Viewing fine-art paintings in a museum usually first calls for letting your eyes adjust to dim lighting. That’s because many historical works use pigments—including the bright red vermilion (α-HgS) made from the mineral cinnabar—that degrade and darken irreversibly over time, and exposure to light can hasten the degradation. Vermilion’s slow transformation from red to a dark gray presumably indicates that elemental liquid mercury is released, but the pathway has been unclear. An international team has now combined computational spectroscopy with high-resolution microscopic x-ray diffraction to shed new light on the processes involved. A mural sample from the Gothic Monastery of Pedralbes in Barcelona, Spain, was taken for x-ray analysis. The figure’s inset shows a cross section of the sample in which the thick red layer is overlain by darker layers that contain, among other things, three different phases of the mineral corderoite (Hg3S2Cl...

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