journal club 29/11/2013
DESCRIPTION
Journal Club 29/11/2013. 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. - PowerPoint PPT PresentationTRANSCRIPT
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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.
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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.
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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
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Results• Mural Samples from the Monastery of Pedralbes,
Spain, 14th Century.
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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
?
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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.
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Results.
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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
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ResultsPhotoactivation mechanism produce structural defects
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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.
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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.