New Chemical Research Uncovers Why Van Gogh Paintings Are Turning Brown
The works of painter Vincent van Gogh are some of the world's most prized pieces of art, partly for their vibrant tones, but some of those colors have been fading in recent years. Art researchers and scientists have been using X-ray microscopes to examine van Gogh paintings that have been losing their color and determine why once brilliant yellows have been turning brown in several key works of van Gogh and other artists of the late 19th century.
The results of the study were published in the journal Analytical Chemistry. The problem comes down to the chromium in a yellow pigment called yellow chrome which undergoes a chemical reaction when exposed to ultraviolet lighting (including sunlight), turning the painting brown. The paint, which gave works including van Gogh's Sunflowers paintings, their sun-splashed beauty has been known to darken under sunlight since the early 19th century. What's new is that now scientists have uncovered why this happens and can work to gain potential clues as to how to prevent it.
The use of synchrotron X-rays at the ESRF in Grenoble (France) helped to unravel the chemical mystery. An X-ray beam of microscopic dimensions allowed scientists to observe a complex chemical reaction taking place in the incredibly thin layer where the paint meets the varnish. The video above shows an animation of the process.
The research team, which included Koen Janssens of Antwerp University in Belgium and Letizia Monico of Perugia University in Italy, discovered that the change was caused when the chromium in the yellow paint tranformed its chemical makeup from chromium (VI) to chromium (III), changing the color of the pigment. Sunlight only penetrates a few micrometers into the paint but it is enough to trigger this chemical reaction.
Pigments were far from standardized in van Gogh's era and paints used varied both from painter to painter and time to time. Researchers had to track down historic tubes of chromium yellow in order to conduct their tests. Chromium yellow is toxic and no longer used. Artists switched from using the paint in the 1950s. The researchers were able to find three tubes of the yellow paint and hastened the aging process by exposing it to 500 hours under a UV lamp. They found that only one of the paint samples turned brown, one belonging to Flemish artist Rik Wouters. The color change was similar to that seen in the Van Gogh painting. During the course of the hurried-up aging process the color became darker and darker. Within three weeks, the bright yellow had become chocolate brown.
Then scientists used the same methods to look at samples from affected areas of two van Gogh paintings, View of Arles with Irises (1888) and Bank of the Seine (1887), shown above, both on display in the Van Gogh Museum in Amsterdam. This part of the experiment was a little more challenging because these samples are multicolored and more difficult to isolate than the artificially aged ones. A variety of analytical tools in various labs had to be used, requiring the samples to travel around Europe. The end result proved that the reduction reaction from Cr(VI) to Cr(III) is also likely to have taken place in the two van Gogh paintings.
This research also uncovered the detail that showed that the Cr(III) was especially prominent in the presence of chemical compounds which contained barium and sulphur leading scientists to speculate that the artist's technique of blending white and yellow paint might be the cause of the darkening of his yellow paint. Barium sulphate is often a component of white pigment for paints.
More experiments are in the works. Koen Janssens from University of Antwerp indicate that future projects include determining the conditions that favor chromium reduction and whether or not it might be possible to reverse the process and "revert pigments to the original state in paintings where it is already taking place."
But for now, the result of this experiment is a key link into understanding how to better care for aging artwork. "For every Italian, conservation of masterpieces has always mattered. I am pleased that science has now added a piece to a puzzle that is a big headache for so many museums" said Letizia Monico from University of Perugia.