Why van Gogh's Sunflowers are wilting

Analysis of chrome based colors at DESY

22-Oct-2015 - Germany

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," explains 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." Lightfast chrome yellow has the chemical formula PbCrO₄, whereas the light-sensitive type has the formula PbCr_1-xS_xO₄, (with x exceeding about 0,4).

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 DESY's X-ray source 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," reports 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," says Monico. This suggests that the Sunflowers may originally have looked different from what we see today.

The scientists used a mobile scanner to identify those parts of the painting which ought to be monitored particularly closely for possible changes. "Since chrome yellow pigments were widely used by late 19th-century painters, this study also has broader implications for assessing the colours of other works of art," emphasises co-author Koen Janssens, from the University of Antwerp.

Original publication

Letizia Monico, Koen Janssens, Ella Hendriks, Frederik Vanmeert, Geert Van der Snickt, Marine Cotte, Gerald Falkenberg, Brunetto Giovanni Brunetti, and Costanza Miliani; "Evidence for Degradation of the Chrome Yellows in Van Gogh's Sunflowers: A Study Using Noninvasive In Situ Methods and Synchrotron-Radiation-Based X-ray Techniques"; Angew. Chem. Int. Ed.; 2015

https://www.chemeurope.com/en/news/155165/why-van-gogh-s-sunflowers-are-wilting.html