Not Fade Away

X-rays show why van Gogh paintings lose their shine

...Scientists have identified a complex chemical reaction responsible for the degradation of two paintings by Vincent van Gogh and other artists of the late 19th century. This discovery is a first step towards understanding how to stop the bright yellow colours of van Gogh’s most famous paintings from becoming covered by a brown shade, and fading over time. In the meantime, the results suggest shielding affected paintings as much as possible from UV and sunlight...

Changes in Van Gogh "Bank of the River Seine" though time

...The fact that yellow chrome paint darkens under sunlight has been known since the early 19th Century. However, not all period paintings are affected, nor does it always happen at the same speed. As chrome yellow is toxic, artists quickly switched to new alternatives in the 1950s. However, Vincent van Gogh did not have this choice, and to preserve his work and that of many contemporaries, interest in the darkening of chrome yellow is now rising again...

...To solve a chemical puzzle nearly 200 years old, the team around Janssens used a two-step approach: first, they collected samples from three left-over historic paint tubes. After these samples had been artificially aged for 500 hours using an UV-lamp, only one sample, from a paint tube belonging to the Flemish Fauvist Rik Wouters (1882-1913), showed significant darkening. Within 3 weeks, its surface of originally bright yellow had become chocolate brown. This sample was taken as the best candidate for having undergone the fatal chemical reaction, and sophisticated X-ray analysis identified the darkening of the top layer as linked to a reduction of the chromium in the chrome yellow from Cr(VI) to Cr(III). The scientists also reproduced Wouters’ chrome yellow paint and found that the darkening effect could be provoked by UV light.

Because the affected areas in these multicoloured samples were even more difficult to locate than in the artificially aged ones, an impressive array of analytical tools had to be deployed which required the samples travelling to laboratories across Europe. The results indicate that the reduction reaction from Cr(VI) to Cr(III) is likely to also have taken place in the two van Gogh paintings.

The microscopic X-ray beam also showed that Cr(III) was especially prominent in the presence of chemical compounds which contained barium and sulphur. Based on this observation, the scientists speculate that van Gogh’s technique of blending white and yellow paint might be the cause of the darkening of his yellow paint.'...

Cool stuff.  I've done a little work with photochemistry of chromium  in the distant past, but nothing this sophisticated.  Chromate, the oxidized form of chromium, commonly with red, orange or yellow colors, is a tremendously strong oxidizing agent.  It would like to oxidize any organic molecule available, but often the kinetics (the speed) of the reaction is very slow.  Light, particularly UV light, can often provide the "kick" required to get over the kinetic hump.  The reduced form of Cr, chromite, is usually green or blue when pure.