Direct evidence for metallic mercury causing photo-induced darkening of red cinnabar tempera paints Elert, Kerstin Pérez Mendoza, Manuel José Cardell Fernández, Carolina FESEM and XPS analyses were performed at the Centro de Instrumentacion Cientifica (University of Granada). We would like to thank C. Rodriguez-Navarro and J. A. R. Navarro for insightful comments. Financial support has been provided through the Research Project EXPOAIR (P12-FQM-1889, European Regional Development Fund), the Research Group RNM-179 (University of Granada), and the Unidades Cientificas de Excelencia (UCE.PP2016.05 and UCE.PP2018.01, University of Granada). K.E. was a post-doctoral fellow in the EXPOAIR Project. Photo-induced darkening of red cinnabar (HgS) has attracted the interest of many researchers as it drastically impacts the visual perception of artworks. Darkening has commonly been related to metallic mercury (Hg0) formation in the presence of chlorides. Based on the study of UV-aged cinnabar pigment and tempera paint we propose an alternative pathway for the blackening reaction of cinnabar, considering its semiconductor properties and pigment-binder interactions. We demonstrate that darkening is caused by the oxidation of cinnabar to mercury sulfates and subsequent reduction to Hg0 via photo-induced electron transfer without the involvement of chlorides, and provide direct evidence for the presence of Hg0 on UV-aged tempera paint. Photooxidation also affects the organic binder, causing a competing depletion of photo-generated holes and consequently limiting but not impeding mercury sulfate formation and subsequent reduction to Hg0. In addition, organics provide active sites for Hg0 sorption, which is ultimately responsible for the darkening of cinnabarbased paint. 2022-01-10T11:38:37Z 2022-01-10T11:38:37Z 2021-12-10 info:eu-repo/semantics/article Elert, K., Pérez Mendoza, M. & Cardell, C. Direct evidence for metallic mercury causing photo-induced darkening of red cinnabar tempera paints. Commun Chem 4, 174 (2021). [https://doi.org/10.1038/s42004-021-00610-2] http://hdl.handle.net/10481/72274 10.1038/s42004-021-00610-2 eng http://creativecommons.org/licenses/by/3.0/es/ info:eu-repo/semantics/openAccess Atribución 3.0 España Nature