Abstract: In this paper, the chromatic alteration of various types of paints, present on mural painting fragments derived from the vaults of The Upper Basilica of Saint Francis of Assisi in Italy (12th-13th century), is studied using synchrotron radiation. Six painted mural fragments, several square centimeters in size, were available for analysis, originating from the ceiling paintings attributed to Cimabue and Giotto; they correspond to originally white, blue/green, and brown/yellow/orange areas showing discoloration. As well as collecting macroscopic X-ray fluorescence and diffraction maps from the entire fragments in the laboratory and at the SOLEIL synchrotron, corresponding paint cross-sections were also analyzed using microscopic X-ray fluorescence and powder diffraction mapping at the PETRA-III synchrotron. Numerous secondary products were observed on the painted surfaces, such as (a) copper tri-hydroxychloride in green/blue areas; (b) corderoite and calomel in vermillion red/cinnabar-rich paints; (c) plattnerite and/or scrutinyite assumed to be oxidation products of (hydro)cerussite (2PbCO(3)center dot Pb(OH)(2)) in the white areas, and (d) the calcium oxalates whewellite and weddellite. An extensive presence of chlorinated metal salts points to the central role of chlorine-containing compounds during the degradation of the 800-year-old paint, leading to, among other things, the formation of the rare mineral cumengeite (21PbCl(2)center dot 20Cu(OH) (2) center dot 6H(2)O).

Abstract: Cadmium red is the name used for denoting a class of twentieth century artists' pigments described by the general formula CdS1-xSex. For their vibrant hues and excellent covering power, a number of renowned modern and contemporary painters, including Jackson Pollock, often used cadmium reds. As direct band gap semiconductors, CdS1-xSex compounds undergo direct radiative recombination (with emissions from the green to orange region) and radiative deactivation from intragap trapping states due to crystal defects, which give rise to two peculiar red-NIR emissions, known as deep level emissions (DLEs). The positions of the DLEs mainly depend on the Se content of CdS1-xSex; thus, photoluminescence and diffuse reflectance vis-NIR spectroscopy have been profitably used for the non-invasive identification of different cadmium red varieties in artworks over the last decade. Systematic knowledge is however currently lacking on what are the parameters related to intrinsic crystal defects of CdS1-xSex and environmental factors influencing the spectral properties of DLEs as well as on the overall (photo)chemical reactivity of cadmium reds in paint matrixes. Here, we present the application of a novel multi-length scale and multi-method approach to deepen insights into the photoluminescence properties and (photo)chemical reactivity of cadmium reds in oil paintings by combining both well established and new non-invasive/non-destructive analytical techniques, including macro-scale vis-NIR and vibrational spectroscopies and micro-/nano-scale advanced electron microscopy mapping and X-ray methods employing synchrotron radiation and conventional sources. Macro-scale vis-NIR spectroscopy data obtained from the in situ non-invasive analysis of nine masterpieces by Gerardo Dottori, Jackson Pollock and Nicolas de Stael allowed classifying the CdS1-xSex-paints in three groups, according to the relative intensity of the two DLE bands. These outcomes, combined with results from micro-/nano-scale electron microscopy mapping and X-ray analysis of a set of CdS1-xSex powders and artificially aged paint mock-ups, indicated that the relative intensity of DLEs is not affected by the morphology, microstructure and local atomic environment of the pigment particles but it is influenced by the presence of moisture. Furthermore, the extensive study of artificially aged oil paint mock-ups permitted us to provide first evidence of the tendency of cadmium reds toward photo-degradation and to establish that the conversion of CdS1-xSex to CdSO4 and/or oxalates is triggered by the oil binding medium and moisture level and depends on the Se content. Based on these findings, we could interpret the localized presence of CdSO4 and cadmium oxalate as alteration products of the original cadmium red paints in two paintings by Pollock.

Abstract: The degradation of cadmium sulfide (CdS)-based oil paints is a phenomenon potentially threatening the iconic painting The Scream (ca. 1910) by Edvard Munch (Munch Museum, Oslo) that is still poorly understood. Here, we provide evidence for the presence of cadmium sulfate and sulfites as alteration products of the original CdS-based paint and explore the external circumstances and internal factors causing this transformation. Macroscale in situ noninvasive spectroscopy studies of the painting in combination with synchrotron-radiation x-ray microspectroscopy investigations of a microsample and artificially aged mock-ups show that moisture and mobile chlorine compounds are key factors for promoting the oxidation of CdS, while light (photodegradation) plays a less important role. Furthermore, under exposure to humidity, parallel/secondary reactions involving dissolution, migration through the paint, and recrystallization of water-soluble phases of the paint are associated with the formation of cadmium sulfates.

Abstract: This chapter provides a description of colour changes in the Amsterdam Sunflowers due to chemical alteration of pigments, with a focus on geranium lakes and chrome yellows. The brilliant and forceful colours of these and other late nineteenth-century synthetic materials offered artists such as Vincent van Gogh new means of artistic expression that exploited a range of contrasting hues and tints. However, geranium lakes have a strong tendency to fade and chrome yellows to darken under the influence of light. Van Gogh, like other artists of his day, was aware of this drawback, yet he continued to favour the use of both pigments up until his death in July 1890 due to the unparalleled effects they gave. In April 1888, Vincent wrote to his brother Theo: Van Gogh's use of unstable colours opens a series of questions regarding the extent to which colour change affects the way his paintings look today, as discussed here in relation to the Amsterdam Sunflowers. Furthermore, given the frequency with which geranium lakes and chrome yellows occur in Van Gogh's paintings of the period 1888–90 and the predominance of chrome yellows in Sunflowers, it becomes important to understand the factors that can drive these processes of deterioration in order to develop appropriate strategies for conserving the artist's works.

Abstract: This chapter explains the materials and techniques employed in the Amsterdam Sunflowers, enabling a comparison with the London version described in chapter 3. Building upon the 2016 article published in the National Gallery Technical Bulletin, it incorporates the latest findings gained by computer-assisted methods used to characterize the canvas support, as well as in-situ campaigns of non-invasive investigation together with further analysis of microscopic paint samples. The chapter sequence follows the steps in Van Gogh's working practice. Starting with the canvas, automated analysis of the weave enables the provenance of the canvas to be traced back to a particular roll of linen ordered by Van Gogh. Combining technical evidence with knowledge of historical manufacturing techniques further allows us to reconstruct the way in which Van Gogh divided his canvas roll into pieces used for Sunflowers and other paintings. We go on to consider how, with the original painting at hand, he used charcoal to transfer the motif of the London Sunflowers onto his blank canvas. Despite careful planning of the composition, an adjustment was required late in the working process, when Van Gogh added a painted wooden strip to extend the background above the flower at the top edge of the canvas. The artist's process of working up the composition in paint is described, paying special attention to his use of colour. The pigments and pigment mixtures used in the Amsterdam Sunflowers have been comprehensively mapped and are compared with the London picture, with discussion of some similarities and differences that account for the distinctive colour scheme of each painting. This understanding of colour application in the Amsterdam Sunflowers lays the foundation for subsequent chapters that will go on to consider the impact of light-induced colour changes that have taken place over time, and the related need to define appropriate lighting guidelines for the future safe preservation of this painting and others made with similar materials (chapters 5 and 7).

Abstract: 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 center dot nH(2)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/Cd0.76Zn0.24S) 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) Cd0.76Zn0.24S is more susceptible to photooxidation than CdS; both compounds can act as photocatalysts for the oil oxidation. 2) The photooxidation of CdS/Cd0.76Zn0.24S to CdSO4 center dot nH(2)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.

Abstract: Environmental factors, such as light, humidity and temperature are triggering agents for the alteration of organic and/or inorganic constituents of oil paintings. The oxidation of the organic material is favored by increasing of relative humidity and temperature, whereas processes involving changes of the oxidation states of a number of inorganic pigments (e.g., vermilion, cadmium yellows, zinc yellows, chrome yellows) are mainly activated by light-exposure. In view of the optimization of the long-term conservation and restoration strategies of paintings it is of relevant interest to establish the consequences of thermal parameters (temperature and relative humidity) on the chemical/photochemical-reactivity and the nature of the alteration products of light sensitive-pigments in oil medium. To this aim here we propose a multi-method analytical approach based on the combination of diffuse reflectance UV-Vis, FTIR, synchrotron radiation (SR)-based micro X-ray fluorescence (mu-XRF)/micro-X-ray absorption neat edge structure ()CANES) and electron paramagnetic resonance (EPR) spectroscopies for studying the effects of different relative humidity conditions before and after light exposure on the reactivity of a series of lead chromate-based pigments [such as PbCrO4 center dot PbO (monoclinic), PbCrO4 (monoclinic) and PbCr0.2S0.8O4 (orthorhombic)] in an oil medium. The investigation of paint models was also compared to that of a late 19th century historical orthorhombic PbCr0.4S0.6O4 oil paint. Diffuse reflectance UV-Vis and FTIR spectroscopies were used to obtain information associated with chromatic changes and the formation of organo-metal degradation products at the paint surface. SR-based Cr K-edge mu-XANES/mu-XRF mapping analysis and EPR spectroscopy were employed in a complementary fashion to determine the amount, nature and distribution of Cr(III) and Cr(V)-based alteration compounds within the paints with micrometric spatial resolution. Under the employed thermal aging conditions, lead(II)-carboxylates and reduced Cr-compounds (in abundance of up to about 35% at the surface) have been identified in the lead chromate-based paints. The tendency of chromates to become reduced increased with increasing moisture levels and was favored for the orthorhombic PbCr0.2S0.8O4 compounds. The redox process gave rise to the formation of Cr(V)-species in relative amount much higher than that was formed in the equivalent paint which was exposed only to light. After light-exposure of the thermally aged paints, compounds ascribable to the oxidation of the organic binder were detected for all the types of pigments. Nevertheless, the previous thermal treatment increased the tendency toward photo-reduction of only the PbCr0.2S0.8O4 pigment. For this light-sensitive compound, the thickness variation of the reduced Cr-rich (ca. 70%) photo-alteration layer with moisture levels could be ascribed to a surface passivation phenomenon that had already occurred before photochemical aging. (C) 2015 Elsevier B.V. All rights reserved.

Abstract: Light-induced redox processes have been established as the cause of the chromatic alterations of a number of artists' pigments used from the 15th to the 20th century. Despite the fact that a general comprehension of the mechanisms has been provided through the characterization of photo-degraded compounds, both exhaustive information on the wavelength-dependence of the alteration process of the pigments and experimental evidence in how visible light may influence the formation pathways of specific secondary compounds are still lacking. Establishing an analytical protocol for the study of wavelength-dependence of pigments on photo-redox pathways is relevant for the safe illumination of paintings, especially in view of the possible use of spectrally tunable light sources such as white light emitting diodes (WLEDs). In this work, we propose an integrated approach based on a combination of diffuse reflectance UV-visible, synchrotron radiation (SR)-based micro X-ray fluorescence (m-XRF)/X-ray absorption near edge structure (m-XANES) and electron paramagnetic resonance (EPR) spectroscopies to study the photo-redox process of Cr(VI) -> Cr(III) for lead chromate yellows (PbCr1-xSxO4, 0 <= x <= 0.8) under exposure to different monochromatic light. In view of the thin (3-5 mm) alteration layer that is formed at the paint surface after light exposure, SR-based Cr K-edge mu-XANES/mu-XRF analysis was employed to obtain information on the abundance, nature and distribution of the alteration of Cr(III)-compounds at the micrometricscale level. On the other hand, EPR spectroscopy was used as a complementary tool to the SR-based X-ray methods due to its sensitivity for revealing species containing one or more unpaired electrons and for distinguishing different coordination geometries of paramagnetic centers, such as Cr(V)-species. Semi-quantitative indications about the darkening of the paint surface were obtained by UV-Vis spectroscopy. An abundance of reduced Cr down to around 50% was detected at the aged surface of chrome yellow paints. The reduction process was favored not only by wavelengths shorter than 460 nm (i.e., where the pigment shows its maximum absorption) but also by light in the 490-530 nm range. The first evidence of the presence of Cr(V)-intermediates in the Cr(VI) -> Cr(III) reduction reaction allowed the risks of inducing photo-degradation of the 490-530 nm wavelength range to be explained.

Abstract: The darkening of lead chromate yellow pigments, caused by a reduction of the chromate ions to Cr(III) compounds, is known to affect the appearance of several paintings by Vincent van Gogh. In previous papers of this series, we demonstrated that the darkening is activated by light and depends on the chemical composition and crystalline structure of the pigments. In this work, the results of Part 2 are extended and complemented with a new study aimed at deepening the knowledge of the nature and distribution of Cr and S species at the interface between the chrome yellow paint and the nonoriginal coating layer. For this purpose, three microsamples from two varnished paintings by Van Gogh and a waxed low relief by Gauguin (all originally uncoated) have been examined. Because nonoriginal coatings are often present in artwork by Van Gogh and contemporaries, the understanding of whether or not their application has influenced the morphological and/or physicochemical properties of the chrome yellow paint underneath is relevant in view of the conservation of these masterpieces. In all the samples studied, microscopic X-ray fluorescence (mu-XRF) and X-ray absorption near edge structure (mu-XANES) investigations showed that Cr(III)-based alteration products are present in the form of grains inside the coating (generally enriched of S species) and also homogeneously widespread at the paint surface. The distribution of Cr(III) species may be explained by the mechanical friction caused by the coating application by brush that picked up and redistributed the superficial Cr compounds, likely already present in the reduced state as result of the photodegradation process. The analysis of the XANES profiles allowed us to obtain new insights into the nature of the Cr(III) alteration products, that were identified as sulfate-, oxide-, organo-metal-, and chloride-based compounds. Building upon the knowledge acquired through the examination of original paint samples and from the investigation of aged model paints in the last Part 4 paper, in this study we aim to characterize a possible relation between the chemical composition of the coating and the chrome yellow degradation pathways by studying photochemically aged model samples covered with a dammar varnish contaminated with sulfide and sulfate salts. Cr speciation results did not show any evidence of the active role of the varnish and added S species on the reduction process of chrome yellows.

Abstract: In this work a reflection mid-infrared spectroscopy study of twelve metal-oxalate complexes, of interest in art conservation science as alteration compounds, was performed. Spectra of the reference materials highlighted the presence of derivative-like and/or inverted features for the fundamental vibrational modes as result of the main contribution from the surface component of the reflected light. In order to provide insights in the interpretation of theses spectral distortions, reflection spectra were compared with conventional transmission ones. The Kramers-Kronig (KK) algorithm, employed to correct for the surface reflection distortions, worked properly only for the derivative-like bands. Therefore, to pay attention to the use of this algorithm when interpreting the reflection spectra is recommended. The outcome of this investigation was exploited to discriminate among different oxalates on thirteen polychrome art-works analyzed in situ by reflection mid-infrared spectroscopy. The visualization of the v(s)(CO) modes (1400-1200 cm(-1)) and low wavenumber bands (below 900 cm(-1)) in the raw reflection profiles allowed Ca, Cu and Zn oxalates to be identified. Further information about the speciation of different hydration forms of calcium oxalates were obtained by using the KK transform. The work proves reflection mid-infrared spectroscopy to be a reliable and sensitive spectro-analytical method for identifying and mapping different metal-oxalate alteration compounds on the surface of artworks, thus providing conservation scientists with a non-invasive tool to obtain information on the state of conservation and causes of alteration of artworks. (C) 2013 Elsevier B.V. All rights reserved.

Abstract: The painter, Vincent van Gogh, and some of his contemporaries frequently made use of the pigment chrome yellow that is known to show a tendency toward darkening. This pigment may correspond to various chemical compounds such as PbCrO4 and PbCr1-xSxO4, that may each be present in various crystallographic forms with different tendencies toward degradation. Investigations by X-ray diffraction (XRD), mid-Fourier Transform infrared (FTIR), and Raman instruments (benchtop and portable) and synchrotron radiation-based micro-XRD and X-ray absorption near edge structure spectroscopy performed on oil-paint models, prepared with in-house synthesized PbCrO4 and PbCr1-xSxO4, permitted us to characterize the spectroscopic features of the various forms. On the basis of these results, an extended study has been carried out on historic paint tubes and on embedded paint microsamples taken from yellow-orange/pale yellow areas of 12 Van Gogh paintings, demonstrating that Van Gogh effectively made use of different chrome yellow types. This conclusion was also confirmed by in situ mid-FTIR investigations on Van Goghs Portrait of Gauguin (Van Gogh Museum, Amsterdam).

Abstract: On several paintings by artists of the end of the 19th century and the beginning of the 20th Century a darkening of the original yellow areas, painted with the chrome yellow pigment (PbCrO4, PbCrO4·xPbSO4, or PbCrO4·xPbO) is observed. The most famous of these are the various Sunflowers paintings Vincent van Gogh made during his career. In the first part of this work, we attempt to elucidate the degradation process of chrome yellow by studying artificially aged model samples. In view of the very thin (1−3 μm) alteration layers that are formed, high lateral resolution spectroscopic methods such as microscopic X-ray absorption near edge (μ-XANES), X-ray fluorescence spectrometry (μ-XRF), and electron energy loss spectrometry (EELS) were employed. Some of these use synchrotron radiation (SR). Additionally, microscopic SR X-ray diffraction (SR μ-XRD), μ-Raman, and mid-FTIR spectroscopy were employed to completely characterize the samples. The formation of Cr(III) compounds at the surface of the chrome yellow paint layers is particularly observed in one aged model sample taken from a historic paint tube (ca. 1914). About two-thirds of the chromium that is present at the surface has reduced from the hexavalent to the trivalent state. The EELS and μ-XANES spectra are consistent with the presence of Cr2O3·2H2O (viridian). Moreover, as demonstrated by μ-XANES, the presence of another Cr(III) compound, such as either Cr2(SO4)3·H2O or (CH3CO2)7Cr3(OH)2 [chromium(III) acetate hydroxide], is likely.