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Blackening of cinnabar (α-HgS) constituted a dilemma for ancient artists and modern conservators as well. In this research program, the mechanisms of HgS phase transformation were studied using several analytical techniques: scanning electron microscopy, energy dispersive spectroscopy, X-ray powder diffraction, Raman spectroscopy, and cathodoluminescence spectroscopy. The experimental study was coupled with advanced computational modeling of materials using Ab initio density functional theory methods. Natural and synthetic standards representing both hexagonal (α-HgS) and cubic (β-HgS) polymorphs of mercuric sulfide were submitted to several kinds of physical treatment to induce transformation in either direction and the products were characterized using appropriate analytical method.

Experimental work shows that physical parameters such as radiation, mechanical activation or thermal treatment can induce the transformation in one or the other direction. The results obtained to date, suggest that blackening is due to amorphization and formation of an intermediate phase, thus following a “nucleation and growth” mechanism. Meanwhile, modeling of the structure shows that the polymorphic difference is primarily associated with the expansion of the c-axis in going from cinnabar to metacinnabar. It also indicates that optical behavior of these two phases is inherently related to the structure of their respective band gaps. The combination of the two approaches has thus proved very powerful in understanding the mechanism of the phase transformation, which is central to any successful preservation of the red pigment and to the restoration of its color once blackened.

This research was made possible through Grant MT-2210-02-NC-12 from the National Center for Preservation Technology and Training (NCPTT).

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