FROM Vincent Van Gogh’s sunflowers to Edvard Munch’s The Scream, there’s no shortage of seminal artwork that was made with a striking hue known as cadmium yellow. But that riot of colour that artists squeezed from their paint tubes isn’t necessarily what museum goers see today: cadmium yellow’s brilliance often diminishes over time, as the paint fades and turns chalky.

And it’s not only centuries-old artworks that are affected. A team of art conservators and scientists recently analysed bits of degraded cadmium yellow paint taken from pieces painted by Spanish artist Joan Miro in the 1970s. One particular brand of paint was likely most responsible for the degradation observed in the Miro pieces, the team concluded in a study published in July in the journal Heritage Science.

Cadmium yellow paint is an amalgam primarily of cadmium and sulphur. It was first commercialised in the 1840s and soon gained renown among artists. Miro described the colour as “splendid”. Tubes of cadmium yellow paint, including Cadmium Yellow Lemon No 1 produced by Parisian manufacturer Lucien Lefebvre-Foinet, litter Miro’s two studios in Mallorca, Spain.

In 2020, Mar Gomez Lobon, an art conservator based in Mallorca, began investigating the paints that Miro used after he settled on the island in the 1950s. An art conservator at the Pilar and Joan Miro Foundation in Mallorca had tipped her off that more than 25 pieces in the foundation’s collection painted in the 1970s showed evidence of degraded yellow paint.

To dig into the cause of the deterioration and whether it could be linked to a particular brand of paint, Gomez Lobon and her colleagues collected tiny flecks of cadmium yellow paint from three untitled pieces that Miro painted between 1973 and 1978. The team also scooped up small samples from three paint tubes from the artist’s Taller Sert and Son Boter studios, a cup used for mixing paint and two palettes. Each sample was roughly the size of a pinhead.

A microscopic sample of paint is enough for many scientific analyses. And there are distinct advantages to analysing just a fleck of paint, said David Muller, a physicist at Cornell University, who was not involved in the Miro research. Transporting a valuable piece of artwork to a laboratory is logistically complicated. “You’ve got this very fancy security procedure,” Muller said. But there’s a lot less pressure to working with a paint sample just a thousandth of an inch wide, which is what Muller and colleagues did when they studied the degradation of cadmium yellow in The Scream.

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Gomez Lobon and her collaborators analysed the nine samples from Miro’s paintings and studio materials by recording how the paint absorbed, reflected and re-emitted different wavelengths of light. That allowed the team to investigate the chemical makeup and crystalline structure of each sample.

The elemental analyses revealed that the degraded paint samples from the three paintings contained primarily cadmium and sulphur, as expected, with traces of zinc. The same mix was found in paint samples from the two palettes and one of the tubes of paint. Furthermore, those six samples – from the degraded paintings, the palettes and the tube of Cadmium Yellow Lemon No 1 by Lucien Lefebvre-Foinet – exhibited poor crystallinity, the team found.

That means that the cadmium and sulphur atoms aren’t perfectly interlocked in their usual hexagonal arrangement, said Daniela Comelli, a material scientist at the Polytechnic University of Milan and a member of the research team. “There’s some disorder.”

Poor crystallinity of cadmium yellow was believed to be partially responsible for the degradation observed in older artworks by Pablo Picasso, Henri Matisse and other artists. (Environmental conditions, particularly humidity and temperature, have also been shown to play a role.) But these new results highlight the fact that the problem persisted well into the mid-20th century, which the researchers found surprising.

“You would think that the paint manufacturers would have corrected the problem,” Gomez Lobon said. Lucien Lefebvre-Foinet was, in addition, a well-regarded brand, she said. “This was a really high-quality paint.”

In the future, Gomez Lobon plans to catalogue the 100 or so tubes of paint still strewed around Miro’s studios. She hopes to precisely age date the Lucien Lefebvre-Foinet tubes and better understand how the brand produced its paint, specifically its cadmium yellow. Miro left behind a treasure trove of supplies that should be studied, Gomez Lobon said. “These studios are like a gold mine. NYTIMES