HgS is the primary component of cinnabar and the pigment vermilion and has been known to photodegrade in the presence of chlorides. To uncover the likely sources of chloride, and compounds responsible for initiating the degradation process, a noninvasive computational probe called DFT was employed. Variations in surface areas, cleavage planes, and surface terminations were explored through comparing adsorption energies. A test set of adsorbate molecules including pollutants, chloride-containing compounds, and water products, with varying oxidation states of the central atoms, were screened to determine which adsorbates cause surface transformations that coincide with lower adsorption energy. Relevant surface interactions were identified by analyzing atomistic and electronic information such as the making and breaking of bonds, bond distances, surface transformations, changes in oxidation states, and adsorption energy. Multiple surface terminations and cleavage planes provided varying degrees of stability, thus comparing surface-adsorbate interactions on multiple surfaces provided a more complete understanding towards the reactivity of different classes of molecules on cinnabar and vermilion. Across multiple surfaces, the oxidation state and species of the adsorbates central atoms contributes to the degree of surface interactions, as well as the magnitude of adsorption energy.

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

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