Sometimes we wonder if modern science is taking the fun out of everything artsy. Mystery upon mystery is unravelled on shows like Mythbusters. Theory after theory about Mona Lisa’s smile and the origin of the subject nearly overwhelm the beauty of it (it was a merchant’s wife! No, it was Da Vinci himself, when younger! No, it was Da Vinci’s favourite assistant and alleged lover! No, you’ve got it all wrong, it’s Jesus!). The theories seem endless and many seem pointless. But the modern scientific aspect of chemicals and how they react hold more practical relevance for oil painting appreciators and practitioners. And the news about some Van Gogh paintings is a little scary.

The pigment in question is a luminous yellow pigment often used by van Gogh in his most famous paintings. The concern was why the yellow pigment turned brown in some paintings, but not in others. A February 2011 study of chrome yellow, a toxic industrial pigment used by many artists in the late nineteenth century, finally yielded the answers. It revealed that the chrome atoms at the heart of the chemical structure of the pigment can be modified, leading to a discoloration from bright yellow to flat brown. This modification occurs when the pigment comes in contact with sun and UV rays, a previously unknown chemical reaction.

Scientists believed this could explain why some Van Gogh paintings, such as the celebrated Sunflowers series, were significantly less bright and much more brown than when they were painted and featured in photographs from just several years ago. What puzzled museums and critics was that only some Van Gogh paintings featuring the yellow changed over time. Professor Koen Janssens at the University of Antwerp in Brussels revealed why: the chrome ions in the pigment underwent inverse oxidisation in the presence of sunlight. In nearly highschool textbook chemistry, he noted that the chemical reduction occurred when the electrons were taken from the oil in oil paint and transferred to the chrome. However, this transfer seemed to only take place when a second white pigment, Barium Sulphate, was also present. The scientists used the ESRF (European Synchroton Radiation Facility) in Grenoble to x-ray analyse sample chrome yellow pigments from old and abandoned Van Gogh paintings tubes. They used the same method to examine the chemical changes in tiny fragments of paint from two Van Gogh paintings, Les bords de Seines (1887) and Vue d’Arles avec Iris (1888). What they found, at amplification of one hundred times smaller than a human hair, showed that the chemical reduction of the chrome took place in a very thin layer between the surface of the paint and the varnish.

The result of this chemical reduction meant damage could also appear more rapidly at higher temperatures. The chemical analysis team recommended that all Van Gogh paintings, or any paintings utilising the same chemical makeup regardless of colour, be stored in a part of the museum that was cool and dark. Common sense, it would seem, but prudent advice nevertheless.

Many Van Gogh paintings feature a luminance of colour specifically chosen by Van Gogh to express spiritual mind states and certain emotions. Van Gogh was very fond of yellow, so it is sad to see so many of his oil paintings turn a bit brown. Rest assured that with proper care, your museum quality oil reproduction paintings will never turn brown. We only use professional grade Winsor & Newton paints that mean your oil paintings will look as good from day 1 as they do at day 10,000.