Pratt Institute fine arts major Sally Pusede preparing frescoes for NMR measurements. Courtesy of Eleonora Del Federico
In Michelangelo's famed Sistine Chapel frescoes, the skies are a divine shade of blue, known as ultramarine. The pigment that produced this color prized was so highly by Medieval and Renaissance artists that it was worth more than its weight in gold.
Slowly but surely, however, the paint is fading. Ultramarine degrades slowly over time, and curators worry that someday the Madonnas captured in the world's frescoes could end up wearing robes of grey.
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Research being done at New York University and the Pratt Institute in Brooklyn, New York, is helping conservators understand how ultramarine goes from gleaming to drab. Eleonora Del Federico, a chemist at Pratt, decided to capitalize on her students' interests by drafting them for research on the science of frescoes.
"My students who are art history or fine arts majors are perfect for this," she said. "We produced the frescoes in the lab and then we aged them."
Del Federico, along with Alexej Jerschow, a chemist at New York University, employed nuclear magnetic resonance (NMR), a technique that reveals the structure and geometry of molecules, to examine the accelerated "wear and tear" frescoes the students had created.
First, the researchers determined the concentration of color-forming pigments in the artwork. Their NMR analysis revealed that the frescoes subjected to fast-forward degradation lost their blue color as the pigment structure broke apart.
According to Jerschow, the color-forming molecule, or "chromophore," is composed of three sulfur atoms plus one electron. Under normal circumstances—or in a well-ventilated fresco—the chromophore remains trapped in a cage of aluminum and silicon atoms.
If the fresco is subjected to very humid or acidic conditions, however, the framework disintegrates.
"There is an aluminum bridged by oxygen to a silicon atom," Jerschow said. "And so when the ultramarine framework breaks apart, some of these bonds are broken. Then color-forming molecules escape and degrade."
The research also revealed that frescoes fade rapidly when wet. Because Renaissance painters often dealt with humid weather, Del Federico believes that even a great master like Michelangelo would have had to go back and touch up his work.
"He seemed to have retouched it on top," she said. "He wouldn't have wanted to do it at all, but he may have had to because the paints had faded."
Now that they have a better understanding of what goes on at the molecular levels of a fresco, Del Federico and Jerschow hope to take the project one step further by exploring whether the degradation can be prevented.
Coating the pigments with a material not susceptible to acidic or alkaline attack is one possibility, Jerschow said, admitting that "these ideas are very preliminary and we will have to bring more collaborators on board."
The use of a portable NMR micro-spectrometer to analyze paintings is a relatively new development in art conservation. It exemplifies a larger movement to analyze artwork at the museum, without having to risk damage by moving it to a laboratory, said Giacomo Chiari, chief scientist at the Getty Museum Conservation Institute in Los Angeles. Later this year conservationists at New York's Metropolitan Museum of Art will use NMR analysis to study the degradation of ink and paint on materials such as paper and parchment.