Researchers have discovered that Van Gogh's famous yellow sunflowers are beginning to lose their shine -- actually, they're turning brown.

A chemical change in one of the yellow pigments favoured by Vincent van Gogh is said to be slowly changing the look of his images.

It's been know for some time that the bright yellows which once featured in van Gogh paintings, are slowly fading and being covered by a brown shade, but no-one knew why.

But art boffins have now used sophisticated X-ray techniques to identify the problem, a "reduction" reaction that alters the chromium in the paint.