Scientists have discovered the chemical reaction that is changing yellow shades to brown on some Van Gogh paintings.
To find out, the researchers obtained three tubes of yellow paint from the Royal Museum of Fine Arts Antwerp that were manufactured around the same time that Van Gogh was working. They spread samples of the still-bright paint onto glass slides and bombarded them with ultraviolet radiation for three weeks to mimic the process of aging.
Only one of the samples browned — and it did so in dramatic fashion, its color turning from daisy to coffee with milk, Janssens said. The brown layer was about a micron thick.The cause?
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."Which leads to an interesting point...
"This is the kind of research that will allow art history to be rewritten," because the colors we observe today are not necessarily the colors the artist intended, said Francesca Casadio, a cultural heritage scientist at the Art Institute of Chicago who was not involved in the work.I imagine most artists and art patrons take the colors they see on a painting for granted as the original, and only have a passing understanding that paint ingredients consist of chemicals with complicated names - never mind the actual chemical properties.
Between this and the evidence for painted Roman statues, is it time assess how many works of art actually look the way they were painted?
...in the meantime, keep your paintings out of the sunlight if you don't know where the paint came from.