A type of bright yellow pigment used by Vincent van Gogh in some of his most famous paintings turns brown in the presence of sunlight, because of a previously unknown chemical reaction, a study has found. Powerful X-ray images of chrome yellow, a toxic industrial pigment used by many artists at the end of the 19th century, revealed that the chromium atoms at the heart of the pigment's complex chemical structure can be changed in a way that causes the colour to turn from bright yellow to dull brown.
Scientists believe the discovery may explain why some of van Gogh's most famous works, including his Sunflowers series, no longer appear as they were intended to look when the Dutch post-Impressionist composed them more than 100 years ago, during a period when he deliberately chose bright colours to convey mood and emotion.
Even van Gogh realised that the chrome yellow he used in his paintings was subject to discoloration, but what has confused art museums is the fact that not all paintings made with chrome yellow seem to be affected. But this latest study may explain why, according to Professor Koen Janssens of Antwerp University in Belgium.
"Van Gogh intended to make his paintings more yellow, but nature is making them more brown. If you want to contemplate turning them back to their original state, you first need to understand why they are turning brown," Professor Janssens said. The study found that the chromium ions in the pigment went through a "reverse oxidation" in the presence of sunlight.
The chemical reduction occurs when electrons are taken from the oil base used in paint oil and passed on to the chromium, but the transfer seems to take place only when a second, white pigment – barium sulphate – is also present, Professor Janssens said.
"Everything tends to oxidise as its gets older, but here it is not oxidation but reduction that is the problem. Normally when you have oxidation something gets burned up and oxygen takes the electrons.
"What is happening here is that the chromium is taking the electrons from the oil," he added. The reaction, however, needs the white pigment used by van Gogh to make the chromium yellow lighter.
The scientists used the Synchrotron Radiation Facility in Grenoble, France, which generates powerful X-rays, to analyse samples of chromium yellow pigment taken from three historic paint tubes left over from the time. They also used the same method to investigate the chemical changes to tiny fragments of paint that came from two van Gogh paintings, Banks Of The River Seine (1887) and View Of Arles With Irises (1888), kept at the Van Gogh Museum in Amsterdam. Microscopic beams of X-rays, 100 times thinner than a human hair, revealed that the chemical reduction of chromium occurs in an exceedingly thin layer between the paint's surface and the varnish.
Professor Janssens said that the reaction can still occur in light, where the harmful ultraviolet component has been filtered out. It may also occur faster at higher temperatures, suggesting that museums should keep paintings in darker, cooler conditions. Ella Kendriks, of the Van Gogh Museum, said: "This type of cutting-edge research is crucial to advance our understanding of how paintings age and should be conserved for future generations."
The study is published in the latest issue of Analytical Chemistry.