Many of Vincent van Gogh's paintings are changing color because his yellow paint reacts to sunlight.

Van Gogh used a pigment called chrome yellow. In 2011, researchers from the University of Antwerp and Delft University of Technology studied this paint using high-intensity X-rays. They found that Van Gogh used a specific version of the paint that is very sensitive to light.When UV light hits the paint, the chromium in the pigment changes its chemical state. It shifts from chromium(VI) to chromium(III). This change turns the bright yellow into a dark green-brown color. The reaction happens faster if the yellow paint was mixed with white paint containing sulfates.In famous paintings like 'Sunflowers,' the bright petals are slowly losing their glow. Museums now use special LED lights to reduce UV exposure and slow this process. Research shows that high humidity can also make the darkening happen faster.Van Gogh chose these paints because they were bright and cheap. He did not know they would change over time. This chemical reaction cannot be reversed. Today, the paintings look much darker than they did when they were first finished.