Non-invasive Investigations of Paintings by Portable Instrumentation: The MOLAB Experience

Time and spatially resolved VIS-NIR hyperspectral imaging as a novel monitoring tool for laser-based spectroscopy to mitigate radiation damage on paintings

The increased adoption of non-invasive laser-based techniques for analysis of cultural assets has recently called into question the non-invasiveness of the techniques in practical operation. The methods to assess the occurrence of radiation-induced alteration on paintings are very limited and none of them can predict damage. Here we present a novel multimodal imaging approach to understand the time and spatial evolution and types of laser-induced surface alterations, through simultaneous monitoring using visible and near infrared (VIS-NIR) reflectance hyperspectral imaging (HSI) and thermal imaging during Raman spectroscopy. The resultant physical and chemical changes were examined in detail by optical coherence tomography and synchrotron based micro-X-ray powder diffraction. HSI was found to be the most sensitive in detecting laser induced alternations compared with conventional methods. It is orders of magnitude more sensitive than Raman spectroscopy and even synchrotron-based micro-X-ray powder diffraction. In cases of thermally driven alterations, transient and reversible reflectance changes were found to be the first indications of laser-induced modifications and can therefore be used as precursors to prevent damage. VIS-NIR reflectance spectroscopy should be used to monitor laser-based analysis and potentially other radiation-based techniques in situ to mitigate laser induced alteration.

Analysis of brazilian paintings of the 20th century: Suspects and authentics through in situ and Non-Invasive techniques

This work studied suspicious and authentic artworks by Brazilian painters Ivan Serpa, Ismael Nery, and Iberê Camargo by XRF, FTIR, OM, and MA-XRF techniques. The studies made it possible to verify that all suspicious artworks are counterfeit artifacts. The analyses were conducted in situ, and different approaches were applied for data treatment. For example, principal component analysis and spectral deconvolution were performed on the XRF data. From these methods, it was possible to verify that the suspect artworks by Ivan Serpa and Iberê Camargo have different materiality than the authentic paintings. Additionally, MA-XRF images did not reveal the presence of a polychrome preparation layer in the suspicious paintings by Ivan Serpa. The suspect artworks from Ismael Nery exhibited a Ca-K/Ti-K ratio that indicates they were created on a low-quality paper support, which is not suitable for paintings. The differences in materials used in the suspicious and authentic artworks are further supported by the FTIR and OM results. In addition to the physicochemical analysis, the paintings were studies graphotechnical examinations, financial evaluations, and artistic analyses that demonstrated they were counterfeit artifacts. The results of the analysis demonstrate how physicochemical techniques can contribute to the forensic investigation of paintings. However, this work highlights the importance of applying distinct treatments to the XRF data in order to accentuate the differences between the suspect and authentic artworks.

Touching the (almost) untouchable: a minimally invasive workflow for microbiological and biomolecular analyses of cultural heritage objects

Microbiological and biomolecular approaches to cultural heritage research have expanded the established research horizon from the prevalent focus on the cultural objects' conservation and human health protection to the relatively recent applications to provenance inquiry and assessment of environmental impacts in a global context of a changing climate. Standard microbiology and molecular biology methods developed for other materials, specimens, and contexts could, in principle, be applied to cultural heritage research. However, given certain characteristics common to several heritage objects-such as uniqueness, fragility, high value, and restricted access, tailored approaches are required. In addition, samples of heritage objects may yield low microbial biomass, rendering them highly susceptible to cross-contamination. Therefore, dedicated methodology addressing these limitations and operational hurdles is needed. Here, we review the main experimental challenges and propose a standardized workflow to study the microbiome of cultural heritage objects, illustrated by the exploration of bacterial taxa. The methodology was developed targeting the challenging side of the spectrum of cultural heritage objects, such as the delicate written record, while retaining flexibility to adapt and/or upscale it to heritage artifacts of a more robust constitution or larger dimensions. We hope this tailored review and workflow will facilitate the interdisciplinary inquiry and interactions among the cultural heritage research community.

Multivariate versus traditional quantitative phase analysis of X-ray powder diffraction and fluorescence data of mixtures showing preferred orientation and microabsorption

In materials and earth science, but also in chemistry, pharmaceutics and engineering, the quantification of elements and crystal phases in solid samples is often essential for a full characterization of materials. The most frequently used techniques for this purpose are X-ray fluorescence (XRF) for elemental analysis and X-ray powder diffraction (XRPD) for phase analysis. In both methods, relations between signal and quantity do exist but they are expressed in terms of complex equations including many parameters related to both sample and instruments, and the dependence on the active element or phase amounts to be determined is convoluted among those parameters. Often real-life samples hold relations not suitable for a direct quantification and, therefore, estimations based only on the values of the relative intensities are affected by large errors. Preferred orientation (PO) and microabsorption (MA) in XRPD cannot usually be avoided, and traditional corrections in Rietveld refinement, such as the Brindley MA correction, are not able, in general, to restore the correct phase quantification. In this work, a multivariate approach, where principal component analysis is exploited alone or combined with regression methods, is used on XRPD profiles collected on ad hoc designed mixtures to face and overcome the typical problems of traditional approaches. Moreover, the partial or no known crystal structure (PONKCS) method was tested on XRPD data, as an example of a hybrid approach between Rietveld and multivariate approaches, to correct for the MA effect. Particular attention is given to the comparison and selection of both method and pre-process, the two key steps for good performance when applying multivariate methods to obtain reliable quantitative estimations from XRPD data, especially when MA and PO are present. A similar approach was tested on XRF data to deal with matrix effects and compared with the more classical fundamental-parameter approach. Finally, useful indications to overcome the difficulties of the general user in managing the parameters for a successful application of multivariate approaches for XRPD and XRF data analysis are given.

Architectural Polychromy on the Athenian Acropolis: An In Situ Non-Invasive Analytical Investigation of the Colour Remains

The preservation of the Athenian Acropolis monuments constitutes an ongoing top-priority national project of global significance and impact. The project concerning the analytical investigation of the polychromy of the Acropolis monuments presented in this paper was part of the Acropolis Restoration Service (YSMA) program (2011–2015), regarding the restoration of the two corners of the west entablature of the Parthenon, which exhibited severe static damage, and a parallel restoration program of the Propylaea. The scope of this research was to investigate the materials in the paint decoration remains on the monuments by applying, entirely in situ, numerous non-invasive techniques on selected architectural members of the Parthenon and the Propylaea. The research focused, mainly, on surfaces where traces of colour or decoration patterns were visible to the naked eye. Furthermore, surfaces that are referred to in the literature as decorated but that are currently covered with weathering crusts (of white or black colour) and/or layers of patina (of yellowish and orange-brown hue), were also examined. The techniques applied in situ on the Acropolis monuments were X-ray fluorescence, micro-Raman, and Fourier Transform InfraRed (FTIR) spectroscopic techniques, conducted with the use of handheld or portable instruments. The scientific data gathered in situ are discussed in this paper to enhance our knowledge of the architectural polychromy of the classical period. Further investigation by applying analytical techniques on a few selected micro-samples would be highly complementary to this present work.

Characterization of Wall Paintings of the Harem Court in the Alhambra Monumental Ensemble: Advantages and Limitations of In Situ Analysis

Non-invasive techniques (X-ray fluorescence, XRF, and Raman spectroscopy) were used for the study of the Hispano Muslim wall paintings. Principal component analysis (PCA) was performed on the semi-quantitative XRF results directly provided by the in-built factory calibrations with minimum user manipulation. The results obtained were satisfactory and highlighted differences and similarities among the measurement points. In this way, it was possible to differentiate the decorations carried out on gypsum plasterwork and the wall paintings over lime plaster. The color palette, revealed by combining the results from XRF and Raman spectroscopies, comprised the pigments hematite, lapis lazuli, cinnabar (in poor conservation state), and possibly, carbon. Evidence of past interventions was also provided by PCA on XRF data, which detected the presence of Pb, Ba, and Zn in some areas. Furthermore, the preparation layers have been studied in detail on cross-sections of two microsamples. Several layers of lime plaster with a compact microstructure have been observed. The characteristic of the pictorial layer and the identification of calcium oxalate point to the use of a secco-technique. The main alteration identified was a gypsum surface layer covering the painting and signs of plaster deterioration due to gypsum migration to more internal areas. Finally, the comparison with the observations made by restorers in previous interventions on these paintings revealed the importance of the representativeness gained with the in situ study, which enabled the analysis of a high number of areas.

Developing Macro-Raman Mapping as a Tool for Studying the Pigment Distribution of Art Objects

Raman spectroscopy is a well-appreciated technique in cultural heritage research for its ability to obtain molecular information nondestructively. During Raman mapping experiments, advantage is taken of the excellent spatial resolution of the approach, allowing to visualize the spatial distribution of the molecules. In the current research, macro-Raman mapping is proposed, allowing us to map large areas of an artwork (typically tens or hundreds of square centimeters). Therefore, a new setup is made, using a commercially available mobile Raman spectrometer and fast translation stages. Moreover, the probe is equipped with a triangulator to measure the distance to the surface of the artwork and thus achieving accurate focusing of the Raman probe. Finally, the correct setup is guaranteed by using a calibration module that is designed to allow for spectral calibration and aligning all components of the probe. The use of the method is demonstrated by three cases, where different data processing techniques are illustrated.

A Combined Non-Destructive and Micro-Destructive Approach to Solving the Forensic Problems in the Field of Cultural Heritage: Two Case Studies

The present paper discusses the importance of non-destructive and micro-destructive technology in forensic investigations in the field of cultural heritage. Recent technological developments and the wide availability of modern analytical instrumentation are creating new possibilities for performing scientific measurements and acquiring data directly on-site—thereby limiting, where possible, sampling activity—as well as learning about the technologies and materials that were employed in the past to create cultural assets. Information on periods, chemical composition, manufacturing techniques, etc., can be gathered more easily. Overall, the benefits of on-site forensic investigations are multiple, including the potential to increase substantially the speed and efficacy of the criminal justice system. However, such benefits are only realized when data quality is guaranteed and findings can be used as forensic evidence in court. The present paper shows data from the non-destructive and micro-destructive analysis of different artworks and objects provided by the Cosenza Carabinieri Unit for the Protection of Cultural Heritage and Anti-Counterfeiting (Calabria, Italy). In particular, two oil paintings on canvas depicting cherubs (Italian: putti), recovered as fragments of larger religious artworks, and two bronze belt and helmet fragments were investigated. In the first case, the research aimed to define the original pictorial layer, identify any reconstruction pictorial areas or pictorial retouching, assess the state of conservation, reconstruct the previous conservation treatments, and provide indications about the chronology of the artworks. In the second case, analysis was performed both to define the bronze chemical composition and the origin of the soil (earth) found within the objects during their recovery. For these purposes, the analytical approach involved the use of non-destructive and micro-destructive analysis as follows: infrared reflectography (IRR), ultraviolet-induced visible fluorescence (UV), X-ray fluorescence analysis (XRF), digital optical microscopy (DOM), scanning electron microscopy equipped with EDX microanalysis (SEM-EDX), and Fourier transform infrared spectroscopy (FT-IR). The results made it possible to collect valuable diagnostic information and answer questions posed by the institutions for the resolution of various doubts about forensic science and cases concerning the seizure, recovery, or return of archaeological or historical-artistic objects of cultural interest.

Camera Color Correction for Cultural Heritage Preservation Based on Clustered Data

Cultural heritage preservation is a crucial topic for our society. When dealing with fine art, color is a primary feature that encompasses much information related to the artwork’s conservation status and to the pigments’ composition. As an alternative to more sophisticated devices, the analysis and identification of color pigments may be addressed via a digital camera, i.e., a non-invasive, inexpensive, and portable tool for studying large surfaces. In the present study, we propose a new supervised approach to camera characterization based on clustered data in order to address the homoscedasticity of the acquired data. The experimental phase is conducted on a real pictorial dataset, where pigments are grouped according to their chromatic or chemical properties. The results show that such a procedure leads to better characterization with respect to state-of-the-art methods. In addition, the present study introduces a method to deal with organic pigments in a quantitative visual approach.

On-site contactless surface analysis of modern paintings from Galleria Nazionale (Rome) by reflectance FTIR and Raman spectroscopies

Seven artworks representing the diversity of paints used around the 1960s and created by German and Italian painters (J. Albers, A. Bonalumi, L. Boille, T. Scialoja and M. Schifano) were studied on-site at the Galleria Nazionale d'Arte Moderna (Rome) with mobile instruments. We present a methodology based on Specular Reflectance Infrared Spectroscopy (SR-FTIR) adapted to unvarnished paintings. Complementary measurements have been performed by Raman spectroscopy. Characteristic bands regarding as-recorded infrared reflectance spectra and Kramers-Kronig Transformation-converted absorbance spectra are identified according to literature and reference spectra recorded on representative commercially available paints. To distinguish the different binders by SR-FTIR, we propose spectroscopic markers as the comparison of the intensity of carbonyl band around 1730-1735 cm^-1 with bands at ~1160 (for acrylic), ~1230 (for PVAc), and 1270 cm^-1 (for alkyds). On the other hand, oil/resin binders are characterized by intense and thin νCH₂, νCH₃ IR absorption bands around 2920-2850 cm^-1, combined with an intense 1260 cm^-1 band and a characteristic concave cradle shape (between ca. 1750 and 1260 cm^-1). The results obtained establish the relevance of the implemented mobile non-invasive infrared spectroscopy analytical approach by successfully identifying acrylic, vinylic, oil media and enamel paints, with or without opacifiers, which is supplemented by Raman analyses for pigment identification.

Plastics in Heritage Science: Analytical Pyrolysis Techniques Applied to Objects of Design

The first synthetic polymers were introduced as constituents of everyday life, design objects, and artworks at the end of the 19th century. Since then, the history of design has been strictly connected with the 20th century evolution of plastic materials. Objects of design from the 20th century are today a precious part of the cultural heritage. They raise specific conservation issues due to the degradation processes affecting synthetic polymer-based plastics. Museums and collections dealing with the conservation of design objects and modern materials need to base their conservation strategies on compositional data that reveal the formulations of historical plastics and their decay processes. Specific and specifically optimized analytical tools are thus needed. We employed flash analytical pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC/MS) and evolved gas analysis coupled with mass spectrometry (EGA-MS) to characterize “historic polymeric materials” (HIPOMS) and heritage plastics at the molecular level with high chemical detail. This approach complements non-invasive spectroscopic diagnosis whenever it fails to obtain significant or complete information on the nature and the state of preservation of the materials under study. We determined the composition of several 20th century design objects (1954–1994) from the Triennale Design Museum of Milan (Triennale Milano - Museo del Design Italiano), which for different morphological, chemical, or physical reasons were unsuitable for characterization by non-invasive spectroscopy. EGA-MS proved capable for the study of the different fractions constituting heterogeneous micro-samples and for gaining an insight into their degradation processes from the contextual interpretation of thermal and mass-spectrometric data.

Recent trends in the application of Fourier Transform Infrared (FT-IR) spectroscopy in Heritage Science: from micro- to non-invasive FT-IR

The relevance of FT-IR spectroscopy in heritage science has experienced a constant grow in the last two decades owing to analytical peculiarities that make it an extremely useful tool to answer the questions posed by the study and conservation of art-historical and archaeological materials. High versatility, sensitivity and molecular specificity are, in fact, all requirements that FT-IR spectroscopy fulfils allowing for the investigation of the chemical properties of heritage materials spanning from the micro- to the macro-scale and offering a variety of approaches to minimize sample manipulation and maximize extracted information. Molecular identification and localisation at high lateral resolution of organic and inorganic components in micro-samples was, over recently, the mostly exploited use of FT-IR in heritage science; however, benefiting from technological progress and advances in optical materials and components achieved in the last decade, it now stands out also for non-invasive surface analysis of artworks by fully portable instrumentation.

Mid and Near-Infrared Reflection Spectral Database of Natural Organic Materials in the Cultural Heritage Field

This study presents mid and near-infrared (7500-375 cm⁻¹) total reflection mode spectra of several natural organic materials used in artworks as binding media, consolidants, adhesives, or protective coatings. A novel approach to describe and interpret reflectance bands as well as calculated absorbance after Kramers-Kronig transformation (KKT) is proposed. Transflection mode spectra have represented a valuable support both to study the distorted reflectance bands and to validate the applicability and usefulness of the KK correction. The aim of this paper is to make available to scientists and conservators a comprehensive infrared reflection spectral database, together with its detailed interpretation, as a tool for the noninvasive identification of proteins, lipids, polysaccharides, and resins by means of portable noncontact FTIR spectrometers.

Role of the Relative Humidity and the Cd/Zn Stoichiometry in the Photooxidation Process of Cadmium Yellows (CdS/Cd1-x Znx S) in Oil Paintings

Cadmium yellows (CdYs) refer to a family of cadmium sulfide pigments, which have been widely used by artists since the late 19th century. Despite being considered stable, they are suffering from discoloration in iconic paintings, such as Joy of Life by Matisse, Flowers in a blue vase by Van Gogh, and The Scream by Munch, most likely due to the formation of CdSO₄ ⋅n H₂ O. The driving factors of the CdYs degradation and how these affect the overall process are still unknown. Here, we study a series of oil mock-up paints made of CdYs of different stoichiometry (CdS/Cd_0.76 Zn_0.24 S) and crystalline structure (hexagonal/cubic) before and after aging at variable relative humidity under exposure to light and in darkness. Synchrotron radiation-based X-ray methods combined with UV-Vis and FTIR spectroscopy show that: 1) Cd_0.76 Zn_0.24 S is more susceptible to photooxidation than CdS; both compounds can act as photocatalysts for the oil oxidation. 2) The photooxidation of CdS/Cd_0.76 Zn_0.24 S to CdSO₄ ⋅n H₂ O is triggered by moisture. 3) The nature of alteration products depends on the aging conditions and the Cd/Zn stoichiometry. Based on our findings, we propose a scheme for the mechanism of the photocorrosion process and the photocatalytic activity of CdY pigments in the oil binder. Overall, our results form a reliable basis for understanding the degradation of CdS-based paints in artworks and contribute towards developing better ways of preserving them for future generations.

Miniaturized Biosensors to Preserve and Monitor Cultural Heritage: from Medical to Conservation Diagnosis

The point-of-care testing concept has been exploited to design and develop portable and cheap bioanalytical systems that can be used on-site by conservators. These systems employ lateral flow immunoassays to simultaneously detect two proteins (ovalbumin and collagen) in artworks. For an in-depth study on the application of these portable biosensors, both chemiluminescent and colorimetric detections were developed and compared in terms of sensitivity and feasibility. The chemiluminescent system displayed the best analytical performance (that is, two orders of magnitude lower limits of detection than the colorimetric system). To simplify its use, a disposable cartridge was designed ad hoc for this specific application. These results highlight the enormous potential of these inexpensive, easy-to-use, and minimally invasive diagnostic tools for conservators in the cultural heritage field.

In situ Raman mapping of art objects

Raman spectroscopy has grown to be one of the techniques of interest for the investigation of art objects. The approach has several advantageous properties, and the non-destructive character of the technique allowed it to be used for in situ investigations. However, compared with laboratory approaches, it would be useful to take advantage of the small spectral footprint of the technique, and use Raman spectroscopy to study the spatial distribution of different compounds. In this work, an in situ Raman mapping system is developed to be able to relate chemical information with its spatial distribution. Challenges for the development are discussed, including the need for stable positioning and proper data treatment. To avoid focusing problems, nineteenth century porcelain cards are used to test the system. This work focuses mainly on the post-processing of the large dataset which consists of four steps: (i) importing the data into the software; (ii) visualization of the dataset; (iii) extraction of the variables; and (iv) creation of a Raman image. It is shown that despite the challenging task of the development of the full in situ Raman mapping system, the first steps are very promising.This article is part of the themed issue 'Raman spectroscopy in art and archaeology'.

Where science meets art

Scientific methods inform conservation efforts in the quest to preserve cultural heritage