Journal Club 29112013 Casting Light on the darkening

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 15 th.

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, -Hg. S) => Rubens, gothic art (12 th - ~late 15 th 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 ( -Hg 3 S 2 Cl 2), Calomel (Hg 2 Cl 2), and mercuric chloride (Hg. Cl 2). – Absence of “black” polymorph cinnabar, metacinnabar ( Hg. S). • Ab-initio theoretical measurements. spectroscopy + XRD – 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, 14 th 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 ?

Results Detail: each layer. Weirdos -> intermediate phases in cinnabar-corderoite-calomel? ? degradations Spectra: ab-initio (ABINIT) calculations and experiment. • DFT: density functional theory; • GW: many-body perturbation theory in G 0 W 0 approximation; • BSE: Bethe-Salpeter equation with excitons.

Results.

Results Photoexcited electrons and holes rise chemical reactions refined 1. Hg is grey/black -Hg. S 2. Cinnabar valence band bellow Cl levels: Favored adsorption -> humidity. Production of Corderoite -Hg 3 S 2 Cl 2 Two key results Band edges of cinnabar with redox reactions (p. H=4)

Results Photoactivation 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: Osubstitution 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.