Van Gogh is famous for his daring use of bright color and the emotional impact of his paintings. But some of his brighter work has lost its sheen, with yellows becoming more of a dull, dirty brown.

The pigment van Gogh used, called chrome yellow, has been known to be unstable since the 19th century, but it took X-ray analysis to be able to identify the problem of fading yellows, because the reaction takes place only in a painting's top micrometer (1,000th of a millimeter).

Research published in the journal Analytical Chemistry showed the chromate in the paint used in works such as "The Banks of the River Seine" loses an oxygen molecule, and becomes green, creating a brownish hue when mixed with the yellow. But with this kind of reaction, a reduction reaction, there always is always an accompanying oxidation reaction. The scientists are still mystified as to where the fugitive oxygen molecule has gone.

"If anything around is likely to get oxidized by the chromate it's the oil," said Koen Janssens, the research leader and a chemistry professor at the University of Antwerp. "We're looking further to see clues."

You get what you pay for

One clue the researchers are examining is the presence of lead sulphate and barium sulphate. Their work suggests that the chrome yellow only reduces with oxygen when one of those materials is also present.

These sulphates were cheap chemicals added to paints to lighten the color and to save artists money. It makes yellow more straw-colored rather than the intense orange of pure chrome yellow.

But with the lower price tag of the pigment, came the unforeseen chemical reaction diminishing the longevity of the color.

"What is happening is the opposite of what van Gogh intended," Janssens said. "It's an inverse situation - as if nature doesn't want to make it lighter."

Janssens' team collected samples from three left-over historic paint tubes and artificially aged them 500 years using an UV lamp. Of the three samples, only one, from a paint tube belonging to the Flemish Fauvist Rik Wouters, showed significant darkening. After examining this sample with X-ray analysis, the research team found that UV light could produce the darkening effect found in van Gogh's work.

But even after clearing up the chemistry, the Janssens said he's wary of reversing the reaction with more chemistry. While he said it could be done in a laboratory, he has doubts about doing experiments on a masterpiece.

"It's a bit of a scary operation, a bit like burning," he said. "All kinds of other things might oxidize. Collateral damage has not been investigated."

Ongoing chemical reaction

The other possibility would be scraping off the micrometer where the reaction has taken place, but that runs against modern curation practices.

"Current ethics mean that you would not remove any original material even if it's deteriorated," said Ella Hendriks, head curator at the van Gogh Museum in the Netherlands, "You might over-paint certain areas to return them to the way they looked before the discoloration took place, but we don't know how the painting looked before the process started so that has other ethical problems."

The best hope is to slow down the process as much as possible, scientists and curators have said.

"It's a very, very thin layer that has formed," Janssens said. "We cannot just say it's happened, it's irreversible. I call this slow chemistry. It's still going on. I'm not pessimistic that by making the reaction slower we can help preserve the paintings."

He said he has a hunch that cooling the paintings by 5 to 10 degrees Celsius may help, but he has not confirmed this yet. But Hendriks sounded a note of caution, pointing out that cooling the paintings could affect them in other ways.

"Chrome yellow is just one example of the many fugitive pigments we have to deal with," she said.

The paintings are already exhibited in micro-climate cases which filter out the UV light and keep the temperature and the humidity stable. The van Gogh Museum is working with the scientists to find out if there is more to be done, and Hendriks said the discovery is useful when it comes to cleaning and preserving the paintings, too.

"It's very useful to know that the brown surface of the paint is an effect of natural ageing and it's not necessarily due to residues of old dirt and varnish left in the paint surface," she said. "It makes you look in an increasingly informed way when you're judging and treating paintings."

Author: Molly Guinness
Editor: Sean Sinico