No one of whatever religious persuasion who visits the Sistine Chapel in Rome can fail to be impressed by the results of a 20-year restoration project that has brought Michelangelo's frescoes back to their original level of artistry. Most notable is the brilliance of the sky blue that almost illuminates the Last Judgement on the altar wall of the chapel. But, recent NMR analysis of the ultramarine pigment used to produce this stunning blue suggests its tendency to fade could see the Last Judgement and other works ultimately perish.

Alexej Jerschow of New York University, Eleonora Del Federico of the Pratt Institute, and their colleagues have now discovered why the blue pigment fades. Their findings could provide art conservationists with vital information on how to protect works of art.

Natural ultramarine is obtained from the semi-precious stone lapis lazuli. Until the nineteenth century it was found only in the quarries of Badakhshan in north eastern Afghanistan, the hardship in obtaining it coupled with its unmatched vibrant blue colour endowed the pigment with an almost divine character, hence its use in the work commissioned by the Pope Clement VII.

Instances of ultramarine fading are well known, but the mechanism was not understood. Jerschow and colleagues examined samples of ultramarine pigments. These are composed of a framework of aluminium and silicon atoms. Sulfur-containing impurities produce the intense blue colour.

"Solid-state ²⁷Al and ²⁹Si NMR allowed us to determine that upon acidic, as well as, alkaline attack, it is aluminium that is first removed from the zeolite framework of the ultramarines," Jerschow told SpectroscopyNOW, "Subsequently, the framework breaks apart and releases the sulfur-containing chromophores, which are then decomposed (into hydrogen sulfide and sulfur)." He explains further that ²⁷Al NMR specifically revealed the appearance of extra-framework aluminium upon degradation in a fresco medium. ²⁹Si NMR allowed them to see indications of framework decomposition.

These unique 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," adds Jerschow.

Related links:

• Inorg Chem, 2006, 45, 1270-1276
• Jerschow Page

Article by David Bradley