| AVS 61st International Symposium & Exhibition | |
| Conservation Studies of Heritage Materials Focus Topic | Thursday Sessions |
| Session CS-ThA |
| Session: | Conservation Studies of Heritage Materials 2 |
| Presenter: | Jennifer Mass, Winterthur Museum |
| Authors: | J.L. Mass, Winterthur Museum E. Pouyet, ESRF, France F. Meirer, Utrecht University, Netherlands M. Cotte, ESRF, France A. Mehta, Stanford Synchrotron Radiation Lightsource |
| Correspondent: | Click to Email |
Cadmium carbonate (CdCO3) has for several years been identified in the altered cadmium yellow (CdS) paints found Impressionist, early modernist, and post-Impressionist works. When it is concentrated at the surface of the painting, CdCO3 appears to be a result of the photo-alteration of CdS, likely through a CdSO4-containing phase. However, in other cases CdCO3 is distributed throughout the paint layer. This is significant because CdCO3 is highly insoluble (Ksp of 1.0 ×10-12), and if it were formed solely as a result of photo-alteration it would not be expected to migrate away from the painting’s surface. In cadmium yellow paints in works such as Edvard Munch’s c. 1910 The Scream (The Munch Museum, Oslo), Plahter et al. have recently proposed that CdCO3 is present because this compound was used in the indirect wet process synthesis of CdS (through, for example, the reaction of CdCO3 and Na2S). This would mean that the CdCO3 is a residual starting reagent rather than a photo-alteration product. Such an interpretation is supported by the identification of CdCO3 in the unaltered cadmium yellow paints of early modernist works such as Pablo Picasso’s The Blue Room (The Phillips Collection, 1901) and Henri Matisse’s Flower Piece (The Barnes Foundation, 1906).
To address this question of CdCO3’s role, a flake of apparently nondegraded cadmium yellow paint was removed from Henri Matisse’s Flower Piece so that the distribution of CdCO3 could be studied, both as a function of depth in the paint layer and in individual pigment particles.
X-Ray microspectroscopy and microdiffraction were respectively carried out at ESRF ID21 and Petra III, supplemented by light microscopy, backscattered electron microscopy with X-ray microanalysis, and Fourier transform infrared spectroscopy. This presentation will focus on results from X-ray microspectroscopy: μ-X-ray fluorescence (XRF scanning) allowed precise mapping of local elemental distribution, and Full Field X-ray Near-Edge Absorption Structure (FF-XANES) for mapping the chemical speciation using the Cd-L3 and S-K edges.
FF-XANES imaging of a 15 micrometer thin section at CdL3-edge revealed the presence of three Cd-based phases: CdS, CdSO4, and CdCO3. The CdSO4 was concentrated on only one surface of the sample, suggesting its role as a photo-alteration product. Cadmium carbonate was found to comprise the bulk of the individual pigment particles, suggesting that it is a synthesis starting reagent. CdS was found to be concentrated on the surface of these CdCO3 particles. CdSO4 could also be observed to surround some of the CdCO3 particles, suggesting the beginnings of photo-oxidation of the thin CdS coating.