X - R A Y N A N O P R O B E
S C I E N T I F I C H I G H L I G H T S
9 0 H I G H L I G H T S 2 0 2 2 I
PRINCIPAL PUBLICATION AND AUTHORS
Application of synchrotron radiation-based micro-analysis on Cadmium Yellows in Pablo Picasso s Femme, M. Ghirardello (a), V. Gonzalez (b), L. Monico (c), A. Nevin (d,e), D. MacLennan (f), C. Schmidt Patterson (f), M. Burghammer (g), M. Réfrégiers (h,i), D. Comelli (a), M. Cotte (g,j), Microsc. Microanal. 28, 1504-1513 (2022); https:/doi.org/10.1017/S1431927622000873 (a) Politecnico di Milano, Milan (Italy) (b) Université Paris-Saclay, Gif-sur-Yvette (France) (c) CNR-SCITEC, Perugia (Italy) (d) IFN-CNR, Milano (Italy) (e) Courtauld Institute of Art, London (UK) (f) Getty Conservation Institute, Los Angeles (USA) (g) ESRF (h) Synchrotron SOLEIL, Gif-sur-Yvette (France) (i) CBM UPR4301, Orléans (France) (j) LAMS, CNRS UMR 8220, Sorbonne Université, Paris (France)
 D. Comelli et al., Anal. Chem. 91(5), 3421-3428 (2019).  A. Veamatahau et al., Phys. Chem. Chem. Phys. 17, 2850- 2858 (2015).  L. Monico et al., Sci. Adv. 6, eaay3514 (2020).
Fig. 81: a) Photomicrograph of the faded paint micro cross- section with indication of the areas analysed. b) RGB composite
μXRF maps of Ba/Pb/S. c) Selection of S K-edge spectra obtained from the degraded yellow layer. d) RGB composite μXRF maps of
Cd/Cl/S and corresponding false-colour image of the PL emission.
as highlighted by µXRD and μXANES analyses, CdS in degraded yellow is in an amorphous or nanocrystalline form. While, in general, amorphous CdS presents a faint optical emission, highly defective CdS nanoparticles present an intense emission from trap states . Therefore, it can be hypothesised that the pigment is based on nanocrystalline CdS.
Although the spatial correlation of the maps obtained with the different techniques is challenging, due to the different volumes probed by each method and the intrinsic sensitivity of each technique to different material properties, the combination of the information offers a unique insight and a better understanding of the sample. In the Femme case, by comparing the µPL and the µXRF maps, areas containing Cd and Cl are more correlated with the distribution in the red, while Cd-rich areas present a NIR emission (Figure 81). Based on this correlation, it is hypothesised that chlorine species affected the paint micro-environment and, consequently, the stability of the highly reactive, poorly crystalline CdS, inducing the formation of a high density of superficial defects.
The poor crystallinity of the faded yellow and the presence of Cd(OH)Cl and CdCO3 can be ascribed to the wet production method of the pigment . Although the two yellow paints have been exposed to the same environmental conditions, the different types of CdS pigment, along with the residual starting reagents (i.e., chlorine species), have influenced the paint stability, leading to degradation where a poorly crystalline CdS pigment accompanied by residual contamination from the synthesis process was used. This finding on the specific case study of Femme agrees with analyses
performed on other degraded cadmium yellow paintings . The occurrence of these two features in different historical degraded paintings confirms their key role in CdS paint degradation and paves the way for future targeted work aimed at clarifying the individual role of these two factors.