Vincent van Gogh’s reds have been turning white, but the exact reason why has remained unclear. Research published last month out of Belgium has identified a rare lead mineral in his paint as the missing link.

As reported this week by Matthew Gunther at the Royal Society of Chemistry’s Chemistry World, a team at the University of Antwerp examined a microscopic sample of van Gogh’s “Wheat Stack Under a Cloudy Sky” (“Heuschober an einem Regentag”) from 1889 at the Kröller-Müller Museum using X-ray powder diffraction tomography, basically focusing beams to reveal crystalline compounds. Van Gogh loved the vibrant lead pigment colors, and the red in “Wheat Stack” turned out to contain a rare mineral lead called plumbonacrite that through light exposure was gradually coated in carbonates that were causing the discoloration. Or, in less science speak, the paint particles are now like if you had a Gobstopper with the red core inside and a light blue layer and then gray layer on the outside of the particle mass.

What the team found was described last month in “Plumbonacrite Identified by X-ray Powder Diffraction Tomography as a Missing Link during Degradation of Red Lead in a Van Gogh Painting” in Angewandte Chemie, published by the German Chemical Society. As they note in their abstract: “This is the first reported occurrence of this compound in a painting dating from before the mid 20th century.” Interestingly, it’s a different issue than the recent analysis of the fading of red in Renoir’s “Madame Léon Clapisson” (1883), where the red lake pigment made of cochineal insects was separating. With van Gogh’s “Roses” (1890) in the National Gallery of Art now flowering in ivory blooms, and those “Wheat Stacks” once surrounded by flourishes of red now muted, the continued research on the chemistry of van Gogh’s pigments could have a wider influence on art conservation. And importantly, it could influence the way his paintings are displayed in light, knowing that the rare mineral in the red may fade from the colors the artist originally envisioned.

“Plumbonacrite Identified by X-ray Powder Diffraction Tomography as a Missing Link during Degradation of Red Lead in a Van Gogh Painting” is available online in Angewandte Chemie.