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.

BBC: Van Gogh's Flowers In A Blue Vase damage seen in X-rays

Although he cannot definitely rule out a direct effect by the pollution on the birds' metabolism of the yellow pigment, or its incorporation into their feathers, the correlation between a shortage of caterpillars and faded plumage is strong evidence of a link between pollution, diet and bird colour.

ECONOMIST: Great tits

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.

BBC: Van Gogh's Flowers In A Blue Vase damage seen in X-rays