Vincent van Gogh’s stunning series of yellow sunflower paintings are slowly withering on the vine. The vibrant hues are progressively fading to an olive-brown color due to the artist’s use of light-sensitive pigments, a group of Dutch and Belgian researchers have discovered.

A team of scientists from the University of Antwerp and the Delft University of Technology spent two years studying an 1889 painting from the artist’s sunflower series in the collection of Amsterdam’s Van Gogh Museum. They found that the artist’s use of chrome yellow paint made certain works particularly susceptible to discoloration.

“Van Gogh used two types of chrome yellow, which is a synthetic pigment that was widely available at the time,” Frederik Vanmeert of the University of Antwerp told artnet News. “One of these types of chrome yellow is quite stable because it has an orange hue, but the other type, which has a pale yellow color, is quite sensitive to degradation, so it will alter its color over time.”

Researchers examined the painting using high-tech techniques including microscopic x-ray imaging and chemical mapping. Thanks to these cutting-edge methods, they were able to understand the painting’s makeup without taking swabs or pigment samples from its surface. (A report on the study first appeared in the Guardian.)

Vincent van Gogh Sunflowers (1889). Photo: courtesy of Van Gogh Museum, Amsterdam.

With the researchers’ findings comes good news and bad news. The good news: The irreversible degradation isn’t yet perceptible to the human eye. The bad news: It is very difficult to predict precisely when these effects will become visible, according to Vanmeert.

It is possible, however, for experts to slow the fading process down. “This process strongly depends on the external factors that the painting has been subjected to,” Vanmeert explained. “For example, the amount of light that has fallen on the painting will have a large effect on the speed.”

This isn’t the first time scientists have discovered Van Gogh’s work transforming significantly with the passage of time. Earlier research found that the artist’s red pigments are also losing their luster and fading to white. (They determined, for example, that the walls in the artist’s 1888 painting The Bedroom were purple before they faded to blue.) As part of a study released three years ago, international scholars also sounded the alarm about the potential for both Van Gogh and Matisse’s cadmium yellow pigment to fade.

In light of these discoveries, the Van Gogh Museum isn’t taking any chances. Curators and conservators are already working to protect the artworks from further deterioration. Five years ago, the museum lowered light levels in its galleries and is currently in the process of reviewing the lighting systems once again.

According to Vanmeert, the new research has implications beyond Van Gogh; similar discoloration could threaten other 19th-century paintings as well. “The exact number is not known, but [the problematic pigment] is a type of yellow Van Gogh frequently employed, so it will be present in a large number of his paintings, and is also presumed to be present in the paintings of a large number of his contemporaries,” he said.

Thankfully, the non-invasive x-ray imaging technique the team developed to conduct its research is easily transferable to the study other paintings and other pigments.