The use of powdered mercury sulfide, HgS, as a brilliant red pigment dates to ancient times in both Europe and Asia. But vermilion, as the pigment is known in Europe, turns gray or black over time. Various analyses have turned up several mercury compounds as possible degradation products, but scientists seeking to understand the causes of pigment darkening and ways to avoid it lacked a mechanism. By combining X-ray diffraction and theoretical studies, a team led by Fabiana Da Pieve of the Free University of Brussels, in Belgium, and Conor Hogan of Italy’s National Research Council now proposes a pathway for the darkening process (Phys. Rev. Lett. 2013, DOI: 10.1103/physrevlett.111.208302). They suggest that Cl– ions carried by water droplets in humid air adsorb onto the surface of HgS and form Hg3S2Cl2. Sulfur in Hg3S2Cl2 then reacts with O2 to form SO2, which escapes as a gas. Mercury atoms left behind form Hg34+ clusters that are stimulated by light to produce Hg and Hg2Cl2, which can liberate additional Hg. Hg is likely the source of the gray color, although additional experiments are necessary to confirm its presence, the researchers note.