NYT: art conservators discovered the cause of fading yellows in painting

Via the New York Times: 

From 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 color 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 analyzed bits of degraded cadmium yellow paint taken from pieces painted by the Spanish artist Joan Miró in the 1970s. One particular brand of paint was likely most responsible for the degradation observed in the Miró pieces, the team concluded in a study published in July in the journal Heritage Science.

...Furthermore those six samples — from the degraded paintings, the palettes and the tube of Cadmium Yellow Lemon No.1 by Lucien Lefebvre-Foinet — all exhibited poor crystallinity, the team found. That means that the cadmium and sulfur atoms aren’t perfectly interlocked in their usual hexagonal arrangement, said Daniela Comelli, a materials scientist at the Polytechnic University of Milan and a member of the research team. “There’s some disorder.”

If you click through to the research article, looks like (unsurprisingly) the sulfur is getting oxidized. I'm just imagining a worker making a random change many years ago that resulted in this lessened crystallinity...