Oct. 13, 2006 — Medieval and Renaissance Madonnas will no longer risk their vibrant blue mantels turning into yellowish grey robes, according to U.S. researchers who have discovered why natural ultramarine blue sometimes fades in frescoes.
Known as "ultramarine sickness," the irreversible form of discoloration has been observed in frescoes at the Church of Saint Augustine in San Gimigniano, near Siena, and in the Basilica of Assisi.
"Our studies explain for the first time the process of fading in ultramarines and may lead to the design of proper art conservation treatments and the development of more-permanent pigments," Alexej Jerschow, an assistant professor of chemistry at NYU, wrote in the journal Inorganic Chemistry, with Eleonora Del Federico, an associate professor of chemistry at the Pratt Institute, and colleagues.
Derived from the semi-precious stone lapis lazuli (literally "blue stone"), ultramarine blue has been one of the most valued pigments by European painters since the late 13th century.
Since the process to extract the pigment from the mineral was very expensive and complicated, ultramarine — from the Latin ultramarinus, meaning "from beyond the sea" — was more precious than gold.
The use of this color was usually reserved to artworks of great importance, such as the funeral mask of Tutankhamen. The pigment found its most extensive use in 14th and 15th century Italian paintings, and was often reserved for the robes and mantels of Christ and the Virgin Mary.
Ultramarine blue was also used by Michelangelo for the skies of the Last Judgment in the Sistine Chapel in Rome.
To discover the mechanism behind ultramarine fading, Jerschow and Del Federico produced frescoes with the pigment and stored them in a chamber at 85 percent relative humidity for two weeks. After that time, the rich and vibrant blue faded to a yellowish grey.
The researchers examined the simulated and degraded fresco samples with nuclear magnetic resonance (NMR), a technology that shows the structure of molecules.
