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Paris, 14 septembre 2012

X-rays elucidate color change in van Gogh painting

Why have parts of van Gogh's painting 'Flowers in a Blue Vase', painted in 1887, changed color over time? Scientists from CNRS, ESRF (European Synchrotron Radiation Facility), the University of Antwerp (Belgium), Delft University of Technology (TU Delft, Netherlands), the Kröller-Müller Museum in Otterlo (Netherlands) and DESY (Deutsches Elektronen SYnchrotron) in Hamburg (Germany) have found the answer. The degradation process, at the interface between paint and varnish, was unknown until now and was identified by using sophisticated techniques involving X-rays in particular. Applied after the painter's death, this supposedly protective varnish turned the bright yellow flowers to an orange-grey color. These results have been published on the website of the journal Analytical Chemistry.

It was in the first half of the twentieth century that the painting, 'Flowers in a Blue Vase', which had been acquired by the Kröller-Müller Museum in Otterlo (Netherlands), was covered with a supposedly protective varnish, like many other paintings by van Gogh, who did not usually varnish his works. Conservation treatment in 2009, revealed the presence of a highly unusual opaque grey crust on the parts of the picture that had been painted with cadmium yellow.
The cadmium yellow (cadmium sulfide, CdS) used by van Gogh was a relatively new pigment, which is now known to oxidize to cadmium sulfate (CdSO4) in air, losing color and brightness. However, the pigments in this painting were covered with an unusual dark, cracked crust instead of the expected transparent oxidized layer.


To identify the degradation process, the museum took two microscopic paint samples from the original artwork, each of which was just a fraction of a millimeter. The scientists studied them with extremely powerful X-ray and infrared beams at ESRF and DESY, which enabled them to analyze their chemical composition and internal structure at the paint-varnish interface. Much to their surprise, they did not find the crystalline cadmium sulfate compounds that should have formed during the oxidation process. Instead, they found anglesite (PbSO4), an opaque compound which was found practically everywhere in the varnish, formed by the reaction of sulfate-type anions with lead ions that probably came from a dryer, or siccative, added to the varnish. At the varnish-paint interface, cadmium ions (coming from the pigment cadmium yellow) combined with degradation products from the varnish itself, forming a layer of cadmium oxalate. It is the cadmium oxalate (CdC2O4), together with the anglesite, that accounts for the opaque, orange-grey crust disfiguring certain parts of the painting at the macroscopic scale.


This work sheds light on the effect that certain varnishes applied subsequently can have on the pigments of the painting and on its current appearance. Many of van Gogh's paintings from the French period were inappropriately varnished, and removing the layers of non-original varnish is bound to have an effect on the painting's original material. This work should help curators to make choices with regard to such complex cleaning treatment. The scientists will now study the effect of conservation conditions inside the museum as well as that of air pollution on cadmium yellow and other sulfide-based pigments used by painters, which affect the life span of paintings. 

References:

Combined use of synchrotron radiation-based μ-XRF, μ-XRD, μ-XANES and μ-FTIR reveals an alternative degradation pathway of the pigment cadmium yellow (CdS) in a painting by Van Gogh. Geert Van der Snickt, Koen H. Janssens, Joris Dik, Wout De Nolf, Frederik Vanmeert, Jacub Jaroszewicz, Marine Cotte, Gerald Falkenberg, and Luuk Van der Loeff Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac3015627 • Publication Date (Web): 30 Aug 2012. Downloaded from View web site on August 31, 2012

Contact information:

CNRS researcher at ESRF l Marine Cotte l T 04 76 88 21 27 l cotte@esrf.fr

CNRS Press Office l Laetitia Louis l T 01 44 96 51 37 l laetitia.louis@cnrs-dir.fr

ESRF Press Office l Claus Habfast l T 04 76 88 21 28/06 66 66 23 84 l claus.habfast@esrf.fr


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