Walking the halls of one of the world's great art museums, it's easy to regard familiar classic paintings as eternal and unchanging. But this is not the case. Paintings are a mix not only of color but of chemistry-and chemistry changes. In some of Vincent van Gogh's works, the striking, sunny yellows have faded and turned brownish, robbing the Dutch master's art of some of its trademark intensity. So a European team of scientists decided to find out exactly what was happening on those canvases.
Using sophisticated X-ray machines, they discovered the chemical reaction to blame - one never before observed in paint. Ironically, van Gogh's decision to use a lighter shade of yellow paint mixed with white is responsible for the unintended darkening, according to a study published online Monday in the journal Analytical Chemistry.
Vincent loved yellow. In particular, he loved chrome yellow, a 19th century invention that shone brighter than previously available hues of paint. Art preservationists have known that the lead-based paint fades under intense sunlight, so they've done what they can to keep van Goghs and similar works out of intense light. What's curious about his paintings, however, is that some yellows have faded while others have not.
To sort this out, the team led by Koen Janssens first acquired paints made around the time van Gogh worked, and then blasted them with UV light to simulate the aging process. When one of Janssens's yellow samples browned the way some van Goghs have, the team examined it with X-ray scans at the European Synchrotron Radiation Facility in Grenoble, France.
The researchers found that a change in the oxidation state of the element chromium (from chromium 6 to chromium 3) was linked to the darkening of chrome yellow paint. The X-ray beam research carried out at ESRF also showed that chromium 3 was especially prominent in the presence of chemical compounds which contained barium and sulphur, according to BBC News.
For comparison, Janssens then looked at tiny paint flecks from two van Goghs: Banks of the Seine (above) and View of Arles with Irises. And in the yellows that had browned, the team found their culprit in forcing the chromium change: barium sulfate.
This suggests that this contaminant, combined with light exposure, is the source of the darkening, Janssens [says]: "We think the barium sulphate could have been part of a paint extender - something used to make the paint go further. The mixture of sulphate and chromate is very sensitive to darkening under UV light. Galleries should keep paintings containing chrome yellow out of any strong light or UV light."