However, scientists led by Koen Janssens at Antwerp University in Belgium have found that by using x-ray techniques, they have identified the cause of the colour change. As an initial test examining the paint itself rather than the canvas, tubes of 100-year-old yellow chrome paint (made from lead chromate and discontinued after the discovery of its toxicity) were aged artificially using UV rays to accelerate the aging process caused by sunlight. It was found that over time, the top nanometre-thin layer of pigment revealed itself to be a darker colour than the pigment underneath, and, when analysed, it was found that the top layer of pigment had undergone a reduction reaction (where a substance is oxidised as a result of an electron gain), changing the composition of the paint from hexavalent to trivalent chromium, which, surprise surprise, is a green colour.
When studies were carried out on the canvas itself, it was found that the darker areas tended to contain barium sulphate, which, when combined with sunlight, also causes discolouration. It is speculated that this may be due to Van Gogh’s penchant for mixing yellow with white paint (containing sulphur and barium) to create the various colours needed for his work, as well as his possible use of a paint extender to make the paint go further. Other possible reasons include Van Gogh’s use of cheaper paints which help explain why other works by different painters using chrome yellow have been left relatively unscathed.
Painting deterioration (rather than discolouration) has also been observed in other famous works such as the Mona Lisa and The Last Supper by Leonardo Da Vinci. Deterioration can occur as a result of changes in humidity, which encourages microbial growth, as well as sunlight, which can fade paintings or cause chemical reactions resulting in discolouration.
Although the discolouration is irreversible, it is hoped that further damage can be prevented to these great works of art, now that the causes are understood.
Image: tonynetone on Flickr