The bright yellow flowers in Van Gogh’s Flowers in a blue vase have faded to orange-grey. Scientists have now shown that a supposedly protective varnish reacting with the original paintwork is to blame.
When Van Gogh painted this still life in Paris in 1887, he left it unvarnished. With time, the chromium yellow pigment (cadmium sulphide, CdS) reacted with air to produce cadmium sulphate (CdSO4), causing a loss of colour and luminosity. Perhaps in response, a layer of varnish was added in the 20th century.
But this solvent-rich varnish dissolved the cadmium sulphate, causing the resulting cadmium and sulphate ions to disperse through the coating, scientists have found. Now it is reported that other compounds have since formed at the interface of the varnish and the breakdown products of the original pigment (Analytical Chemistry, doi: 10.1021/ac3015627).
‘It emerged that the sulphate anions had found a suitable reaction partner in lead ions from the varnish and had formed anglesite [PbSO4],’ explains co-author Gerald Falkenburg at the German Electron Synchrotron facility. This opaque compound occurs throughout the varnish. Also, at the boundary of art and varnish, cadmium ions and degradation products formed a thin layer of cadmium oxalate (CdC2O4), further obscuring the pigment.
Study co-author Marine Cotte of the European Synchrotron Radiation Facility in Grenoble, France, says this cadmium oxalate discovery ‘was partially a surprise because it was not observed in a previous study of cadmium yellow [in a James Ensor painting].’ On the other hand, ‘oxalates seem to be a quite frequent end product of long-term degradation,’ she notes, adding she had previously observed them on Egyptian mummy skins.
The key to detecting the thin disfiguring layer was the use of powerful X-ray and infrared beams at synchrotron facilities in France and Germany, which combined excellent local resolution, non-invasiveness and specific chemical analysis.
Many of Van Gogh’s French period paintings have been inappropriately varnished and their removal is challenging. ‘Before doing any restoration, it is very important to know the conservation condition of the painting,’ says chemist Costanza Miliani of the CNR Institute for Science and Technology, Italy. ‘The first step is the formation of cadmium sulphate, which then reacts with the varnish to form cadmium oxalate. But the initial alteration of cadmium sulphide [cadmium yellow] can be slowed by controlling humidity and light.’
Other paintings from this period face similar issues. ‘In this period new types of pigments came out and the artists were drawn to use these new materials, but they were not tested with respect to their stability. A lot of them turned out not to be stable to light,’ Miliani explains.
The removal of original paint during a treatment is undesirable. So the interface between varnish and pigment poses a conundrum, as conservators must now take into account that the varnish and crust contain original cadmium yellow oil paint used by Van Gogh.
In the future, the hope is that this degradation process can be inhibited or even prevented thanks to novel preservation and conservation techniques, the researchers point out.
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