Journal Club29/11/2013

Casting Light on the darkening of colors in historial paintings.

Fabiana Da Pieve et al. Phys. Rev. Lett., 111, 208302 (2013)

Viewpoint of Physics and Editor suggestion.Published November 15th.

Motivation

• Archeological sites, cathedral wall paintings, and famous masterpieces are affected by light-induced degradation of pigments– Chrome yellow=> Van Gogh.– Mineral cinnabar (mercury sulfide, -HgS) => Rubens,

gothic art (12th - ~late 15th centuries).• Appearence of gray or black layers, with whitish streaks.

• Interconnection between factors of degradation, pathway are not understood, affecting protection protocols.

State of “art” vs. this paper• Detection of secondary compounds through x-ray

absorption near-edge spectroscopy, Raman, and miscroscopy:– Corderoite (-Hg3S2Cl2), Calomel (Hg2Cl2), and mercuric

chloride (HgCl2).– Absence of “black” polymorph cinnabar, metacinnabar (-HgS).

• Ab-initio theoretical spectroscopy + XRD measurements.– Composition of degraded areas.– Theoretical treatment unveils the path of degradation:

• Chemical: photoreductive dissolution• Physical: structural instabilities

Deposition of dark elemental HG

Results• Mural Samples from the Monastery of Pedralbes,

Spain, 14th Century.

Results

(a) Degraded sample. Analyzed area as seen under ordinary light, side view.

b) High-resolution -XRD color map, with false color, indicating the five minerals, side view.

Grey area

?

ResultsDetail: each layer.

Spectra: ab-initio (ABINIT) calculations and experiment.

Weirdos -> intermediate phases in cinnabar-corderoite-calomel?? degradations

• DFT: density functional theory;• GW: many-body perturbation

theory in G0W0 approximation;• BSE: Bethe-Salpeter equation with

excitons.

Results.

Results

Photoexcited electrons and holes rise chemical reactions

Two key resultsBand edges of cinnabar with redox reactions (pH=4)

refined 1. Hg is grey/black

2. Cinnabar valence band bellow Cl levels: Favored adsorption -> humidity.

Production of Corderoite-Hg3S2Cl2

-HgS

ResultsPhotoactivation mechanism produce structural defects

The Sequence

1. Salt in the surroundings provides chloride ions to convert vermilion to corderoite.

2. Exposure to air allows the creation of defects: O-substitution and S-vacancy .

3. Light convert mercury ions into metallic mercury.4. The mercury would collect on the surface in the

form of small liquid droplets, which appears black.

Conclusions

• Clarification of composition of degraded layers of painting.

• Consistent model (supported by measurements and theoretical analysis) for the pathway of degradation:– Illumination– Humidity– Exposure to air

• Decomposition=>layered succession of alteration products (XRD maps) photoactivated.

• Hipothesis of darkening: presence of Hg, not already measured.– Low-temperature x-ray analysis for detection of Hg.