UCLA

The red mineral pigment mercuric sulfide (HgS) was commonly employed in Roman wall paintings. Fresco artists of the period favored this pigment for its striking red color. Upon excavation and exposure to air and light, however, cinnabar pigmented surfaces recovered from archaeological contexts often proved unstable. Mural paintings colored with cinnabar that have been exposed in the open air frequently demonstrate a disfiguring, irreversible darkening of the surface. Traditionally, scholars have attributed this alteration to a light-induced phase change from red cinnabar to black meta-cinnabar (Gettens et al. 1972). While this transformation has not been totally excluded, the prevailing view among conservation scientists is that chlorine plays a key role in the darkening process (Spring and Grout 2002, Keune and Boon 2005, Cotte et al. 2006, Cotte et al. 2008, Cotte et al. 2009 and Radepont et al. 2011), through the formation of light-sensitive mercury-chloride compounds, or as a catalyst in the photochemical redox of Hg(II)S into Hg(0) and S(0). Using laboratory-based experiments and thermodynamic modeling, this paper attempts to further clarify the mechanism(s) and kinetics of cinnabar alteration in fresco applications, specifically the role of light, humidity and chlorine ions. Additionally, it explores possible pathways, during or immediately following excavation, to inhibit or retard darkening of cinnabar pigmented fresco surfaces.