Analytical Imaging Studies Clarifying the Process of the Darkening of Vermilion in Paintings Katrien Keune and Jaap J. Boon* Molecular Paintings Research Group, FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands 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 se- quentially. 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 accumulat- ing 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 (HgCl 2 ), are formed. Blackening of vermilion (HgS, trigonal), a light-induced degradation phenomenon of HgS, is observed on the raw mineral cinnabar as well as on vermilion paint in works of art. The color change may strongly disfigure the painted image. 1 The highlights in a detail of Triumphal Procession with Sacrificial Bull by P. de Grebber (1650) (Oranjezaal, Huis Ten Bosch Palace, The Hague, The Netherlands) should have been vivid red, but are transformed into grayish strokes (Figure 1). This irreversible surface degrada- tion process takes place irrespective of the origin of the vermilion and the type of binding medium. 1 It does not matter whether vermilion has been processed wet or dry or is derived from a natural source as cinnabar. 1 Until recently, the black product on vermilion was thought to be meta-cinnabar (HgS, cubic). 2-5 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. 6 McCormack and others have shown recently that halogen impurities in and around vermilion particles play a dominant role in the darkening process. 4,7,8 Small concen- trations 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. 7 Not only chlorine but also other halogens (mainly iodine) trigger the blackening of vermilion. 4,5,7 Recently, Spring et al. observed two degradation products in blackened vermilion, 9 a black and white one, which were assumed to be formed simultaneously. It was inferred that vermilion is first transformed into the photosensitive mineral corderoite (Hg 3 S 2 - Cl 2 ) after exposure to humidity and chloride ions. This corderoite * To whom correspondence should be addressed. E-mail: boon@amolf.nl. (1) Grout, R.; Burnstock, A. Z. Kunsttechnol. Konserv. 2000, 1, 15-22. (2) Feller, R. L. Studies on the Darkening of Vermilion by Light, Report and Studies in the History of Art 1967, National Gallery of Art, Washington, DC, 1967. (3) Gettens, R. J.; Feller, R. L.; Chase, W. T. In Artist’s pigments; Roy A., Ed.; Oxford University Press: New York, 1993; Vol. 2, pp 167-168. (4) Daniels, V. In Recent Advances in the Conservation and Analysis of Artifacts; Black J., Ed.; Summer School Press: London, 1987; pp 280-282. (2) Davidson, R. S.; Willsher C. J. J. Chem. Soc., Dalton Trans. 1981, 3, 833- 835. (3) Dreyer, R. M. Am. Mineral. 1938, 23, 457-460. (4) McCormack, J. K. Miner. Deposita 2000, 35, 796-798. (5) Davidson, R. S.; Willsher, C. J.; Morrison, C. L. J. Chem. Soc., Faraday Trans. 1 1982, 78, 1011-1019. (6) Spring, M.; Grout, R. Natl. Gallery Technol. Bull. 2002, 23, 50-61. Figure 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. These highlights consist of blackened vermilion. Photo: L. Speleers, Stichting Restauratie Atelier Limburg (SRAL), Maastricht, The Netherlands. Anal. Chem. 2005, 77, 4742-4750 4742 Analytical Chemistry, Vol. 77, No. 15, August 1, 2005 10.1021/ac048158f CCC: $30.25 © 2005 American Chemical Society Published on Web 06/22/2005