and addressing the sampling to the more significant and complex areas.

Time-of-flight secondary ion mass spectrometry imaging in cultural heritage: A focus on old paintings

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging is a surface analysis technique that identifies and spatially resolves the chemical composition of a sample with a lateral resolution of less than 1 μm. Depth analyses can also be performed over thicknesses of several microns. In the case of a painting cross section, for example, TOF-SIMS can identify the organic composition, by detecting molecular ions and fragments of binders, as well as the mineral composition of most of the pigments. Importantly, the technique is almost not destructive and is therefore increasingly used in cultural heritage research such as the analysis of painting samples, especially old paintings. In this review, state of the art of TOF-SIMS analysis methods will be described with a particular focus on tuning the instruments for the analysis of painting cross sections and with several examples from the literature showing the added value of this technique when studying cultural heritage samples.

Nondestructive Analysis of Wall Paintings at Ostia Antica

Roman wall paintings at Ostia Antica were studied for the first time in situ in an integrated approach using nuclear magnetic resonance (NMR) depth profiling, portable X-ray fluorescence (XRF), and visible induced luminescence (VIL) in order to explore the materials used in their construction and gain insight into the evolution of the Roman painting technique over time. NMR revealed the signatures of covered wall paintings through details of the structure of the top painted mortar layers, and the loss of this information that can be encountered when paintings are detached from the wall for preservation purposes. XRF provided information about the pigment composition of the paintings, and VIL was used to identify Egyptian Blue. Egyptian Blue was only found in the earlier wall paintings studied dating from 1st century B.C.E. to the 1st century C.E. The pigment palette seems to become limited to iron-based pigments in the later paintings, whereas the palette of the earlier paintings appears to be more varied including mercury, lead, and copper-based pigments.

Investigation of Ancient Wall Painting Fragments Discovered in the Roman Baths from Alburnus Maior by Complementary Non-Destructive Techniques

In this study, several wall painting fragments discovered in the Roman baths from the archeological site Alburnus Maior (Roşia Montană, Romania) were analyzed with the aim to investigate the material composition of both plasters and pictorial layers. Dated from the beginning of the second century AD, these rare findings stand among the oldest examples of preserved decorative polychrome paintings on plaster excavated thus far in the former territory of the Roman province of Dacia. A non-destructive multi-analytical approach based on complementary techniques was considered: Fourier transform infrared (FTIR) spectroscopy, X-ray fluorescence (XRF), X-ray diffraction (XRD), UV fluorescence, and hyperspectral imaging (HSI). The obtained results highlight a common Roman color palette mainly based on naturally occurring earth pigments. Red ochre, yellow ochre, manganese-rich ochres/wads, carbon black, and calcite were identified. A traditional two-layer sequence of plasters was found—arriccio (based on lime and siliceous sands), and intonaco (pure lime). The presence of an organic protein binder, identified via FTIR analysis, and sustained by combined imaging documentation, indicates that the pigments were applied a secco. The obtained results are discussed in relation to previous published data, and they can be considered as valuable archeological indicators that contribute to the understanding of the painting techniques and the materials used in the Roman provinces.

The Irreplaceable Contribution of Cross Sections Investigation: Painted Plasters from the Sphinx Room (Domus Aurea, Rome)

Fragments and micro-fragments of painted plasters from the Sphinx Room, recently discovered in the Domus Aurea Esquiline wing (Rome, Italy), were studied. They were respectively taken from the debris in the vicinity of the walls and from already damaged edges of the decorated walls. A previous study, mainly based on non-destructive and non-invasive investigations, proved effective in giving a general overview of the employed palette, allowing the comparison with paintings from the same palace and other coeval contexts, and also providing some preliminary hints concerning the pictorial technique. Nevertheless, some issues remained unsolved, concerning the pigment mixtures (e.g., lead-/iron-based yellow to red colors; copper-/iron-based green/greyish areas), and the a fresco/a secco painting technique debate. Therefore, cross sections of the above-mentioned fragments were observed by means of Optical (OM) and Scanning Electron (SEM) Microscopy and analyzed with micro-Raman spectroscopy and Energy Dispersive X-Ray Spectroscopy (EDS), with the aims of: studying the paint layer composition through point analysis and mapping of the elemental distribution; comparing these in relation to what was observed with previously reported non-destructive analyses; studying the stratigraphy of the painted plaster, focusing on the contact between the plaster and the paint.

Microbial biofilm community structure and composition on the lithic substrates of Herculaneum Suburban Baths

In this work, we want to investigate the impact of different substrates and different environmental condition on the biofilm communities growing on plaster, marble, and mortar substrates inside the Herculaneum Suburban Baths. To do so, we measured environmental conditions and sampled biofilm communities along the walls of the baths and used culture-dependent and -independent molecular techniques (DGGE) to identify the species at each sampling sites. We used the species pool to infer structure and richness of communities within each site in each substrate, and confocal light scanning microscopy to assess the three-dimensional structure of the sampled biofilms. To gather further insights, we built a meta-community network and used its local realizations to analyze co-occurrence patterns of species. We found that light is a limiting factor in the baths environment, that moving along sites equals moving along an irradiation gradient, and that such gradient shapes the community structure, de facto separating a dark community, rich in Bacteria, Fungi and cyanobacteria, from two dim communities, rich in Chlorophyta. Almost all sites are dominated by photoautotrophs, with Fungi and Bacteria relegated to the role of rare species., and structural properties of biofilms are not consistent within the same substrate. We conclude that the Herculaneum suburban baths are an environment-shaped community, where one dark community (plaster) and one dim community (mortar) provides species to a “midway” community (marble).

Identification of pigments in the Annunciation sculptural group (Cordoba, Spain) by micro-Raman spectroscopy

A highly flexible method based on micro-Raman spectroscopy was used to examine pigment traces on an Annunciation sculpture group dating from the late Middle Ages. The group comprises a statue of the Archangel Gabriel and another of the Virgin Mary. Both are currently housed in the Archaeological Museum of Cordoba, southern Spain. Information about the pigment palette used by the artists of the time to decorate religious limestone sculptures was for the first time obtained. The pigments found included vermilion (HgS), hematite (α-Fe₂O₃), azurite [Cu₃(CO₃)₂(OH)₂], cerussite (PbCO₃) and anatase (TiO₂). None was directly applied to the sculptures; rather, the limestone was coated with a primer containing calcium carbonate (possibly chalk or half-chalk). The polychromy on both sculptures, which was originally applied in the XV century and seriously damaged in the XVIII, had been treated with an ochre-coloured priming layer of gypsum to make it more uniform and optically similar to the underlying stone.

An Enhanced GC/MS Procedure for the Identification of Proteins in Paint Microsamples

The chemical characterization of materials used in works of art is extremely useful for gaining a better knowledge of the artistic heritage and to guarantee its preservation. A derivatization GC/MS procedure for the identification of proteins in a microsample from painted works of art has been optimized. The amino acid fraction is derivatized using anhydrous dimethylformamide (DMF) as solvent instead of pyridine (Py), commonly used to facilitate the reaction. Although pyridine is often considered a silylation catalyst, there are many instances in which silylation reactions actually are slower in pyridine than other solvents. In addition, pyridine also may have other undesirable effects such as the promotion of secondary products and other chromatographic anomalies. Using DMF, the formation of artifacts is limited and the derivatization yield of hydrophilic amino acids such as proline and hydroxyproline has improved, thus making the identification of organic paint media more straightforward. The method has been validated and successfully applied to identify the binder of the sample taken from the pictorial cycle of the 12th century monastery of Santa Maria delle Cerrate (Lecce, Italy), thus highlighting the use of eggs as a binding medium.

Portable Raman, DRIFTS, and XRF Analysis to Diagnose the Conservation State of Two Wall Painting Panels from Pompeii Deposited in the Naples National Archaeological Museum (Italy)

This work presents a methodology that combines spectroscopic speciation, performed through portable Raman spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and energy dispersive X-ray fluorescence spectrometry (ED-XRF) working in situ, and thermodynamic speciation to diagnose the environmental impacts, induced by past and current events, on two wall painting panels (Nos. 9103 and 9255) extracted more than 150 years ago from the walls of a Pompeian house (Marcus Lucretius House, Regio IX, Insula 3, House 5/24) and deposited in the Naples National Archaeological Museum (MANN). The results show a severe chemical attack of the acid gases that can be explained only by the action of H2S during and just after the eruption of the Vesuvius volcano, that expelled a high concentration of sulfur gases. This fact can be considered as the most important process impacting the wall painting panels deposited in the museum, while the rain-wash processes and the colonization of microorganisms have not been observed in contrast to the impacts shown by the wall paintings left outside in the archaeological site of Pompeii. Moreover, the systematic presence of lead traces and strontium in both wall paintings suggests their presence as impurities of the calcite mortars (intonacco) or calcite binder of these particular fresco Pompeian murals.