The umber tinge accenting the tips of many of Vincent van Gogh's famous sunflowers has vexed conservators and chemists alike, who know that this hue was not what the artist had first daubed on his canvases. Now, after subjecting several samples of 19th-century paints to finely tuned x-rays at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, scientists think they have cracked the chemical code behind this famous fading.

Most paintings tend to lose their luster over the centuries, whether due to accumulated grime or environmental exposures. But the chrome yellow family of paints—especially popular in the 1800s—is particularly prone to discoloration.

To try to find out why these yellows dared to dull so quickly, a team of European researchers artificially aged chromium-based yellow paint taken from historic paint tubes (from the workshop of Flemish painter Rik Wouters, among others) with ultra-violet light.* At ESRF the researchers focused beams of x-ray fluorescence and x-ray absorption near-edge spectroscopy on the paint samples and found that the tawny top layer of chromium in the paint had degraded from Cr(VI) to the more stable Cr(III).

And with the help of synchrotron radiation x-ray diffraction, Raman and mid-Fourier transform infrared spectroscopy, the researchers then compared the chemical profile of these examples with those of flecks taken from van Gogh's Bank of the Seine (1887) and View of Arles with Irises (1888) and found the same transformation had occurred in the darker sections of the artist's yellows.

The findings are published in two papers in the February 15 issue of Analytical Chemistry.

"This type of cutting edge research is crucial to advance our understanding of how paintings age and should be conserved for future generations," Ella Hendriks, of the van Gogh Museum in Amsterdam and co-author of one of the new studies, said in a prepared statement.

The researchers also found that Cr(III) was more prevalent when there was also barium or sulfur present, suggesting that "sulfate anions likely play a key role in this alteration process," they noted in their second paper. This connection means that van Gogh's frequent use of white paint—likely intended to brighten the shades—might actually be speeding the yellow's darkening.

This level of technical analysis "has opened the door to a whole new world of discovery for art historians and conservators," Marine Cotte, an ESRF researcher also affiliated with the Musée du Louvre and who was a co-author on both papers, said in a prepared statement. Many other 19th-century painters, including Paul Cézanne, John Constable, Georges Seurat and J.M.W. Turner, incorporated chrome yellows in their work, making its preservation a looming concern for many museums.

The researchers are now investigating more closely the forces likely contributing to the yellows' fading. "We want to understand which conditions favor the reduction of chromium," Koen Janssens, of the University of Antwerp and co-author of the new papers, said in a prepared statement. And that will help his team to see "whether there is any hope to revert pigments to the original state in paintings where it is already taking place," he said.