Abstract: The darkening of the original yellow areas painted with the chrome yellow pigment (PbCrO4, PbCrO4·xPbSO4, or PbCrO4·xPbO) is a phenomenon widely observed on several paintings by Vincent van Gogh, such as the famous different versions of Sunflowers. During our previous investigations on artificially aged model samples of lead chromate, we established for the first time that darkening of chrome yellow is caused by reduction of PbCrO4 to Cr2O3·2H2O (viridian green), likely accompanied by the presence of another Cr(III) compound, such as either Cr2(SO4)3·H2O or (CH3CO2)7Cr3(OH)2 [chromium(III) acetate hydroxide]. In the second part of this work, in order to demonstrate that this reduction phenomenon effectively takes place in real paintings, we study original paint samples from two paintings of V. van Gogh. As with the model samples, in view of the thin superficial alteration layers that are present, high lateral resolution spectroscopic methods that make use of synchrotron radiation (SR), such as microscopic X-ray absorption near edge (μ-XANES) and X-ray fluorescence spectrometry (μ-XRF) were employed. Additionally, μ-Raman and mid-FTIR analyses were carried out to completely characterize the samples. On both paint microsamples, the local presence of reduced Cr was demonstrated by means of μ-XANES point measurements. The presence of Cr(III) was revealed in specific areas, in some cases correlated to the presence of Ba(sulfate) and/or to that of aluminum silicate compounds.

Abstract: Over the past years a number of studies have described the instability of the pigment cadmium yellow (CdS). In a previous paper we have shown how cadmium sulfide on paintings by James Ensor oxidizes to CdSO4 center dot H2O. The degradation process gives rise to the fading of the bright yellow color and the formation of disfiguring white crystals that are present on the paint surface in approximately 50 mu m sized globular agglomerations. Here, we study cadmium yellow in the painting “Flowers in a blue vase” by Vincent van Gogh. This painting differs from the Ensor case in the fact that (a) a varnish was superimposed onto the degraded paint surface and (b) the CdS paint area is entirely covered with an opaque crust. The latter obscures the yellow color completely and thus presents a seemingly more advanced state of degradation. Analysis of a cross-sectioned and a crushed sample by combining scanning microscopic X-ray diffraction (mu-XRD), microscopic X-ray absorption near-edge spectroscopy (mu-XANES), microscopic X-ray fluorescence (mu-XRF) based chemical state mapping and scanning microscopic Fourier transform infrared (mu-FT-IR) spectrometry allowed unravelling the complex alteration pathway. Although no crystalline CdSO4 compounds were identified on the Van Gogh paint samples, we conclude that the observed degradation was initially caused by oxidation of the original CdS pigment, similar as for the previous Ensor case. However, due to the presence of an overlying varnish containing lead-based driers and oxalate ions, secondary reactions took place. In particular, it appears that upon the photoinduced oxidation of its sulfidic counterion, the Cd2+ ions reprecipitated at the paint/varnish interface after having formed a complex with oxalate ions that themselves are considered to be degradation products of the resin and/or oil in the varnish. The SO42- anions, for their part, found a suitable reaction partner in Pb2+ ions stemming from a dissolved lead-based siccative that was added to the varnish to promote its drying. The resulting opaque anglesite compound in the varnish, in combination with the underlying CdC2O4 layer at the paint/varnish interface, account for the orange-gray crust that is disfiguring the painting on a macroscopic level. In this way, the results presented in this paper demonstrate how, through a judicious combined use of several microanalytical methods with speciation capabilities, many new insights can be obtained from two minute, but highly complex and heterogeneous paint samples.

Abstract: The darkening due to chemical alteration of chrome yellows (PbCrO4/PbCr1-xSxO4) is a phenomenon threatening a large number of 19th-20th century paintings, including the Amsterdam Sunflowers by Vincent van Gogh. Our earlier studies have proven that the alteration is due to a Cr(VI) -> Cr(III) reduction with Cr(V)-species that are formed as long-lived intermediates and that bCr(1-x)S(x)O(4) (0 < x <= 0.8) types undergo reduction more readily than monoclinic, S-free, PbCrO4. In this context, there is still lack of knowledge about the effects of the chemical properties of the binding medium (i.e., chemical composition and drying process) and the solubility of chrome yellows on the overall reduction pathways. Here, we study a series of naturally and photochemically aged mock-up paints prepared by mixing chrome yellow powders (PbCrO4/PbCr0.2S0.8O4) with either linseed oil or a water-based acrylic emulsion as the binding medium. Equivalent paints made up of the highly soluble K2CrO4 were also investigated and used as benchmarks to provide a more in-depth understanding of the influence of the solubility on the chromate reduction pathways in the two different binders. A combination of synchrotron radiation-based Cr K-edge X-ray absorption near edge structure (XANES), electron paramagnetic resonance (EPR), and UV-Visible spectroscopy measurements shows that: (1) the Cr(VI) reduction results from the interaction between the pigment and the binder; (2) the process is more significant in oil, giving rise to Cr(V)- and Cr(III)-species as well as oxidized organic compounds; (3) the lightfastness of the chrome yellow pigment is enhanced in the acrylic binder; and (4) the tendency toward chromium reduction increases with increasing solubility of the pigment. Based on our findings, we propose a scheme for the mechanism of the (photo)reduction process of chrome yellows in the oil and acrylic binder. Overall, our results provide new insights into the factors driving the degradation of lead chromate-based paints in artworks and contribute to the development of strategies for preserving them over time.

Abstract: Imaging of crystalline phase distributions in heterogeneous materials, either plane projected or in virtual cross sections of the object under investigation, can be achieved by scanning X-ray powder diffraction employing X-ray micro beams and X-ray-sensitive area detectors. Software exists to convert the two-dimensional powder diffraction patterns that are recorded by these detectors to one-dimensional diffractograms, which may be analysed by the broad variety of powder diffraction software developed by the crystallography community. However, employing these tools for the construction of crystalline phase distribution maps proves to be very difficult, especially when employing micro-focused X-ray beams, as most diffraction software tools have mainly been developed having structure solution in mind and are not suitable for phase imaging purposes. XRDUA has been developed to facilitate the execution of the complete sequence of data reduction and interpretation steps required to convert large sequences of powder diffraction patterns into a limited set of crystalline phase maps in an integrated fashion.

Abstract: Recent studies in which X-ray beams of macroscopic to (sub) microscopic dimensions were used for non-destructive analysis and characterization of pigments, paint micro samples and/or entire paintings by Vincent van Gogh are concisely reviewed. 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 and macroscopic XRF are variants of the method that are well suited to visualize the elemental distribution of key elements, mostly metals, present in paint multi layers, either on the length scale from 1–100 μm inside micro samples taken from paintings or on the 1–100 cm length scale when the (subsurface) distribution of specific pigments in entire paintings is concerned. In the context of the characterization of van Gogh's pigments subject 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 (μ-FTIR) spectroscopy since this method delivers complementary information at more or less the same length scale as the X-ray microprobe techniques. Also in the context of macroscopic imaging of works of art, the complementary use of X-ray based and infra-red based imaging appears very promising; some recent developments are discussed.

Abstract: Synchrotron radiation micro-X-ray fluorescence and micro-X-ray diffraction analysis, micro-computed tomography and transmission electron microscopy were used to investigate the microstructure of turbine blades made of a second generation nickel based superalloy CMSX-4 manufactured by investment casting process via Bridgman technique. The dependence of microstructure on process parameters, among others, on the withdrawal rate was analysed. The results set the stage for optimising the microstructure of the blades in terms of their mechanical properties.