Casting Light on the Darkening of Colors in Historical Paintings

The degradation of colors in historical paintings affects our cultural heritage in both museums and archeological sites. Despite intensive experimental studies, the origin of darkening of one of the most ancient pigments known to humankind, vermilion ($\alpha$-HgS), remains unexplained. Here, by combining many-body theoretical spectroscopy and high-resolution microscopic x-ray diffraction, we clarify the composition of the damaged paint work and demonstrate possible physicochemical processes, induced by illumination and exposure to humidity and air, that cause photoactivation of the original pigment and the degradation of the secondary minerals. The results suggest a new path for the darkening process which was never considered by previous studies and prompt a critical examination of their findings.

Figure 1

(a) Degraded sample from the Monastery of Pedralbes, analyzed area as seen under ordinary light, side view; (b) high-resolution $\mu$-XRD color map, with false color, indicating the five minerals, side view; (c) $\mu$-XRD maps corresponding to Fig. 1b, albeit separately for each of the five minerals; (d) optical absorption spectra of $\alpha$-HgS compared with experiment [33].

Figure 2

Band edge positions for the $\alpha$-$\mathrm{HgS}(10\overline{1}0)$ surface (within DFT and after addition of $GW$ (i.e., $G_0{W}_0$) corrections, ${\mathrm{E}}_{\mathrm{BGC}}$ denotes the band gap center), and free reaction energies of ${\mathrm{Cl}}^{-}$ oxidation and Hg(II) reduction ($p\mathrm{H}=4$, see text).

Figure 3

(a) Atomic arrangements around a S atom in pure corderoite, around a S vacancy and around an O impurity in defective corderoite, and (b) their density of states. (c) Schematic of Jahn-Teller distortion of $\mathrm{Hg}_3{}^{4+}$-like cluster [27] in defective corderoite, leading to ejection of metallic Hg.

Figure 4

$G_0{W}_0$ electronic levels of the surface of the secondary minerals: $\alpha$-${\mathrm{Hg}}_3{\mathrm{S}}_2{\mathrm{Cl}}_2$ and ${\mathrm{Hg}}_2{\mathrm{Cl}}_2$, compared with relevant redox potentials.