Washington: Scientists have discovered why some paintings of Vincent van Gogh and other artists of the late 19th century are losing their shine.
Using sophisticated X-rays, they have identified a complex chemical reaction responsible for the degradation of the works.
They found that the Dutch master’s decision to use yellow paint mixed with white is responsible for the unintended darkening.
The finding is a first step to understanding how to stop the vibrant yellow colours of van Gogh’s most famous paintings from being covered by a brown shade, and fading over time.
The results suggested shielding the affected paintings as much as possible from UV and sunlight.
The research was carried out by an international team of scientists, led by Koen Janssens of Antwerp University in Belgium. Letizia Monico, an Italian chemist, headed the experiments. Scientists from Italy, France and the Netherlands were also part of the team.
Uncovering the secrets of the chemical reaction required the scientists to use an array of analytical tools, with synchrotron X-rays at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, providing the final answers.
Sunlight can penetrate only a few micrometers 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.
The vibrancy of new industrial pigments such as chrome yellow allowed van Gogh to achieve the intensity of, for example, his series of Sunflowers paintings.
He started to paint in these bright colours after leaving his native Holland for France where he became friends with artists who shared his new ideas about the use of colours.
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.
To solve a chemical puzzle nearly 200 years old, the team 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 chromium 6 to chromium 3.
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), both on display in the Van Gogh Museum in Amsterdam.
The X-ray beam research carried out at ESRF indicated that the reduction reaction from chromium 6 to chromium 3 is likely to also have taken place in the two paintings.
It also showed that chromium 3 was especially prominent in the presence of chemical compounds that 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,” said Janssens.