The colour of Vincent van Gogh's famous Sunflowers is changing over time, because of the mixture of pigments used by the Dutch master in his painting, a new study has found.
The study suggests that the Sunflowers painting may originally have looked different from what we see today, researchers said.
Evidence for the process comes from a detailed spectroscopic investigation of the Sunflowers version at the Van Gogh Museum in Amsterdam.
A group of scientists headed by Letizia Monico from the Institute of Molecular Science and Technology (CNR-ISTM) of Perugia, the University of Perugia and the University of Antwerp shone X-rays through tiny particles of paint taken from the painting.
The study also identified areas of the painting that should be monitored particularly closely for any changes.
Vincent van Gogh (1853-1890) is famous for his use of bright yellow colours. The Dutch painter used so-called chrome yellows, a class of compounds consisting of lead, chromium and oxygen.
"There are different shades of the pigment, and not all of them are photochemically stable over time," said Monico.
"Lighter chrome yellow has sulphur mixed into it, and is susceptible to chemical degradation when exposed to light, which leads to a darkening of the pigment," said Monico.
The scientists examined a Sunflowers painting, which dates back to 1889, to determine whether van Gogh had used different types of chrome yellow when painting it.
He produced three versions of the painting, one of which is on display at the National Gallery in London, one at the Seji Togo Memorial Sompo Japan Nipponkoa Museum of Art in Tokyo and one at the Van Gogh Museum in Amsterdam.
Two small paint samples, measuring less than 1 millimetre each, were taken from the painting in Amsterdam and examined using X-ray source from Deutsches Elektronen-Synchrotron (DESY)'s lightsource PETRA III.
"The analysis shows that the orange-yellow hues mainly contain the lightfast version of chrome yellow, whereas the light-sensitive type is mainly found in the pale yellow areas," said co-author Gerald Falkenberg, who is in charge of DESY's beamline P06, where the X-ray diffraction measurements were carried out.
At the European Synchrotron Radiation Facility (ESRF) in Grenoble, the team examined the chemical state of the paint samples. When light sensitive chrome yellow darkens, the chromium is reduced from its highest oxidation state CrVI to CrIII.
The scientists were indeed able to detect a relative proportion of 35 per cent CrIII on the surface of the paint.
"At least at the two sites from which the paint samples were taken, a colour change has occurred in the Sunflowers as a result of the reduction of chrome yellow," said Monico.
The study was published in the journal Angewandte Chemie.
