A complex chemical reaction is to blame for the discolouration of yellows on canvases painted by Vincent van Gogh and James Ensor.

Research, published in Analytical Chemistry, by an international team at Antwerp University in Belgium shows that the chromium used in yellow paints at the end of the 19th century is responsible for the change.

The researchers artificially aged paint from tubes of the period, owned by the painter Rik Wouters, using techniques including ultraviolet rays.

They also extracted tiny bits of paint from two paintings by Van Gogh, Field with flowers close to Arles (1888) and Bank of the Seine (1887), both in the Van Gogh Museum in Amsterdam.

The specimens from Van Gogh and the tubes from Wouters were then examined at the European Synchrotron Radiation Facility research institute in Grenoble. One specimen changed “from bright yellow to chocolate brown in three weeks,” according to one researcher.

The discovery is the first step in finding a way to halt or slow the discolouration in some of Van Gogh’s works.