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Journal Club 29/11/2013

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  1. Journal Club29/11/2013 Casting Light onthedarkeningofcolors in historial paintings. Fabiana Da Pieveet al. Phys. Rev. Lett., 111, 208302 (2013) Viewpoint ofPhysicsand Editor suggestion. PublishedNovember 15th.

  2. Motivation • Archeological sites, cathedralwallpaintings, and famousmasterpieces are affectedby light-induceddegradationofpigments • Chromeyellow=> Van Gogh. • Mineral cinnabar (mercurysulfide, -HgS) => Rubens, gothicart (12th - ~late 15th centuries). • Appearenceofgrayorblacklayers, withwhitishstreaks. • Interconnectionbetweenfactorsofdegradation, pathway are notunderstood, affectingprotectionprotocols.

  3. Stateof “art” vs. thispaper • Detectionofsecondarycompoundsthrough x-rayabsorptionnear-edgespectroscopy, Raman, and miscroscopy: • Corderoite (-Hg3S2Cl2), Calomel(Hg2Cl2), and mercuricchloride (HgCl2). • Absenceof “black” polymorphcinnabar, metacinnabar (-HgS). • Ab-initiotheoreticalspectroscopy + XRD measurements. • Compositionofdegradedareas. • Theoreticaltreatmentunveilsthe path ofdegradation: • Chemical: photoreductivedissolution • Physical: structuralinstabilities • Depositionofdarkelemental HG

  4. Results • Mural SamplesfromtheMonasteryofPedralbes, Spain, 14th Century.

  5. Results • Degraded sample. Analyzed area as seen under ordinary light, sideview. • High-resolution-XRD color map, with false color, indicatingthefiveminerals, sideview. Greyarea ?

  6. Results Detail: eachlayer. Weirdos -> intermediatephases in cinnabar-corderoite-calomel?? degradations Spectra: ab-initio(ABINIT)calculations and experiment. • DFT: densityfunctionaltheory; • GW: many-bodyperturbationtheory in G0W0approximation; • BSE: Bethe-Salpeterequationwithexcitons.

  7. Results.

  8. Results Photoexcitedelectrons and holesrisechemicalreactions refined 1. Hg isgrey/black 2. Cinnabarvalencebandbellow Cl levels: Favoredadsorption-> humidity. ProductionofCorderoite -Hg3S2Cl2 -HgS Twokeyresults Band edgesofcinnabarwith redox reactions (pH=4)

  9. Results Photoactivationmechanismproducestructuraldefects

  10. The Sequence • Salt in the surroundings provides chloride ions to convert vermilion to corderoite. • Exposure to air allows the creation of defects: O-substitution and S-vacancy . • Light convert mercury ions into metallic mercury. • The mercury would collect on the surface in the form of small liquid droplets, which appears black.

  11. Conclusions • Clarificationofcompositionofdegradedlayersofpainting. • Consistentmodel (supportedbymeasurements and theoreticalanalysis) for thepathwayofdegradation: • Illumination • Humidity • Exposuretoair • Decomposition=>layeredsuccessionofalterationproducts (XRD maps) photoactivated. • Hipothesisofdarkening: presenceof Hg, notalreadymeasured. • Low-temperature x-rayanalysis for detectionof Hg.