Paintings turning black? Blame mercury
Davide Castelvecchi
The pigment vermilion has been a favourite of artists for centuries, but it is notorious for turning black as it degrades over time. But the source of the black colouring, found in works ranging from ancient Roman frescoes to the baroque paintings by Peter Paul Rubens, has been unclear. Now researchers have shown that the culprit is elemental mercury, which forms under exposure to light and chlorine ions in the air. The finding is the first step to devising a way for conservators to prevent further degradation of historic masterpieces.
Vermilion, also known as cinnabar, is a mineral made of the elements sulphur and mercury. In 2005 Katrien Keune, a chemist at the University of Amsterdam, and her colleagues showed that the compound can be broken down through a series of chemical reactions initiated by chlorine ions — which are especially abundant in the air near the sea — together with light.
Keune and her team suggested that the ultimate product of this reaction was pure mercury, the liquid used in old-fashioned thermometers. Its accumulation in microscopic pores and crevices in the paint could be responsible for the blackening effect.
But their theory was hard to prove — metallic mercury is volatile, so determining whether it has formed in a given reaction is difficult, says Koen Janssens, a chemist at the University of Antwerp in Belgium, and a co-author of the latest study2. “It’s gone before you are able to analyse it,” he says.
more
http://www.nature.com/news/paintings-turning-black-blame-mercury-1.13887
= new reply since forum marked as read
