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Casting Light on the Darkening of Colors in Historical Paintings

Casting Light on the Darkening of Colors in Historical Paintings

By F. Da Pieve, C. Hogan, D. Lamoen et al.

Physical Review Letters, Vol.111:20 (2013)

Ravages of time. Researchers took a small flake (leaving a white spot) from the dark region (formerly red) just below the lower of two reddish, flower-shaped elements at the left of this mural. The mural is in the 14th century Monastery of Pedralbes in Spain. X-ray diffraction along with first-principles calculations led to a proposed sequence of chemical reactions that explains the darkening of the red color over time. Photo courtesy American Physical Association
Ravages of time. Researchers took a small flake (leaving a white spot) from the dark region (formerly red) just below the lower of two reddish, flower-shaped elements at the left of this mural. The mural is in the 14th century Monastery of Pedralbes in Spain. X-ray diffraction along with first-principles calculations led to a proposed sequence of chemical reactions that explains the darkening of the red color over time. Photo courtesy American Physical Association

Abstract: 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 (-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.

Introduction: Many archeological sites, cathedral wall paintings, and famous masterpieces from major painters are affected by a slow, irreversible, light-induced degradation of pigments. Well-known examples of such phenomena are the discoloration of ‘‘chrome yellow,’’ the pigment mostly favored by Van Gogh, and the darkening of vermilion, the red bright pigment also known as the mineral cinnabar (mercury sulfide, -HgS), favored by Rubens and Italian masters and present in archeological sites worldwide. In spite of intensive experimental studies, the complex physicochemical processes underlying the degradation of vermilion remain a mystery. Factors such as light, impurities, humidity, and exposure to air are known to play a role, but the reasons behind their importance and the way they interconnect are not understood.

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Vermilion degradation manifests through the appearance of gray or black layers, often accompanied by whitish streaks. Besides cinnabar, a number of secondary compounds have been detected in degraded samples through x-ray absorption near-edge spectroscopy, Raman, and microscopy: these include corderoite (-Hg3S2Cl2), calomel (Hg2Cl2), and mercuric chloride (HgCl2), although the presence of the latter seems to depend strongly on the sample investigated. Notably absent from these studies is the black polymorph of cinnabar, metacinnabar (-HgS), as ruled out by the most recent experimental findings [3–5]. Instead, some studies have suggested that metallic Hg is responsible for the dark luster, but its presence is highly controversial, due to the difficulty in observing crystalline elemental mercury.

Click here to read this article from EMAT (Electron Microscopy for Materials Science) 

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