Pompeiian wall paintings colored with cinnabar have darkened since their excavation. “Art preservationists have been uncertain why the degradation occurs, but have suspected that sunlight causes the mercury sulfide to change crystalline phases, to a form called metacinnabar. But an analysis using the European Synchrotron Radiation Facility in Grenoble, France, shows that there is no metacinnabar to be found. Instead, Marine Cotte of the synchrotron facility and colleagues found two other degradation processes at work, probably caused at least in part by chlorine.”