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How varnish dimmed Van Gogh's yellow

The colours Dutch master Vincent Van Gogh used in his work were brilliant. But in 'Flowers in a blue vase', which he painted in 1887, some of the bouquet's bright yellow blossoms are now orange-grey. Why? A European team of researchers has discovered the answer: varnish. The f...

The colours Dutch master Vincent Van Gogh used in his work were brilliant. But in 'Flowers in a blue vase', which he painted in 1887, some of the bouquet's bright yellow blossoms are now orange-grey. Why? A European team of researchers has discovered the answer: varnish. The finding is presented in the journal Analytical Chemistry. Van Gogh did not usually apply varnish to his works, but varnish was applied to this painting for protection after he died. Led by the University of Antwerp in Belgium, the researchers in Germany, France and the Netherlands used sophisticated X-ray analysis to investigate how the varnish caused the discoloration. 'A conservative treatment in 2009 revealed an unusual grey opaque crust on parts of the painting with cadmium yellow paint,' said Margje Leeuwestein, a paintings conservator from the Netherlands-based Kröller-Müller Museum, which acquired the painting in the early 20th century. The experts said that the cadmium yellow (cadmium sulphide, CdS) used in this painting was a fairly new pigment. Researchers had already discovered that it oxidises with air (to cadmium sulphate, CdSO4) in unvarnished paintings. The outcome is that pigments lose colour and luminosity. 'We identified this process a few years ago, and the observation that instead of a slightly off-white, transparent oxidation layer, the pigments in this painting were covered with a dark, cracked crust intrigued us very much,' explained Dr Koen Janssens, also from Antwerp. 'The removal of the orange-grey crust and discoloured varnish was not possible without affecting the very fragile original cadmium yellow paint on these parts,' Ms Leeuwestein added. The group from the museum used two microscopic paint samples, obtained from the original painting, to determine what happened. Each sample was just a fraction of a millimetre in size and was sent to Dr Janssens for analysis. X-ray beams at the European Synchrotron Radiation Facility (ESRF) in France and Deutsches Elektronen-Synchrotron (DESY) in Germany were used on the samples. Here, the researchers observed the samples' chemical composition and internal structure at the interface between varnish and paint. What they found was a surprise. Crystalline cadmium sulphate compounds were not formed in the oxidation process, something they thought would occur. 'It emerged that the sulphate anions had found a suitable reaction partner in lead ions from the varnish and had formed anglesite,' said Gerald Falkenberg from DESY. Anglesite (PbSO4) is an opaque compound that was found nearly everywhere throughout the varnish. 'The source of the lead probably is a lead-based siccative that had been added to the varnish,' he said. For her part, ESRF scientist Marine Cotte said: 'At the interface between paint and varnish, the cadmium ions together with degradation products from the varnish itself also formed a layer of cadmium oxalate.' Anglesite and the cadmium oxalate (CdC2O4) together are responsible for the opaque, orange-grey crust disfiguring parts of the painting on a macroscopic level, according to the researchers. This latest finding will compel painting conservators in other museums to consider how best to restore Van Gogh paintings. Dr Janssens concluded: 'Once again, we find that paintings by Vincent van Gogh are not static entities for decades and centuries to come. Over a period of 100 years, they can actually be considered a fairly reactive cocktail of chemicals that behaves in unexpected manners.'For more information, please visit:University of Antwerp:http://www.ua.ac.be/main.aspx?c=.ENGLISHEuropean Synchrotron Radiation Facility:http://www.esrf.eu/Analytical Chemistry:http://pubs.acs.org/journal/ancham

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Belgium, Germany, France, Netherlands