Colour darkening in historical paintings

FOR over two millennia, vermilion (mercury sulphide) has been the painter’s finest red. But in many old paintings, the pigment has discoloured to a drab brownish black.

The reason for the change has been controversial, but a team of researchers from Belgium, Italy and France now claims to have figured out the cause of such degradation. The authors applied X-ray techniques to examine the composition of a discoloured vermilion paint layer from a medieval mural in a Catalan monastery.

Guided by simulations, they suggest in a recent paper in the journal Physical Review Letters that the pigment darkens via a sequence of light-initiated chemical reactions that involve the formation of metallic mercury.

The finding clarifies the connection between degradation and environmental conditions and may lead to better ways of protecting paintings through the proper choice of lighting and humidity conditions.