Role of the Relative Humidity and the Cd/Zn Stoichiometry in the Photooxidation Process of Cadmium Yellows (CdS/Cd_1−xZn_xS) in Oil Paintings

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 CdSO ⋅n H 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 Zn 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 Zn S is more susceptible to photooxidation than CdS; both compounds can act as photocatalysts for the oil oxidation. 2) The photooxidation of CdS/Cd Zn S to CdSO ⋅n H 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.

Selective Excitation of CdSe/CdS Dot‐in‐Rod Nanocrystals and Distinct Roles of Electrons and Holes in Photophysical and Photochemical Interactions with Ambient Air

“…These kinds of interactions can be chemical reactions, which convert surface CdS into other specimens and therefore change the confining potentials. Photooxidation is generally observed in CdSe and CdS nanomaterials, [9,14,15,36] converting surface CdS (CdSe) into CdSO x (CdSeO x ) and CdO via the reaction with adsorbed oxygen and water molecules, which requires the participation of holes. The spectral results in Figure 3b suggest that CdS surface reacts with adsorbed ambient molecules due to the presence of holes, which may produce CdSO x and CdO at the surface.…”

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

“…SR‐μXRD mapping also has been extensively used to identify alteration products in artistic materials, and most often in paintings. As mentioned earlier, this is the case for the chemical degradation of chrome yellow, cadmium yellow, cinnabar and vermilion, copper‐based pigments, arsenate‐based pigments, but also for silver foils and lead‐based driers . In the case of HgS degradation, μXRD could even differentiate two forms of the same degradation product: corderoite (α‐Hg 3 S 2 Cl 2 ) and its metastable polymorph, kenhsuite ( γ ‐Hg 3 S 2 Cl 2 ).…”

Development of a multi-method analytical approach based on the combination of synchrotron radiation X-ray micro-analytical techniques and vibrational micro-spectroscopy methods to unveil the causes and mechanism of darkening of “fake-gilded” decorations in a Cimabue painting

“…In chemical sciences the study of the causes triggering the color change of paintings is a grand challenge. Over the past decades, investigations of micro-samples taken from historical paintings and articially aged paint mock-ups have established that redox processes driven by environmental agents (light, humidity, temperature and/or atmospheric pollutants) are the main responsible for irreversible chromatic alterations of a number of inorganic pigments, such as cadmium yellows (Cd 1Àx Zn x S, with 0 # x# 0.3), [1][2][3][4][5][6] chrome yellows (PbCr 1Àx S x O 4 , with 0 # x# 0.8), [7][8][9][10][11][12][13] vermilion red (HgS), 14,15 Prussian blue (MFe III [Fe II (CN) 6 ] Â H 2 O, with M ¼ K + , NH 4 + or Na + ) 16,17 and zinc yellow (K 2 O$4ZnCrO 4 $3H 2 O). 18 Similar to the above-mentioned compounds, also the yellow pigment orpiment (As 2 S 3 ) is characterized by a low chemical stability.…”

“…[23][24][25][26][27] From a methodological point of view, synchrotron radiation (SR)-based X-ray methods employing micro-probes, including micro X-ray uorescence (m-XRF), micro X-ray diffraction (m-XRD) and micro X-ray absorption near edge structure (m-XANES) spectroscopy (in mapping and point analysis mode), were used for their capabilities to provide spatially resolved elemental speciation and structural information down to the (sub) micrometer scale length. 1,2,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][21][22][23][24][25][26] The specicity of vibrational spectroscopy techniques (i.e., IR and Raman) to render complementary molecular information with respect to SR-based X-ray methods were also sometime exploited to study the chemical nature of secondary compounds associated with the degradation of different pigments. 3,5,6,12,[20][21][22][23][24][25] In addition, electrochemical techniques were proposed as alternative methods for a fast monitoring of the photo-degradation mechanism of a selection of semiconductor pigment powders.…”

“…1,2,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][21][22][23][24][25][26] The specicity of vibrational spectroscopy techniques (i.e., IR and Raman) to render complementary molecular information with respect to SR-based X-ray methods were also sometime exploited to study the chemical nature of secondary compounds associated with the degradation of different pigments. 3,5,6,12,[20][21][22][23][24][25] In addition, electrochemical techniques were proposed as alternative methods for a fast monitoring of the photo-degradation mechanism of a selection of semiconductor pigment powders. 21,22,28,29 In the context of orpiment-based paints, despite a large quantity of data on the general characterization and distribution of degraded compounds has been recently acquired, [19][20][21][22][23][24][25][26]…”