Letizia Monico investigated the cause in her doctoral thesis.
"That kind of yellow paint is very sensitive to green and blue. I would advise museums not to use LED bulbs any more as they emit at lot of blue light.”
The researchers used powerful x-rays to examine a series of paintings by Van Gogh.
"We discovered that the Dutch master didn’t always use the same type of chrome-yellow paint.“
Van Gogh also used a lighter lemon-yellow and a lighter-still primula-yellow.
"These varieties have a different crystal structure, albeit combined with higher sulphur content. They react quiet adversely to light rays. After just a few days of our tests they changed colour from to brown and olive green. We discovered this unstable form of chrome yellow paint in a number of very well-known paintings like the 'Portrait of Gauguin” and the ‘Sunflowers’.
The researchers are keen to stress that not only Van Gogh’s paintings, but also paintings by the French master Paul Cézanne contained both stable and unstable types of chrome-yellow paint.
The discovery will have consequences for the many museums that use led-bulbs in lights above their paintings.