To figure out how red lead was changing colors, the Belgium researchers took a tiny white piece of van Gogh’s Wheat stack under a cloudy sky. They examined the white speck under a microscope and fired x-ray lasers through the sample to determine what minerals it contained.

Turns out that speck of paint was originally red.

The team of scientists note that, “this is the first reported occurrence of this compound in a painting dating from before the mid 20th century.” But this isn’t the same reason why Renior’s 1883’s Madame Léon Clapisson is fading. The red color used for that painting comes from an insect known as a cochineal, which, when exposed to light separates it’s organic and inorganic compounds. That turns the red in the Renior’s work into a mellow gray color.

The next step for scientists: Develop a way in which the reds can be restored to the paintings.