Scientists have identified why the bright yellows in some of Vincent van Gogh's paintings have turned brown. A complex chemical reaction is behind the deterioration of the works. The finding is a first step to understanding how to stop some of the Dutch master's most famous paintings from fading over time. The results, published in the journal Analytical Chemistry, suggest shielding the affected paintings as much as possible from UV and sunlight. Sunlight can penetrate only a few micrometres into the paint, but over this short distance, the researchers found it could trigger a hitherto unknown chemical reaction turning chrome yellow into brown pigments, altering the original composition. The scientists employed a microscopic X-ray beam to reveal a complex chemical reaction taking place in the incredibly thin layer where the paint meets the varnish.