Portrait of a young woman Peter Paul Rubens (1620s)  Mauritshuis, The Hague inv. no. 251

The Rubens classic, Portrait of a Young Lady, is losing its color thanks to a small quantity of chloride in the red pigment. The Peter Paul Rubens painting is in the Mauritshuis Museum in The Hague but areas are slowly turning black and white under the influence of light. Now, Dutch researcher Katrien Keune of the Foundation for Fundamental Research on Matter Institute for Atomic and Molecular Physics (AMOLF) has determined the cause and course of the discoloration.

Keune and supervisor Jaap Boon investigated the red pigment vermilion from the Rubens painting. Vermilion contains mercury sulfide and was originally just as red as the upper stripe of the Dutch flag. However, in the painting 'Portrait of a Young Lady', by Peter Paul Rubens, this clear red color has discolored under the influence of light and exhibits black and white spots. Keune has now demonstrated that theories about how and why this decoloration has occurred do not hold up against current evidence. Earlier theories suggested that the black and white decoloration is occurring simultaneously. Keune has found this not to be the case.

Katrien Keune

She found that two-thirds of the thickness of the paint layer was discolored but some vermilion particles were only partially affected: the upper part was black whereas the lower part of the particle was still red. This, she explains, suggests that the particles must first turn black and then white: a two-step degradation process.

She has also now described the chemistry taking place to explain the changes that are happening to the painting. Using spatially-resolved mass spectrometry she discovered that intact vermilion contained traces of chloride, whereas the damaged vermilion contained higher quantities of chloride and mercury chloride complexes. This led to the conclusion that the traces of chloride in the red vermilion are acting as a catalyst in the photochemical degradation of vermilion. The result is the formation of nanoscopic particles of metallic mercury, which completely absorb light and show up as black spots. The long-held view that these spots are a black form of mercury sulphide has to now be rejected. The metallic mercury then reacts with excess chloride to form a white mercury chloride compound.

Unfortunately, it appears that this two-stage photo-damage is irreversible. However, now that Keune has enlightened science as to the underlying cause it might be possible to preserve other paintings with chloride contaminants before they too become damaged beyond repair.

http://www.amolf.nl/research/fill_fr_main_grp.php?P_gid=10005 

http://www.nwo.nl/demayerne

http://en.wikipedia.org/wiki/Vermilion

http://www.mauritshuis.nl/english