If you were to compare Vincent van Gogh’s painting Wheat Stack Under a Cloudy Sky today with how it looked when it was originally completed in 1889, you might notice a subtle difference. The fall leaves floating on the surface of the pond in the artwork aren’t as red as they once were. They now look more like a reflection of the white clouds painted in the sky above.
Scientists recently figured out what caused the color change. A pigment the artist used to color his paints is slowly breaking down, turning from a once-brilliant red to white.
Van Gogh used a red pigment called red lead, or minium, in many of his paintings. Scientists have long known that minium whitens over time. But they didn’t know why.
To learn what was causing the color to fade, researchers from the University of Antwerp in Belgium examined a microscopic paint sample taken from Wheat Stack Under a Cloudy Sky. They zapped it with a focused beam of X-rays, a type of high-energy wave, to study its chemical composition. From the chemicals detected, they determined that minium undergoes a series of reactions when exposed to sunlight and air, forming white crystals that have altered the Van Gogh painting.
The findings are important because understanding how pigments break down could help museums better identify a painting’s original colors so they can be restored. It could also help them better preserve artworks, keeping paintings from fading in the future. Changing the environment in which paintings that contain minium are kept, for example, could prevent the pigment from turning white.
The scientists’ work may explain why colors in other priceless Van Gogh paintings, like Sunflowers and The Bedroom, have lightened too. “Normally, the idea is these paintings are there for a hundred years, or five hundred years, and they’re static—nothing really changes,” says Koen Janssens, who led the study. “But the opposite is actually true when you look in detail.”