Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Spectromicroscopic Methods. 4. Artificial Aging of Model Samples of Co-Precipitates of Lead Chromate and Lead Sulfate

Effect of low molecular weight organic acids on the lead and chromium release from widely-used lead chromate pigments under sunlight irradiation

Lead chromate pigments are commonly used yellow inorganic pigments. They can pose environmental risks as they contain toxic heavy metals lead and chromium. Low molecular weight organic acids (LMWOAs), as widespread dissolved organic matter (DOM), affect the lead and chromium release from the pigment in water. In this work, the role of LMWOAs in the photodissolution of commercial lead chromate pigment was investigated. The pigment underwent significant photodissolution under simulated sunlight exposure with LMWOAs, and subsequently released Cr(III) and Pb(II). The photodissolution process is caused by the reduction of Cr(VI) by photogenerated electrons of the lead chromate pigment. The LMWOAs promoted photodissolution of the pigment by improving the electron-hole separation. The formation of Cr(III)-contained compounds leads to a slower release of chromium than lead. The photodissolution kinetics increase with decreasing pH and increasing LMWOAs concentration. The photodissolution of lead chromate pigment was basically positively related to the total number of hydroxyl and carboxyl groups in LMWOAs. The LMWOAs with stronger affinity to lead chromate pigment, lower adiabatic ionization potential (AIP) and higher energy of the highest occupied molecular orbital (EHOMO) are favorable to Cr(VI) reduction by photogenerated electrons and pigment photodissolution. 2.39% of chromium and 10.34% of lead released from the lead chromate pigment in natural conditions during a 6-h sunlight exposure. This study revealed the photodissolution mechanism of lead chromate pigment mediated by LMWOAs with different molecular structures, which helps understand the environmental photochemical behavior of the pigment. The present results emphasize the important role of DOM in the heavy metals release from commercial inorganic pigments.

The "Historical Materials BAG": A New Facilitated Access to Synchrotron X-ray Diffraction Analyses for Cultural Heritage Materials at the European Synchrotron Radiation Facility

The European Synchrotron Radiation Facility (ESRF) has recently commissioned the new Extremely Brilliant Source (EBS). The gain in brightness as well as the continuous development of beamline instruments boosts the beamline performances, in particular in terms of accelerated data acquisition. This has motivated the development of new access modes as an alternative to standard proposals for access to beamtime, in particular via the "block allocation group" (BAG) mode. Here, we present the recently implemented "historical materials BAG": a community proposal giving to 10 European institutes the opportunity for guaranteed beamtime at two X-ray powder diffraction (XRPD) beamlines-ID13, for 2D high lateral resolution XRPD mapping, and ID22 for high angular resolution XRPD bulk analyses-with a particular focus on applications to cultural heritage. The capabilities offered by these instruments, the specific hardware and software developments to facilitate and speed-up data acquisition and data processing are detailed, and the first results from this new access are illustrated with recent applications to pigments, paintings, ceramics and wood.

Damages Induced by Synchrotron Radiation-Based X-ray Microanalysis in Chrome Yellow Paints and Related Cr-Compounds: Assessment, Quantification, and Mitigation Strategies

Synchrotron radiation (SR)-based X-ray methods are powerful analytical tools for several purposes. They are widely used to probe the degradation mechanisms of inorganic artists' pigments in paintings, including chrome yellows (PbCr_1-xS_xO₄; 0 ≤ x ≤ 0.8), a class of compounds often found in Van Gogh masterpieces. However, the high intensity and brightness of SR beams raise important issues regarding the potential damage inflicted on the analyzed samples. A thorough knowledge of the SR X-ray sensitivity of each class of pigment in the painting matrix is therefore required to find analytical strategies that seek to minimize the damage for preserving the integrity of the analyzed samples and to avoid data misinterpretation. Here, we employ a combination of Cr K-edge X-ray absorption near-edge structure spectroscopy, Cr-K_β X-ray emission spectroscopy, and X-ray diffraction to monitor and quantify the effects of SR X-rays on the stability of chrome yellows and related Cr compounds and to define mitigation strategies. We found that the SR X-ray beam exposure induces changes in the oxidation state and local coordination environment of Cr ions and leads to a loss of the compound's crystalline structure. The extent of X-ray damage depends on some intrinsic properties of the samples (chemical composition of the pigment and the presence/absence and nature of the binder). It can be minimized by optimizing the overall fluence/dose released to the samples and by working in vacuum and under cryogenic conditions.

Investigation of mineralization products of lead soaps in a late medieval panel painting

Metal soaps pose significant concerns in the preservation of paintings made with oil as a binding medium. In highly alkaline environments, metal soap aggregates may undergo mineralization processes with the formation of new phases, such as carbonates, chlorides, oxides and sulfates of the metal cations that are present in pigments or paint additives. In this work, we report new examples of the mineralization of lead soaps in paint-cross sections taken from a late-medieval panel painting owned by the Museum of Cultural History at the University of Oslo. Scientific analyses were carried out with optical microscopy under UV and visible light, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and Raman spectroscopy to investigate and characterize the nature of the compounds present in the samples. Lead (II,IV) oxide, which is presumed to be a secondary product in the mineralization of lead soaps, has been identified in lead-based paint.

X-ray Diffraction Mapping for Cultural Heritage Science: a Review of Experimental Configurations and Applications

X-ray diffraction (XRD) mapping consists in the acquisition of XRD patterns at each pixel (or voxel) of an area (or volume). The spatial resolution ranges from the micrometer (μXRD) to the millimeter (MA-XRD) scale, making the technique relevant for tiny samples up to large objects. Although XRD is primarily used for the identification of different materials in (complex) mixtures, additional information regarding the crystallite size, their orientation, and their in-depth distribution can also be obtained. Through mapping, these different types of information can be located on the studied sample/object. Cultural heritage objects are usually highly heterogeneous, and contain both original and later (degradation, conservation) materials. Their structural characterization is required both to determine ancient manufacturing processes and to evaluate their conservation state. Together with other mapping techniques, XRD mapping is increasingly used for these purposes. Here, the authors review applications as well as the various configurations for XRD mapping (synchrotron/laboratory X-ray source, poly-/monochromatic beam, micro/macro beam, 2D/3D, transmission/reflection mode). On-going hardware and software developments will further establish the technique as a key tool in heritage science.

Pigment darkening as case study of In-Air Plasma-Induced Luminescence

We introduce the use of an In-Air Plasma-Induced Luminescence (In-Air-PIL) spectroscopy as an alternative to classical chemical and crystallographic methods used in materials science. The In-Air-PIL is evaluated on a case study investigating the effect of light aging on the darkening of five pristine yellow pigments commonly used in artworks. We show that the darkening is not associated to changes in the chemical composition, but to a loss in crystallinity, indicating an amorphization process of the pigments induced and catalyzed by the light irradiation. This favors the interaction of the pigment molecules with oxygen and carbon adsorbed from the environment or solved in the binding agent, subsequently leading to the formation of oxalates and carbonates as observed in other works. We demonstrate that the In-Air-PIL results are in perfect agreement with more complex classical materials science analysis methods, making our plasma-driven method a potentially easier and faster technique.

Sunlight-Mediated Lead and Chromium Release from Commercial Lead Chromate Pigments in Aqueous Phase

Lead chromate pigments are included in a group of the most widely used pigments, which account for 3% of worldwide lead consumption. This study reports the photoactivity of commercial lead chromate pigment (i.e., chrome yellow) under simulated sunlight. It underwent photodissolution in the presence of organic acid and dissolved organic matter in the aqueous phase, releasing Pb(II) and Cr(III). Pb(II) was released more readily than Cr(III) which mainly formed hydroxides and oxides. The photodissolution can be activated by light with a wavelength <514 nm. The reaction is mediated by the reduction of Cr(VI) in the pigment by self-generated electrons. The kinetics were mainly affected by the electron-hole separation efficiency which can be enhanced by electron donors. The reaction rate decreases with increasing solution pH as the photodissolution process consumes protons. The photodissolution of the chrome yellow pigment was further confirmed in a river water sample under natural sunlight, with 11.28% of lead and 2.56% of chromium released in 7 h. This study highlights the importance of considering photochemical processes in risk assessments and regulations of commercial semiconductor pigments, which are currently based on their solubility.

Photochemistry of Artists' Dyes and Pigments: Towards Better Understanding and Prevention of Colour Change in Works of Art

The absorption of light gives a pigment its colour and its reason for being, but it also creates excited states, that is, new molecules with an energy excess that can be dissipated through degradation pathways. Photodegradation processes provoke long-term, cumulative and irreversible colour changes (fading, darkening, blanching) of which the prediction and prevention are challenging tasks. Of all the environmental risks that affect heritage materials, light exposure is the only one that cannot be controlled without any impact on the optimal display of the exhibit. Light-induced alterations are not only associated with the pigment itself but also with its interactions with support/binder and, in turn, are further complicated by the nature of the environmental conditions. In this Minireview we investigate how chemistry, encompassing multi-scale analytical investigations of works of art, computational modelling and physical and chemical studies contributes to improve our prediction of artwork appearance before degradation and to establish effective preventive conservation strategies.

Time-Dependent ATR-FTIR Spectroscopic Studies on Fatty Acid Diffusion and the Formation of Metal Soaps in Oil Paint Model Systems

The formation of metal soaps (metal complexes of saturated fatty acids) is a serious problem affecting the appearance and structural integrity of many oil paintings. Tailored model systems for aged oil paint and time-dependent attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy were used to study the diffusion of palmitic acid and subsequent metal soap crystallization. The simultaneous presence of free saturated fatty acids and polymer-bound metal carboxylates leads to rapid metal soap crystallization, following a complex mechanism that involves both acid and metal diffusion. Solvent flow, water, and pigments all enhance metal soap crystallization in the model systems. These results contribute to the development of paint cleaning strategies, a better understanding of oil paint degradation, and highlight the potential of time-dependent ATR-FTIR spectroscopy for studying dynamic processes in polymer films.

"Paintings Fade Like Flowers": Pigment Analysis and Digital Reconstruction of a Faded Pink Lake Pigment in Vincent van Gogh's Undergrowth with Two Figures

Color fading in Vincent van Gogh's Undergrowth with Two Figures was studied chemically to facilitate the creation of a digital reconstruction of fugitive colors . The painting contains a field of white, green, orange, and yellow flowers under a canopy of poplar trees with two central figures-a man and a woman, arms entwined. From Van Gogh's letters, however, it is known that he painted the picture with some pink flowers, which appear to have altered, presumably to white. Raman spectroscopy was applied to microsamples of paint to identify the faded pigment as geranium lake, which in this painting consists of the dye, eosin (2',4',5',7'-tetrabromofluorescein). For the first time, lead(II) sulfate has been specifically identified as the likely inorganic substrate for a geranium lake used by Van Gogh in the last months of his life. Microfocus X-ray fluorescence (MXRF) spectroscopy was subsequently used in situ to analyze the white flowers to identify bromine as a proxy for eosin, thus indicating an original pink coloration. Of the 387 white flowers analyzed, 37.7% contained measurable bromine and were, therefore, originally pink. Several cross-sections from these formerly pink areas were assessed using a combination of visual inspection and microcolorimetry to create a colored mask in Adobe Photoshop to digitally reconstruct a suggestion of the original appearance of the painting with regard to the faded flowers. Additionally, microfadeometry was undertaken for the first time on a painting cross-section sample to understand the actual fading kinetics of the underlying bright pink geranium lake used by Van Gogh. A combination of Raman microspectroscopy, MXRF, and scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) were utilized in situ and on paint microsamples to identify the complete palette used to create Undergrowth with Two Figures.

Non-Invasive and Non-Destructive Examination of Artistic Pigments, Paints, and Paintings by Means of X-Ray Methods

Recent studies are concisely reviewed, in which X-ray beams of (sub)micrometre to millimetre dimensions have been used for non-destructive analysis and characterization of pigments, minute paint samples, and/or entire paintings from the seventeenth to the early twentieth century painters. The overview presented encompasses the use of laboratory and synchrotron radiation-based instrumentation and deals with the use of several variants of X-ray fluorescence (XRF) as a method of elemental analysis and imaging, as well as with the combined use of X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Microscopic XRF is a variant of the method that is well suited to visualize the elemental distribution of key elements, mostly metals, present in paint multi-layers, on the length scale from 1 to 100 μm inside micro-samples taken from paintings. In the context of the characterization of artists' pigments subjected to natural degradation, the use of methods limited to elemental analysis or imaging usually is not sufficient to elucidate the chemical transformations that have taken place. However, at synchrotron facilities, combinations of μ-XRF with related methods such as μ-XAS and μ-XRD have proven themselves to be very suitable for such studies. Their use is often combined with microscopic Fourier transform infra-red spectroscopy and/or Raman microscopy since these methods deliver complementary information of high molecular specificity at more or less the same length scale as the X-ray microprobe techniques. Since microscopic investigation of a relatively limited number of minute paint samples, taken from a given work of art, may not yield representative information about the entire artefact, several methods for macroscopic, non-invasive imaging have recently been developed. Those based on XRF scanning and full-field hyperspectral imaging appear very promising; some recent published results are discussed.

Development of a Hydrogen Sulfide End-of-Service-Life Indicator for Respirator Cartridges Using Cobinamide

An inexpensive paper-based sensor was developed for detecting low ppm concentrations of hydrogen sulfide gas. A piece of filter paper containing aquohydroxocobinamide [OH(H2O)Cbi] was placed on the end of a bifurcated optical fiber, and the reflectance spectrum of the OH(H2O)Cbi was monitored during exposure to 10.0 ppm hydrogen sulfide gas (NIOSH recommended exposure limit). Reaction of sulfide (HS-) yielded an increase in reflectance from 400-450 nm, and decrease from 470-550 nm. Spectral changes were monitored as a function of time at 25, 50, and 85% relative humidity. Spectral shifts at high-er humidity suggested reduction of the Cbi(III) compound. The sensor was used to detect hydrogen sulfide breakthrough from respirator carbon beds and results correlated well with a standard electrochemical detector. The simple paper-based sensor could provide a real-time end-of-service-life alert for hydrogen sulfide gas.

Ab initio study of PbCr(1-x)S x O4 solid solution: an inside look at Van Gogh Yellow degradation

Van Gogh Yellow refers to a family of lead chromate pigments widely used in the 19th century and often mixed with lead sulfate to obtain different yellow hues. Unfortunately, some paintings, such as the famous Sunflowers series, suffered degradation problems due to photoactivated darkening of once bright yellow areas, especially when irradiated with UV light. Recent advanced spectroscopic analyses have proved that this process occurs mostly where the pigment presents a sulfur-rich orthorhombic phase of a PbCr(1-x)S x O4 solid solution, while chromium-rich monoclinic phases are lightfast. However, the question of whether degradation is purely a surface phenomenon or if the bulk properties of sulfur-rich pigments trigger the process is still open. Here, we use first-principles calculations to unveil the role of sulfur in determining important bulk features such as structure, stability, and optical properties. From our findings, we suggest that degradation occurs via an initial local segregation of lead sulfate that absorbs at UV light wavelengths and provides the necessary energy for subsequent reduction of chromate ions into the greenish chromic oxide detected in experiments. In perspective, our results set reliable scientific foundations for further studies on surface browning phenomena and can help to chose the best strategy for the proper conservation of art masterpieces.

The efficiency of micro-Raman spectroscopy in the analysis of complicated mixtures in modern paints: Munch's and Kupka's paintings under study

Twenty one mock-up samples containing inorganic pigments primarily used at the turn of the 19th and 20th century were selected for comparative study and measured by micro-Raman and portable Raman spectrometers. They included pure grounds (chalk-based, earth-based and lithopone-based), grounds covered by resin-based varnish, and different paint layers containing mixtures of white, yellow, orange, red, green, blue and black pigments, usually in combination with white pigments (titanium, zinc and barium whites or chalk). In addition, ten micro-samples obtained from seven paintings of two world-famous modern painters Edvard Munch and František Kupka have been investigated. Infrared reflection spectroscopy (FTIR), portable X-ray fluorescence (XRF) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) were used as supplementary methods. The measurements showed that blue pigments (ultramarine, Prussian blue and azurite), vermilion and ivory black in mixture with whites provided characteristic Raman spectra, while Co-, Cd- and Cr- pigments' bands were suppressed by fluorescence. The best success rate of micro-Raman spectroscopy has been achieved using the 780 nm excitation, however, the sensitivity of this excitation laser in a portable Raman instrument significantly decreased. The analyses of micro-samples of paintings by E. Munch and F. Kupka showed that micro-Raman spectroscopy identified pigments which would remain unidentified if analyzed only by SEM-EDS (zinc yellow, Prussian blue). On the other hand, chromium oxide green and ultramarine were not detected together in a sample due to overlap of their main bands. In those cases, it is always necessary to complement Raman analysis with other analytical methods.

Structural and electronic properties of the PbCrO4 chrome yellow pigment and of its light sensitive sulfate-substituted compounds

Chrome Yellows (CY) are a family of synthetic pigments of formula (PbCr_(1−x)S_xO₄) used by van Gogh. We investigate structure/property relations in CY by first-principles methods, providing insight into their possible degradation mechanisms.

Development of a cobinamide-based end-of-service-life indicator for detection of hydrogen cyanide gas

We describe an inexpensive paper-based sensor for rapid detection of low concentrations (ppm) of hydrogen cyanide gas. A piece of filter paper pre-spotted with a dilute monocyanocobinamide [CN(H₂O)Cbi] solution was placed on the end of a bifurcated optical fiber and the reflectance spectrum of the CN(H₂O)Cbi was monitored during exposure to 1.0-10.0 ppm hydrogen cyanide gas. Formation of dicyanocobinamide yielded a peak at 583 nm with a simultaneous decrease in reflectance from 450-500 nm. Spectral changes were monitored as a function of time at several relative humidity values: 25, 50, and 85% relative humidity. With either cellulose or glass fiber papers, spectral changes occurred within 10 s of exposure to 5.0 ppm hydrogen cyanide gas (NIOSH recommended short-term exposure limit). We conclude that this sensor could provide a real-time end-of-service-life alert to a respirator user.

Raman spectroscopic signatures of the yellow and ochre paints from artist palette of J. Matejko (1838-1893)

The Raman and complementary spectroscopic analyses were performed using the exceptional possibility of research on the XIX c. original paint materials of the artist palette of J. Matejko stored in the National Museum in Cracow. The yellow and ochre-based paints characteristic for Matejko's workshop and selected from the ensemble of 273 labelled tubes (brand of R. Ainé/Paris) supplied during the period of 1880-1893 were investigated. Highly specific Raman spectra were obtained for paints containing mixtures of the Zn- and Sn-modified Pb-Sb pigment, and also for the ochre-based ones. A clear pigment discrimination of the mixture of cadmium yellow (CdS), cinnabar (HgS) and lead white (2PbCO3⋅Pb(OH)2) was possible by means of Raman data collected under different excitations at 514 nm and 785 nm. It was shown that the Raman spectra complemented by the XRF, SEM-EDX and in some cases also by the LIPS and FTIR data ensure reliable pigment identification in multi-component paints containing secondary species and impurities. The reported spectral signatures will be used for non-destructive investigation of the collection of about 300 oil paintings of J. Matejko. In view of the comparative research on polish painting which point out that richness of modified Naples yellows clearly distinguish Matejko's artworks from other ones painted in the period of 1850-1883, the Raman data of these paints can provide support in the authentication studies.

Evaluation of surface-enhanced Raman scattering detection using a handheld and a bench-top Raman spectrometer: a comparative study

Surface enhanced Raman scattering (SERS) detection using a handheld Raman spectrometer and a bench-top Raman spectrometer was systemically evaluated and compared in this study. Silver dendrites were used as the SERS substrate, and two pesticides, maneb and pyrrolidine dithiocarbamate-ammonium salt (PDCA) were used as the analytes. Capacity and performance were evaluated based on spectral resolution, signal variation, quantitative capacity, sensitivity, flexibility and intelligence for SERS detection. The results showed that the handheld Raman spectrometer had better data consistency, more accurate quantification capacity, as well as the capacity of on-site and intelligence for qualitative and semi-quantitative analysis. On the other hand, the bench-top Raman spectrometer showed about 10 times higher sensitivity, as well as flexibility for optimization of the SERS measurements under different parameters such as laser power output, collective time, and objective magnification. The study on the optimization of SERS measurements on a bench-top spectrometer provides a useful guide for designing a handheld Raman spectrometer, specifically for SERS detection. This evaluation can advance the application of a handheld Raman spectrometer for the on-site measurement of trace amounts of pesticides or other chemicals.

Nanoscale investigation of the degradation mechanism of a historical chrome yellow paint by quantitative electron energy loss spectroscopy mapping of chromium species

Getting the picture: The investigation of 100 year old chrome yellow paint by transmission electron microscopy and spectroscopy has led to the identification of four types of core-shell particles. This nanoscale investigation has allowed a mechanism to be proposed for the darkening of some bright yellow colors in Van Gogh's paintings (e.g. in Falling leaves (Les Alyscamps), 1888).

SR-FTIR imaging of the altered cadmium sulfide yellow paints in Henri Matisse's Le Bonheur de vivre (1905-6)--examination of visually distinct degradation regions

SR-FTIR imaging has been used to map the mid-IR active photo-degradation phases in two thin sections of cadmium yellow paint removed from Henri Matisse's Le Bonheur de vivre (1905-1906, The Barnes Foundation). These samples represent both the darkened cadmium yellow foliage in the upper left of the work and the lightened cadmium yellow field beneath the central reclining figures. The altered cadmium yellow paints from both regions were found to contain cadmium carbonate (CdCO3), cadmium sulphate (CdSO4), and cadmium oxalate (CdC2O4). Each of these phases was imaged to determine their positions as a function of depth, with the aim of better understanding the role of each phase in the degradation mechanism. This speciation mapping is critical because cadmium oxalate was used in this period as an additive in cadmium yellow light. In addition, cadmium carbonate and cadmium sulphate were synthesis starting materials for cadmium yellow, and so their distribution throughout the paint layer can provide an indication of their roles. It was established that cadmium oxalate is localized at the surface of the paint layer, cadmium carbonate is found deeper in the layer but still enriched at the surface, and cadmium sulphate is distributed throughout the layer. This distribution, along with the chloride content of the paint suggesting a cadmium chloride starting material, is consistent with an alteration mechanism in which the cadmium sulphide is oxidized to sulphate and this is then converted to carbonate and oxalate. The relative solubilities of the three photo-degradation products are also relevant to their locations in the paint film.

The use of synchrotron radiation for the characterization of artists' pigments and paintings

We review methods and recent studies in which macroscopic to (sub)microscopic X-ray beams were used for nondestructive analysis and characterization of pigments, paint microsamples, and/or entire paintings. We discuss the use of portable laboratory- and synchrotron-based instrumentation and describe several variants of X-ray fluorescence (XRF) analysis used for elemental analysis and imaging and combined with X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Macroscopic and microscopic (μ-)XRF variants of this method are suitable for visualizing the elemental distribution of key elements in paint multilayers. Technical innovations such as multielement, large-area XRF detectors have enabled such developments. The use of methods limited to elemental analysis or imaging usually is not sufficient to elucidate the chemical transformations that take place during natural pigment alteration processes. However, synchrotron-based combinations of μ-XRF, μ-XAS, and μ-XRD are suitable for such studies.