One of the features of Vincent Van Gogh's art that set him apart was his use of bright colors, made possible by the invention of industrial pigments such as chrome yellow. But in the century since, many of these colors, including the bright yellows of his famous sunflowers, have faded, turning brown after exposure to sunlight.
A group of chemists set out to discover what was happening with the paints, with the hope that they might one day be able to reverse the process; their study appears in Analytical Chemistry. They started by artificially aging paint samples taken from historic paint tubes by exposing them to light from a UV lamp for 500 hours. One sample, from a tube that had belonged to Flemish painter Fauvist Rikk Wouters, quickly turned brown. X-ray analysis revealed that the oxidation state of the chromium atoms had changed from Cr(VI) to Cr(III), a more stable form of the atom and one that appears green instead of yellow.
The chemists then applied their X-ray analysis to two Van Gogh paintings, View of Arles with Irises and Bank of the Seine, that reside at the Van Gogh Museum in Amsterdam. That analysis revealed that the change in oxidation state tended to occur when the chromium was mixed with compounds containing barium sulfate. Barium sulfate was a major component in lithopone, a white pigment commonly used during Van Gogh's time, although there is no record of him using that pigment. The chemists speculate that Van Gogh mixed lithopone into his yellow paint, possibly as an extender to get more use out of it. He may have stretched his paint, but it appears he also lessened how long it would shine so brightly.