Direct evidence for metallic mercury causing photo-induced darkening of red cinnabar tempera paints.

ABSTRACT: 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 cinnabar-based paint.

SUBMITTER: Elert K 

PROVIDER: S-EPMC9814095 | biostudies-literature | 2021 Dec

REPOSITORIES: biostudies-literature