Fading colors in masterpieces are a universal headache for museums and galleries, but one particularly problematic blue pigment is smalt, which was commonly used during the 16th, 17th and 18th centuries across Europe. The pretty blue pigment originates from ground-up glass which artists would then mix into their paint. Smalt was a poor man’s option compared to relatively pricy ultramarine. Now scientists know exactly why the cheaper blue pigment fades away, turning artwork a miserable brown with time.
I wrote a detailed news story here describing how the color change occurs. Pretty much what happens is this: Smalt’s blue hue comes from cobalt ions found in a glass that is otherwise composed of silicon, oxygen and potassium.
Like a little ecosystem, the cobalt ions need potassium ions to shine blue. But with time, potassium ions leach away, changing the chemical environment of the glass around the cobalt ions, and presto, the blue color fades away.
Many a museum scientists had suspected this to be the case—in fact, here’s an informative post from 2007 about smalt from the Tate–but nobody had conclusively proved the theory in samples taken from a fading masterpieces… It’s not as if museums like to let scientists scrape off paint samples from their already hurting art.
But luckily enough, there were already paint samples from several fading, smalt-containing masterpieces at the U.K.’s National Gallery and France’s Louvre, that were then scrutinized at SOLEIL, a big synchrotron in France. And presto: one mystery of a disappearing blue pigment is solved.
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