Image caption Microscopic samples of the work were carefully extracted in two places

Researchers have spotted a never-before-seen chemical effect in Vincent Van Gogh's Flowers In A Blue Vase that is dulling the work's vibrant yellows.

It seems a layer of varnish added later to protect the work is in fact turning the yellow to a greyish-orange colour.

High-intensity X-ray studies described in Analytical Chemistry found compounds called oxalates were responsible.

But atoms from the original paint were also found in the varnish, which may therefore be left in place.

It is not the first time that the bright yellows that Van Gogh preferred have been examined with X-rays.

In 2011, an article in the same journal from a team led by the University of Antwerp's Koen Janssens reported that a pigment Van Gogh favoured called chrome yellow degraded when other, chromium-containing pigments were present.

The new work was begun during a conservation treatment in 2009, when conservators found that the yellows in Flowers In A Blue Vase - this time from a pigment called cadmium yellow - had turned greyish and cracked.

Normally, cadmium yellow grows paler and less vibrant as it ages.

So the team again took tiny samples of the work to some of Europe's largest sources of X-rays: the ESRF in France and Desy in Germany. Both use vast particle accelerators to speed up electrons, which spray out X-rays as they pass around the accelerators.

The purpose was to determine not only what was in the samples in terms of atoms and molecules, but also the precise structures in the interface layer between the original paint and the varnish.

That is where the team was shocked to find a compound called cadmium oxalate as the cause of the grey-orange pallor.

Image caption The cadmium oxalate was a surprise find in the layer between paint and varnish

"The contact layer between the varnish and the paint, where the cadmium oxalate is found, is micrometer thin," Dr Janssens told BBC News.

"If we had not used methods that allow us to interrogate this very thin layer, we would never have noticed that there were oxalates there."

Oxalates are commonly found in much older works, and in association with different pigments. This is the first time that cadmium has been seen to form oxalates within the varnish - a protective measure that was added much later.

"Van Gogh didn't like to varnish his paintings - he liked them, let's say, rough," Dr Janssens said.

"It was only after he died that these paintings found their way into the art market and into private and public collections and individual conservators would say 'we're going to varnish it because we do that with all our paintings'."

Image caption The samples extracted from the work and placed in the X-ray beams were almost too small to see

That some of the Van Gogh's paint has been drawn into the varnish creates a troubling problem for conservators, who of course want to prevent any further degradation but are duty-bound not to remove any original material.

The particular chemical reaction may be putting the yellows of other works at risk by Van Gogh and others, but it does not happen with every type of varnish.

For now, perhaps the world's best collection of Van Gogh's work - at the Van Gogh Museum in the Netherlands - seems safe.

"I don't anticipate it will be a wide-scale problem for our particular collection, given its conservation history," said Ella Hendriks, the museum's head of conservation.

"But of course it's always good to be aware of the possibility that you could come across this in other paintings," she told BBC News.

"This type of information for conservators is very valuable because it helps us understand the condition of the paintings and make the right choices about how we can best conserve them."