Abstract:A study has been made of the irradiation of red mercury(@ sulphide electrodes in water, acetonitrile, pyridine and methanol containing dissolved tetra-alkylammonium salts. Photocurrents were found to be due to solvent oxidation, iodide oxidation and lattice decomposition, but which of these processes actually operates is determined by the individual solvent, solvation of the iodide ion and adsorption of the tetra-alkylammonium cation on the semiconductor surface. The length of the alkyl chain of the substitute… Show more
“…Our energy level diagram indicates that the cinnabar valence band edge computed with G 0 W 0 lies well below the (Cl À =Cl 2 ) level by about 0.8 eV, allowing for oxidation of chloride impurities by photogenerated valence band holes at the cinnabar surface, and facilitating the adsorption of chlorine on the surface itself. This finding describes quantitatively the effect of humidity, as given by Cl salts [1]: a surface modification of the original red pigment takes place, due to quantum-allowed electron transfer mechanisms, explaining the initiation of reported transformations [1,[22][23][24]. Although the details of the crystal growth are beyond the scope of this work, our preliminary studies show that Cl preferentially adsorbs on the cinnabar surface at a site consistent with its final position in the corderoite lattice [18].…”
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 (α-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.
“…Our energy level diagram indicates that the cinnabar valence band edge computed with G 0 W 0 lies well below the (Cl À =Cl 2 ) level by about 0.8 eV, allowing for oxidation of chloride impurities by photogenerated valence band holes at the cinnabar surface, and facilitating the adsorption of chlorine on the surface itself. This finding describes quantitatively the effect of humidity, as given by Cl salts [1]: a surface modification of the original red pigment takes place, due to quantum-allowed electron transfer mechanisms, explaining the initiation of reported transformations [1,[22][23][24]. Although the details of the crystal growth are beyond the scope of this work, our preliminary studies show that Cl preferentially adsorbs on the cinnabar surface at a site consistent with its final position in the corderoite lattice [18].…”
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 (α-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.
“…Until recently, the black product on vermilion was thought to be meta-cinnabar (HgS, cubic). − However, Dreyer already suggested in 1938 that the blackening resulted from a superficial layer of colloidal mercury in solid solution on the cinnabar and that the blacking process was accelerated by impurities . McCormack and others have shown recently that halogen impurities in and around vermilion particles play a dominant role in the darkening process. ,, Small concentrations of chlorine in cinnabar (average between 0.05 and 1 wt %) can cause blackening, whereas cinnabar with a chlorine concentration less than 0.01 wt % remains unaffected by sunlight ,, 1 Detail of “Triumphal Procession with Sacrificial Bull” by P. de Grebber (1650) (Oranjezaal, Huis Ten Bosch Palace, The Hague, The Netherlands) showing grayish strokes on top of the red dress.…”
Imaging secondary ion mass spectrometry (SIMS) is applied for the first time to paint cross sections with degraded vermilion (red mercury sulfide) paint to cast new light on the well-known problem of its light-induced darkening. The static SIMS data are combined with light microscopic, electron microscopic studies and energy-dispersive X-ray analysis to identify and localize the various reaction products. The spatial distribution of atomic and molecular species in paint cross sections of the native vermilion and the reaction products leads to the formulation of a new hypothesis on the reaction mechanism of the photodegradation of vermilion where two black and white reaction products are formed sequentially. Under the influence of light, some of the vermilion (HgS) is converted into Hg(0) and S(0). In this process, the chlorine ions, present in the native vermilion, act as a catalyst. We propose that the Hg(0) is deposited on the surface of the remaining HgS as elementary mercury nanoparticles, which turns the vermilion black. Chloride, derived from an external source, is accumulating in the black phase. The metallic mercury and the remaining HgS react away with the excess of chloride. Two intermediate products and a white end product, mercuric chloride (HgCl2), are formed.
“…This pigment is considered generally stable, when it is pure and in absence of particular agents determining the so-called phenomenon of Vermilion darkening [2,6,7]. Indeed, there are specific impurities which may cause the effect of HgS degradation and, as evidenced in the literature, catalytic elements (halides for examples) accelerate the darkening process [2,5,8]. The mentioned degradation affected numerous artifacts during the time and involved not only paintings, but it is quite evident also in mural frescoes, obviously exposed to external agents (light, relative humidity and salt content in the atmosphere) [1,2,5,6].…”
Darkening of Red Vermilion (cinnabar) is a well-known phenomenon which needs, at the moment, a full comprehension of its nature. Some paintings reveal a disfiguring process of blackening degradation of Red Vermilion, albeit other relics preserve the original color during the time. The presence of halide elements like Cl − ions, in addition to the light exposure, represents the most alleged cause to explain the darkening phenomenon. However, the real effect of chlorine impurities is still not completely understood. In this study, starting from a multi-technique characterization, we propose a kinetic model of mutual composition of alpha/beta cinnabar derived by the experimental darkening of pure alpha synthetic cinnabar intentionally doped with Cl − ions and treated under prolonged UV light exposure. The model was further applied to ancient samples by analyzing the Raman spectra of antique cinnabar pigments belonging to manuscripts of XIII-XVII centuries.
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