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 have revealed for the first time.
Vincent Van Gogh (1853-1890) painted “Flowers in a blue vase” in 1887 in Paris, and in the early 20th century, the painting was acquired by the Kröller-Müller Museum. The master usually did not varnish his works, but this painting was later covered with a supposedly protective varnish, like many other Van Gogh paintings in the first half of the 20th century. "A conservation treatment in 2009 revealed an unusual grey opaque crust on parts of the painting with cadmium yellow paint," says paintings conservator Margje Leeuwestein from the Kröller-Müller Museum.
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 oxidises with air (to cadmium sulphate; CdSO4) making the pigments lose colour and luminosity.
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.
“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,” says ESRF scientist Marine Cotte. Together with the anglesite, the cadmium oxalate (CdC2O4) accounts for the opaque, orange-grey crust disfiguring parts of the painting on a macroscopic level.
After this discovery, conservators in many museums will have to newly address the question of restoring Van Gogh paintings. "This study on the deterioration of cadmium yellow is 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," says Ella Hendriks, Head of Conservation of the Van Gogh Museum in Amsterdam.
In the next four years, the research group plans to study how museum indoor conditions and air pollution affect cadmium yellow and related sulphide-containing pigments used by artists.
This story is reprinted from material from ESRF, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.