When paintings go bad

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 (Hg₃S₂Cl₂). None of the compounds found in the x-ray analysis are intrinsically gray-black; being a liquid, Hg was invisible. Detailed ab initio calculations provided some missing pieces of the puzzle—such as optical absorption, electron bandgaps, and excitation levels—and led to the following scenario. Impurities—notably chlorine and oxygen—in humid air react with the vermilion and begin to generate secondary mineral crystals like corderoite. Both sunlight and museum lamps can then photoactivate those secondaries and induce structural defects in the crystals. When the crystals collapse, elemental Hg is released and migrates to the surface, where its dark gray appearance is manifest. The researchers think that controlling humidity and tailoring the frequencies of museum lamps could slow the degradation. (F. Da Pieve, C. Hogan et al., Phys. Rev. Lett. 111, 208302, 2013, doi:10.1103/PhysRevLett.111.208302 .)