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.
Highlights
The X-ray diffraction (XRD) pattern of the Chinese cinnabar pigment used in this study was in very good agreement with a natural reference cinnabar (International Centre for Diffraction Data (ICDD) file number: 897103)
Our results show that cinnabar pigment underwent important mineralogical changes upon UV aging, involving the formation of mercury sulfates (i.e., HgSO4·H2O and schuetteite (Hg3(SO4)O2))
It might be argued that mercury sulfates would only form under the extreme experimental conditions during UV aging in this study involving UV-C radiation
Summary
The authors concluded that, even though photooxidation might occur, a direct photo-induced reduction to metallic mercury was not possible because the redox potential was more negative than the conduction band energy, the energy of photo-excited electrons being insufficient to directly reduce Hg2+ to Hg0. The authors proposed that the reduction to elemental mercury in cinnabar would only proceed in the presence of chlorides This theory was justified by the fact that the redox potential of various mercury chloride compounds falls within the band gap of cinnabar (i.e., the potentials of HgCl42−(ads) + 2e− ↔ Hg0(ads) + 4Cl− and Hg2Cl2 + 2e− ↔ 2Hg0 + 2Cl− are 0.48 and 0.27 V vs NHE, respectively) and could, be reduced to metallic mercury via photo-induced electron transfer[1,14].
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