Van Gogh Painting Discoloration Blamed On Varnish
September 15, 2012

Chemical Cause Of Van Gogh Painting Color Change Discovered

redOrbit Staff & Wire Reports - Your Universe Online

A reaction between paint and a protective varnish applied following the artist's death is to blame for a color change in the Van Gogh painting "Flowers in a Blue Vase," claims a new study in the journal Analytical Chemistry.

In a discovery that the Daily Mail's Mark Prigg says "could have major implications for artistic conservation", researchers used x-ray scanners at the European Synchrotron Radiation Facility (ESRF) and Deutsches Elektronen-Synchrotron (DESY) to discover why bright yellow flowers in the painting have turned orange-grey over time.

The painting was completed, using cadmium yellow paint (a relatively new pigment at the time) by Van Gogh in 1887, and in the early 1900s, it was acquired by the Kröller-Müller Museum, Prigg explained. While the painter did not varnish his works personally, a protective varnish was later discovered on "Flowers in a Blue Vase" -- a coating that was likely applied following the master's death.

"A conservation treatment in 2009 revealed an unusual grey opaque crust on parts of the painting with cadmium yellow paint," Kröller-Müller Museum paintings conservator Margje Leeuwestein told the Daily Mail on Friday. "The removal of the orange-grey crust and discolored varnish was not possible without affecting the very fragile original cadmium yellow paint on these parts. The research into this hitherto unknown degradation process of varnished cadmium yellow oil paint allows to better understand the current appearance of the painting."

Lead researcher Koen Janssens of the University of Antwerp discovered a few years ago that the cadmium yellow paint used by Van Gogh oxidizes with air in unvarnished paintings, resulting in the loss of color and luminosity.

"To identify what had happened, the museum took two microscopic paint samples -- each only a fraction of a millimeter in size -- from the original painting and sent them to Janssens for a detailed investigation," a September 14 ESRF press release explained. "The scientists studied the samples using powerful X-ray beams at the ESRF and at DESY's PETRA III, revealing their chemical composition and internal structure at the interface between varnish and paint. To their surprise, they did not find the crystalline cadmium sulphate compounds that should have formed in the oxidation process."

They discovered that the sulphate anions reacted with the lead ions to form an opaque compound known as anglesite, which was noticeable throughout the varnish. The source of the lead, DESY scientist Gerald Falkenberg explained, was likely a lead-based substance added to the mixture in order to promote drying. Likewise, cadmium ions mixed with substances produced by the breaking down of the varnish to form a layer of cadmium oxalate, and together the two substances are being blamed for the painting's orange-grey crust.

"The research into this hitherto unknown degradation process of varnished cadmium yellow oil paint allows to better understand the current appearance of the painting," Leeuwestein said.

Likewise, Ella Hendriks, Head of Conservation at Amsterdam's Van Gogh Museum, who was not involved in the work, called the research "an excellent example of how collaboration between scientists and conservators can help to improve our understanding of the condition of Van Gogh's paintings and lead to better preservation of his works“¦ Many of Van Gogh's French period paintings have been inappropriately varnished in the past and removal of these non-original varnish layers is one of the challenges facing conservators on a world-wide basis today."