Mercury sulfide, HgS, the primary component of cinnabar and the pigment vermilion, has been found to photodegrade in the presence of chlorides and other small molecules. To uncover the potential mechanisms of degradation, the noninvasive computational probe density functional theory (DFT) was implemented to map out atomistic information about the initial degradation steps of cinnabar and vermilion in the context of art conservation. A range of vermilion chemical environments were probed by adding a wide set of test adsorbates to the surfaces and investigating their adsorption reactivity for different surface areas, cleavage planes, and surface terminations. Surface transformations were investigated using comparative electronic band analyses structure and a first-principles thermodynamics analysis that includes solvation. Different surface terminations and cleavage planes were found to have varying degrees of stability that depended on surface area and adsorbate density, where the variable stability of different surfaces might contribute to the photo-reactivity of mercury sulfide in the presence of chlorides and small molecules. These results suggest that come cleavage planes of HgS crystal might be more unstable than others, and thus having a higher propensity towards reacting with atmospheric adsorbates to cause degradation.

First-principles calculations as a noninvasive probe of vermilion and cinnabar surfaces in art conservation science

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