The brilliant blues in Michaelangelo’s Last Judgment, which covers the western wall of the Sistine Chapel, come from ultramarine. The once rare and precious pigment has appeared on Western canvases since the 13th Century. The name comes from the Italian for “beyond the seas,” and the pigment itself is derived from the semi-precious stone lapis lazuli. And as it turns out, ultramarine fades over time.

A new collaborative study among the NYU Chemistry Department, the Pratt Institute, and the Metropolitan Museum of Art has established the chemical mechanism for the pigment’s slow change in color. Using nuclear magnetic resonance, the researchers found that ultramarine’s blueness comes from sulfur atoms trapped inside a lattice of aluminum and silicon; when the sulfur siggles free, ultramarine loses its blue. Armed with this knowledge, conservationists can now begin to develop means for preserving works.

On the visualziation side of things, I have to admit that I’d like to see a cartoon or rendering that shows the aluminum-silicon structure and how the sulfur atoms fit in. But if I have to settle for Michaelangelo (an image similar to the above appeared with the NYU press release), so be it.

In case this all sounds intriguing… Victoria Finlay’s brilliant book, Color: A Natural History of the Palette, has a chapter about blue that includes many intriguing details. The story of pigments used in art—and how those pigments and other chemicals interact and change over time—contains many fascinating twists and turns. Also, a couple of years ago, the American Folk Art Museum ran an exhibit about “blue” that examined some of the science behind pigmentation; I seem to recaall a catalog, but I can’t find any evidence for one.