How A Red Lady Becomes Black And White

At the FOM institute for Atomic and Molecular Physics Keune investigated the pigment vermilion from a painting by Rubens. Originally this mercury sulphide containing pigment was 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 colour has discoloured under the influence of light and exhibits black and white spots.

The researcher discovered that current hypotheses about the progress of the discoloration were not true. In the painting that the chemist investigated, two-thirds of the thickness of the paint layer was discoloured. Some vermilion particles were only partially affected: the upper surface was black whereas the lower surface was still red. This revealed that the particles first of all discolour into black and only then into white: a two-step degradation process. This goes against the prevailing view that the black and white colours develop at the same time.

Also the idea about exactly which reaction takes place was found not to be true. With a spatially- resolved mass spectrometer the researcher discovered that the intact vermilion contained traces of chloride, whereas the damaged vermilion contained higher quantities of chloride and mercury chloride complexes. From this Keune concluded that the traces of chloride in the red vermilion act as a catalyst in the reaction between vermilion and light. This results in nanoscopic particles of metallic mercury that completely absorbs the light and is visible on the painting as black spots. With this the long-held view that the black product was a black form of mercury sulphide was rejected. This metallic mercury subsequently reacts with the excess chloride to form a white mercury chloride compound. The degradation process is unfortunately irreversible.

Battling against the aging process

Keune's research was part of the De Mayerne research programme. This programme was set up to obtain scientific insights into the molecular changes in old works of art due to the ageing process, painting techniques and the effects of restorations carried out in the past.

Katrien Keune's research was funded by the Foundation for Fundamental Research on Matter (FOM).