X-Rays show why van Gogh paintings lose their shine

14 February 2011

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 to understanding how to stop the bright yellow colours of van Gogh’s most famous paintings from being 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. The results are published online on 14 February 2011 in the journal Analytical Chemistry.

It was the vibrancy of new industrial pigments such as chrome yellow which allowed van
Gogh to achieve the intensity of, for example, his series of Sunflowers paintings.
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 comtemporaries, interest in the darkening of chrome yellow is now rising again.

Crime scene investigation
Uncovering the secrets of the chemical reaction needed deployment of an impressive arsenal of analytical tools, with synchrotron X-rays at the ESRF in Grenoble (France) providing the final answers. The experiment reads like a crime scene investigation. The scientists employed an X-ray beam of microscopic dimensions to reveal a complex chemical reaction taking place in the incredibly thin layer where the paint meets the varnish.

 
Chemical puzzle
To solve a chemical puzzle nearly 200 years old, the scientists 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.
In the second step, the scientists used the same methods to examine samples from affected
areas of two van Gogh paintings, View of Arles with Irises (1888) and Bank of the Seine
(1887).

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

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.

“Our next experiments are already in the pipeline. Obviously, we want to understand which
conditions favour the reduction of chromium, and whether there is any hope to revert
pigments to the original state in paintings where it is already taking place.”, summarises
Koen Janssens from University of Antwerp.

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