LOS ANGELES – It’s hard to imagine some of Vincent van Gogh’s signature works without the vibrant strokes of yellow that brightened the sky in “Starry Night” and drenched his sunflowers in color. But the yellow hues in some of his paintings have mysteriously turned to brown, and a team of European scientists has figured out why.
Using X-rays, they found the chemical reaction to blame – one never before seen in paint. 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.
In some of Van Gogh’s paintings, the yellow has dulled to coffee-brown, and in about 10, the discoloration is serious, said Koen Janssens, an analytical chemist at Antwerp University in Belgium who co-authored the study.
The root of the problem is the lead-chromate paint he used.
The researchers obtained three tubes of yellow paint from the Royal Museum of Fine Arts of Antwerp that were manufactured around the same time that Van Gogh was working. They spread samples onto glass slides and bombarded them with ultraviolet radiation for three weeks to mimic the process of aging.
Only one of the samples turned brown, Janssens said. Then they hit the paint with a high-intensity X-ray, and found that that the colorfast samples were made of chromium in its pure, crystalline form.
The darkened sample contained sulfates, which are associated with white pigment. For now, there’s no way to stop the darkening – only to slow it down.