Analytical Imaging Studies Clarifying the Process of the Darkening of Vermilion in Paintings

• Published in: Analytical chemistry (Washington) Vol. 77; no. 15; pp. 4742 - 4750
• Main Authors: Keune, Katrien, Boon, Jaap J
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.08.2005
• Subjects: Analytical chemistry; Chemical reactions; Chemistry; Chlorine; Exact sciences and technology; Light; Mass spectrometry; Mercury; Paints; Spectrometric and optical methods; Chlorine; Cross section; Nanoparticle; Reaction product; Secondary ion mass spectrometry; Paint; X ray analysis; Spatial distribution; Chlorides; Imaging; Reaction mechanism; Mercury sulfide; Darkening; Photochemical degradation
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac048158f

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.