Probing the chemistry of CdS paints in The Scream by in situ noninvasive spectroscopies and synchrotron radiation x-ray techniques

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

Chemical Mapping by Macroscopic X-ray Powder Diffraction (MA-XRPD) of Van Gogh's Sunflowers : identification of areas with higher degradation risk

The discoloration rate of chrome yellow (CY), a class of synthetic inorganic pigments (PbCr1−xSxO4) frequently used by Van Gogh and his contemporaries, strongly depends on its sulfate content and on its crystalline structure (either monoclinic or orthorhombic). Macroscopic X‐Ray powder diffraction imaging of selected areas on Van Gogh's Sunflowers (Van Gogh Museum, Amsterdam) revealed the presence of two subtypes of CY: the light‐fast monoclinic PbCrO4 (LF‐CY) and the light‐sensitive monoclinic PbCr1−xSxO4 (x≈0.5; LS‐CY). The latter was encountered in large parts of the painting (e.g., in the pale‐yellow background and the bright‐yellow petals, but also in the green stems and flower hearts), thus indicating their higher risk for past or future darkening. Overall, it is present in more than 50 % of the CY regions. Preferred orientation of LS‐CY allows observation of a significant ordering of the elongated crystallites along the direction of Van Gogh's brush strokes

Chromium speciation methods and infrared spectroscopy for studying the chemical reactivity of lead chromate-based pigments in oil medium

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 (μ-XRF)/micro-X-ray absorption near edge structure (XANES) 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 PbCrO$_4$∙ PbO (monoclinic), PbCrO$_4$ (monoclinic) and PbCr$_{0.2}$S$_{0.8}$O$_4$ (orthorhombic)] in an oil medium. The investigation of paint models was also compared to that of a late 19th century historical orthorhombic PbCr$_{0.4}$S$_{0.6}$O$_4$ 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 μ-XANES/μ-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 PbCr$_{0.2}$S$_{0.8}$O$_4$ 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 PbCr$_{0.2}$S$_{0.8}$O$_4$ 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