Van Gogh used chromium yellow pigment in works such as Sunflowers, and recent tests have revealed that it fades from yellow to brown when exposed to ultraviolet light.
Credit: Wikimedia
WASHINGTON: A previously unknown chemical reaction that has caused the once-vivid yellows in Vincent van Gogh's paintings to turn brown has been identified by an international team of scientists.
A super-sensitive microscopic X-ray has revealed a chemical reaction that takes place where the paint meets the varnish, and is triggered by sunlight to cause yellow to fade, said the findings in the journal Analytical Chemistry.
"This type of cutting edge research is crucial to advance our understanding of how paintings age and should be conserved for future generations," said Ella Hendriks of the Van Gogh Museum Amsterdam.
Discovering why discolouration was inconsistent
The chromium yellow pigment used by Van Gogh in works such as Sunflowers was known to take on a brown discolouration under the influence of sunlight. However the mechanism that had caused the paintings to experience differing degrees of discolouration remained a mystery.
In order to investigate, scientists took a microscopic piece out of two paintings, Banks Of The River Seine (1887) and View Of Arles With Irises (1888), as well as paint samples taken from paint tubes used by early 20th century Flemish artist, Rik Wouters, that had been exposed to UV light.
Using the X-ray from the European Synchrotron Radiation Facility in Grenoble, France, the researchers showed that a reduction in chromium "was especially prominent in the presence of chemical compounds which contained barium and sulphur".
New perspectives for conservators
The study found that the chromium ions in the pigment went through a ‘reverse oxidation’ in the presence of sunlight. The reduction occurs when electrons are taken from the paint’s oil base and passed on to the chromium only when a second, white pigment – barium sulphate – is also present.
That observation led the scientists to believe that "Van Gogh's technique of blending white and yellow paint might be the cause of the darkening of his yellow paint”, according to the paper. The best way to avoid such deterioration is to shield vulnerable artwork from ultraviolet rays and sunlight, the authors said.
"Our X-ray beam is 100 times thinner than a human hair, and it reveals subtle chemical processes over equally minuscule areas," said Marine Cotte, a scientist with the research institute in Grenoble. "Making this possible has opened the door to a whole new world of discovery for art historians and conservators."
Cool, dark conditions needed
According to the researchers, the reaction can still occur in light even where the harmful ultraviolet component has been filtered out, and it may also occur faster at higher temperatures, suggesting that museums should keep paintings in darker, cooler conditions.
The research was 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.
Van Gogh, known for his bold, tempestuous brushwork and having cut off his own ear, died at Auvers-sur-Oise near Paris in July 1890 after shooting himself in the chest.
