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Researchers from New York University and the Pratt Institute say the pigment, obtained from the semi-precious stone lapis lazuli, has been most valued by European painters since the late 13th century.

Instances of fading of ultramarine pigments are known, but the mechanism of color alteration of the pigments is not understood.

Alexej Jerschow, an assistant professor of chemistry at NYU, Eleonora Del Federico, an associate professor of chemistry at the Pratt Institute, and colleagues noted ultramarine pigments consist of a framework of aluminum and silicon atoms. The intense blue color is formed by small molecules made up of sulfur trapped within that framework.

The scientists discovered that upon color degradation, the framework breaks apart and releases the color-forming molecules.

"Apart from the scientific interest in this work, these ... investigations promise to have tremendous impact on our understanding and prevention of the chemical processes that underlie the slow -- often irreversible -- decay of our cultural heirlooms", said Jerschow.

The research team is also investigating lead-soap formation in traditional oil paintings.