(Don’t) Fade to Grey

The yellow paint beneath the central reclining figures was fading to a dirty ivory color; simultaneously, the upper layer was flaking away to reveal the original bright-yellow color beneath. Jennifer Mass, a senior scientist at the Scientific Research and Analysis Laboratory Conservation Department, Winterthur Museum, Delaware, USA, led the international research project that followed. “The code of ethics of the American Institute for Conservation of Artistic and Historic Works (AIC), challenges chemists in the field of art conservation to do our work either noninvasively using standoff methods or in as minimally invasive a manner as possible,” she says. To that end, the team initially chose to examine the painting using portable X-ray fluorescence spectroscopy (pXRF), which revealed that both cadmium yellow and chromium yellow pigments had been used, but only the cadmium yellow paints were undergoing a physical and chemical breakdown.

Next, microsamples of paint were removed to probe composition as a function of depth, using synchrotron-based FTIR, XRF, and XANES (X-ray absorption near-edge structure) imaging. “The analyses revealed that the off-white crust was composed predominately of cadmium carbonate, cadmium sulfate, and cadmium oxalate,” says Mass. Studying the composition of the paint as a function of depth helped identify the compounds as photo-oxidation products rather than residual starting reagents or fillers. The resulting cadmium compounds are all white, which explains the fading of the paint from yellow to ivory – and, as the pigment degrades, the sulfates seem to cause acid hydrolysis of the oil binder, resulting in the crumbling and flaking.

Sadly, it’s not an uncommon problem. “The phenomena described above are observed predominately in cadmium yellow paints prepared between the 1880s and the 1920s, which unfortunately means that degradation of cadmium yellow paints is found in the works of the impressionist, post-impressionist, and early modernist masters, including Claude Monet, Pablo Picasso, Vincent Van Gogh, and Henri Matisse. Literally billions of dollars of global cultural heritage is at risk,” says Mass.

On the bright side (pun intended), the new knowledge gained is likely to lead to recommendations for preservation. Next, the group is interested in detecting at-risk paintings before the degradation of occurs. “Preliminary work has revealed that cadmium yellow pigments that are beginning to degrade have unique fluorescence properties. Ongoing work will attempt to reproduce these early findings, and determine under what conditions they can be predictive,” concludes Mass.