A spray of vivid yellow flowers painted by Vincent Van Gogh have taken on a grayish-orange hue as a result of a chemical effect.

Researchers have discovered through X-rays that a thin layer of varnish causes a chemical change that deteriorates the cadmium yellow used by Van Gogh.

Microscopic samples of his "Flowers in a Blue Vase" showed that varnish added after Van Gogh's death contained cadmium oxalate which altered the artist's intended color over time.

Van Gogh did not like to use varnish, said University of Antwerp's Koen Janssens to the BBC.