Losing The Blues

Throughout the 16th to 18th centuries, to create their brilliant blues, many watercolor and oil painters opted for a glassy blue pigment called smalt instead of the more costly azurite or ultramarine. But this cheaper choice came at the expense of longevity: Over time, the blue smalt degraded and the paintings' bright hues faded, turning a drab brown.

Now European researchers have confirmed what scientists had long postulated about the chemical mechanism behind the fading blues: With age, potassium atoms in the pigment seep away, changing the chemical environment around the blue cobalt ion (Anal. Chem., DOI: 10.1021/ac200184f).

Old masters produced smalt by grinding blue glass into tiny shards, which they then added to the paint. Their blue glass was primarily composed of silicon, oxygen, and potassium, but it also contained cobalt ions that provided the blue hue.

To solve the mystery of smalt's disappearing color, Laurianne Robinet, a researcher at IPANEMA, a center for ancient-materials research at the SOLEIL Synchrotron in Saint-Aubin, France, collaborated with scientists at several museums across Europe. The team collected samples of fading blue paint from masterpieces in the Louvre's collection in Paris and in the National Gallery's collection in London.

The samples were no greater than 400 µm in diameter, and had been taken from the artwork during previous restoration projects, says Marika Spring, a senior scientist at the National Gallery, who participated in the research.

Shining a X-ray beam just 2 µm wide from the SOLEIL synchrotron at the shards of smalt glass in the paint, the researchers studied the chemical structure of the pigments when they were still blue and at different stages of fading.

As scientists had previously predicted, Robinet found that faded pigment particles lacked potassium. More important, she says, was the information they gleaned about the cobalt ion's geometry and orientation relative to other atoms in the glass—its so-called coordination number.

When potassium is present, it stabilizes the cobalt ion in a tetrahedral coordination, she explains, which means the cobalt binds to four oxygen atoms with each at the corner of a tetrahedron. This tetrahedral coordination provides the blue color.

When potassium leaches out of the pigment, the researchers found, cobalt adopts a higher coordination number, binding with more oxygen atoms, sometimes up to six. The switch in the cobalt's chemical environment changes its color—to the regret of curators and museumgoers worldwide.

Scientists had also proposed that changes in oxidation states or loss of cobalt ions could have led to the color change, Robinet says. But, "we conclusively prove in real samples of smalt pigment," that discoloration comes from cobalt's coordination change due to the leaching of potassium, she adds.

Joyce H. Townsend, a senior conservation scientist at the Tate Britain, in London, is happy to see confirmation of the potassium leaching hypothesis. She adds that this better understanding of the mechanism could "lead to better risk assessments for conservation treatments, and better preventive conservation."