With a sophisticated X-ray analysis scientists have identified why parts of the Van Gogh painting “Flowers in a blue vase” have changed colour over time: a supposedly protective varnish applied after the master’s death has made some bright yellow flowers turn to an orange-grey colour. The origin of this alteration is a hitherto unknown degradation process at the interface between paint and varnish, which studies at the European Synchrotron Radiation Facility ESRF in Grenoble (France) and at Deutsches Elektronen-Synchrotron DESY in Hamburg (Germany) have revealed for the first time.
The results are published in an upcoming issue of Analytical Chemistry, the first author of which is Geert Van der Snickt, who received a PhD in Conservation and Restauration from the University of Antwerp (Belgium) for this work. The research team was led by Koen Janssens from Antwerp and also comprised scientists from TU Delft (Netherlands), the French CNRS, the Kroller-Müller Museum in Otterlo (Netherlands), the ESRF and DESY.
The cadmium yellow (cadmium sulphide, CdS) used by Van Gogh was a relatively new pigment, of which it has recently been discovered that in unvarnished paintings, it oxidizes with air (to cadmium sulphate; CdSO4) making the 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,” says Janssens. “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,” adds Leeuwestein.
To identify what had happened, the museum took two microscopic paint samples – each only a fraction of a millimetre in size – from the original painting and sent them to Janssens for a detailed investigation. 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. “It emerged that the sulphate anions had found a suitable reaction partner in lead ions from the varnish and had formed anglesite,” explains DESY scientist Gerald Falkenberg. 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,” adds Falkenberg.