One Mystery Solved About Pigments That Get The Blues

Microscopic image of blue glass pigments called smalt that fade with time. © Analytical Chem.

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.

The Consecration of Saint Nicholas by Paolo Veronese helped solve the mystery. © National Gallery, London

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.

Landscape with the Expulsion of Harpies by Paolo Fiammingo also helped solved smalt's mystery. © National Gallery, London

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.

Author: Sarah Everts

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3 Comments

  1. Is there any way you could reverse the process by somehow re-introducing potassium ions?

    • Good question… I’ve asked the conservators and museum scientists to chime in on it. I would suspect that even if re-introducing potassium ions did bring back the blue color in the glass, it might be hard to get this to happen in the dried paint mixture given its heterogeneousness. Or maybe it would work like a charm…

  2. No, unfortunately it would not be possible to re-introduce potassium in the glass, and that is for 2 reasons.
    First, introduction of potassium in a glass requires an exchange with another ion, usually sodium, and implies placing the glass in a molten potassium salt bath at temperature over 400 degres Celsius for several hours or days.
    The second most important reason is that the silicate glass structure has re-organised itself following the loss potassium creating new bonds leading to a more condensed network. In this new network there is no room anymore for the potassium ion and the bonds created are very strong and cannot be broken easily.