- Favourite yellows of Van Gogh turn brown or green under ‘soft’ lighting
- Museums warned to think again about some types of LED lighting
- Powerful X-ray beams used to analyse make-up of paint fragments
Yellow paint used in several of his most famous pictures, including Sunflowers and Portrait of Gauguin, is unstable and can turn to shades of green or brown.
Scientists have now discovered that one of the reasons why the yellow paint could be darkening is the lights used by museums and art galleries.
Portrait of Gauguin by Vincent Van Gogh was shown to contain unstable shades of yellows
They fear that dozens of masterpieces by Van Gogh, Paul Cézanne and other artists may be at risk of irrevocable deterioration under the soft lighting of museums and art galleries.
LED lights had been thought an ideal way to protect the pictures because it avoided the damage caused by the glare of natural and types of illumination.
But the study of a type of yellow paint introduced in the nineteenth century and beloved by artists such as Van Gogh and Cézanne is a cause of deep concern to art conservators.
It began because curators had become worried that some of their most valuable masterpieces were changing colour.
Researchers studying Van Gogh’s pictures concentrated on his use of chrome yellow paint, lead chromate, which is richer in sulphates than most others.
It is the high proportion of sulphates that is believed by the European team of researchers to be responsible for the darkening of the yellows under light.
Chrome yellow paint (starting from far left) before aging and as it is exposed to different types of light
Tiny fragment of paint from “Portrait of Gauguin” that proved to contain unstable variants of lead chromate
‘Both stable and unstable forms of chrome yellow paint were used not only by Vincent Van Gogh but also by Paul Cézanne,’said Professor Koen Janssens, of the University of Antwerp, who headed the research team.
‘We advise musea that own such masterpieces to ascertain whether the unstable forms of chrome yellow were used and if needed to tailor their lighting systems accordingly.’
Three shades of chrome yellow were considered in the study, published in the journal Analytical Chemistry – lemon yellow, middle yellow and primrose yellow.
Using the powerful X-ray beams at the Deutsches Elektronen-Synchrotron (DESY) in Germany and at the European Synchrotron Radiation Facility (ESRF) in France, researchers discovered that middle yellow was stable but the primrose and lemon shades were not.
Sunflowers by Van Gogh is one of the masterpieces containing yellow paint that can darken under museum lights
Exposed to the ‘green/blue’ light emitted by some LED light sources – and previously believed to be safe for artworks – the primrose and lemon yellow began turning brown or olive green.
Professor Jenssens added: “Some LED Lights may emit much more blue/green light than ‘normal’ light sources.
‘We have observed that these wavelengths can speed up the discoloration of some very sensitive variants of the yellow pigment lead chromate.
‘We recommend Musea to become aware of the problem and, in those cases where it is required, choose LEDs that do not contain ‘extra’ blue/green in their emission spectrum.’
Costanza Miliani, of the University of Perugia in Italy, said: ‘We were surprised to note that already under conditions of illumination currently considered safe, some of our test samples started changing colour quite quickly.’
Bruno Brunetti, also from the University of Perugia, added: ‘We could show that by means of relatively simple equipment based on reflection of infra-red light, that may be operated in the museum galleries themselves, it is possible to establish which type of chrome yellow is present in a painting.’
Ella Hendriks, Head of Conservation at the Van Gogh Museum in Amsterdam, welcomed the findings: ‘Studies like these are very important to make museum curators aware that, even under ambient light conditions, the degradation of some sensitive materials proceeds continuously.
‘Musea should carefully consider the potential impact of, for example, the new, LED-based, lighting systems that are now being installed in collections.’