International scientists have discovered a chemical reaction that has caused the once-vivid yellows in Vincent van Gogh’s paintings to turn brown, according to a study published in the US.

A super-sensitive microscopic X-ray has revealed a chemical reaction taking place where the paint meets the varnish, triggered by sunlight which causes yellow to fade, said the findings in the journal Analytical Chemistry.

“This type of cutting edge research is crucial to advance our understanding of how paintings age and should be conserved for future generations,” said Ella Hendriks of the Van Gogh Museum Amsterdam.

The X-ray from the European Synchrotron Radiation Facility in Grenoble, France also showed a reduction in chromium “was especially prominent in the presence of chemical compounds which contained barium and sulphur”.

That observation led scientists to believe that “Van Gogh’s technique of blending white and yellow paint might be the cause of the darkening of his yellow paint”, the study said.