Imagine Van Gogh’s paintings of yellow sunflowers. Now imagine the flowers brown. Eh, not so good. Sadly, we may someday see what that would look like. Because the bright yellow paints on the canvas are slowly turning muddy. Now, an international team of scientists has pinpointed the chemical reaction that drives this degradation—the first step toward stopping it. Their findings appear in the journal Analytical Chemistry. [Letizia Monico et al., Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Synchrotron X-ray Spectromicroscopy and Related Methods. 1. Artificially Aged Model Samples and 2. Original Paint Layer Samples]

Van Gogh’s use of strong bright colors to convey mood and emotion marks a major milestone in art history. And his Sunflower series wouldn’t be the same without the vibrant “chrome yellow” pigment that was new at the time. But this color darkens when exposed to the sun.

To find out why, researchers used sophisticated x-ray analyses to determine the exact chemical composition of the paint as it browns. First, they exposed three tubes of chrome yellow to UV radiation until one turned a chocolate color. Analyzing that sample, they identified specific changes in the paint’s chromium compounds. They then found the same chemical signature in the darkened areas of two van Gogh paintings. Until a chemical antidote is found, the researchers have some ironic advice: keep sunlight away from van Gogh’s sunflowers.