The Blackening of Vermilion: An Analytical Study of the Process in Paintings

Marika Spring and Rachel Grout
Technical Bulletin Volume 23, 2002

Abstract

Ten paintings that predate the introduction of wet-process vermilion, known to blacken as a result of impurities, were examined and analysed. Vermilion had deteriorated to different extents on the same painting and had turned completely grey in some areas, lilac-grey in others. The discolouration consists of a thin blackened layer overlying coarser white and black altered paint, with unaltered red beneath. This optical layering gives the range of discoloration observed in different areas. Raman spectroscopy showed that the white material is mercury(I) chloride, calomel, while the black is likely to be metacinnabar, indistinguishable from vermilion itself by Raman techniques.

It is suggested that the chlorine derives from dirt, since the unaltered vermilion was chlorine-free, and the paintings in question have been described as unusually 'dirty' in the past. The origin may be sodium chloride from skin debris; dirt analysed at the Gallery and elsewhere contained sodium chloride absorbed onto calcium-rich particles.

A degradation mechanism for vermilion altering in the presence of sodium chloride to corderoite Hg5S2Cl2, itself altered to calomel on exposure to light, is proposed. Glazing applied over still-red vermilion has protected it from both dust and light, and the presence of varnish is also (partially at least) protective against discolouration.

Keywords 

chemical analysis, discoloration, light, paintings, vermilion

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To cite this article we suggest using

Spring, M., Grout, R. 'The Blackening of Vermilion: An Analytical Study of the Process in Paintings'. National Gallery Technical Bulletin Vol 23, pp 50–61.
http://www.nationalgallery.org.uk/technical-bulletin/spring_grout2002


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