CONCOURS D’INGENIEURS DE LABORATOIRE
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DU 22 JANVIER 2014
Concours interne : spécialité chimie analytique
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Mercury's dark influence on art
The brilliant red pigment vermilion has been widely used since antiquity on monastic
murals and baroque paintings. But a chemical mystery has dogged this paint, as
when it ages it darkens. Scientists have long suspected a thin film of mercury may be
responsible for the blackening, but have been unable to prove that this was the case.
Now two papers from researchers in Belgium have begun to unravel just what is
going on at the surface of these works of art.
Vermilion was first made from the mineral cinnabar, mercury sulfide, but was
subsequently produced synthetically. First used in 7000–8000BC, it was highly prized
in ancient Rome and was the principal red pigment used by European painters during
the Renaissance until the 20th century, when it was finally superseded by the
cheaper and safer cadmium red.
In 2005, Katrien Keune and Jaap Boon of the FOM Institute for Atomic and Molecular
Physics in Amsterdam proposed that chlorine salts in the air catalysed the
photodecomposition of mercury sulfide into elemental mercury, which appears black
in thin layers. Mercury, however, is extremely difficult to detect because, as a liquid
metal, it does not have a regular crystal structure to diffract radiation, making it
invisible to x-ray diffraction and absorption spectroscopy.
Back to black
To investigate this hypothesis, researchers at the University of Antwerp dipped a
piece of platinum covered with vermilion into an aqueous solution of chloride ions
and then irradiated it with laser light. Sure enough, it turned black. By applying a
voltage to the blackened pigment, they found that mercury ions were released into
the solution. The voltage needed to achieve this was exactly the voltage required to
liberate mercury ions from metallic mercury.
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In parallel, European researchers led by theoretician Fabiana Da Pieve, of the
University of Antwerp and Vrije Universiteit Brussel, used density functional theory
and other theoretical techniques to calculate how mercury might end up on the
surface of degraded paint. They confirmed, in work accepted for publication in
Physical Review Letters, that mercury sulfide ought to attract chlorine atoms from
chloride salts commonly found in humid air. They concluded, however, that even in
the presence of these, the photons absorbed by vermilion would have insufficient
energy to cause direct decomposition of the mercury sulfide into elemental mercury.
They calculated that the chlorine atoms could, however, be absorbed into the
mercury sulfide lattice, forming the mineral corderoite (Hg3S2Cl2). The oxygen in the
air could react with this to form sulfur dioxide, leaving behind an unstable defect in
the corderoite lattice that, when hit by a photon, would collapse, releasing an atom of
metallic mercury. Furthermore, the mineral calomel (Hg2Cl2) left behind can then
dissociate to yield more metallic mercury.
Evidence builds
To confirm this, the researchers looked at micrometre-scale samples of degraded
vermilion paint from murals in the Monastery of Pedralbes in Catalonia. Highresolution x-ray diffraction showed that a layer of corderoite lay on top of the original
mercury sulfide, and on top of this was a layer of calomel, implying that each layer
had formed from chemical degradation of the layer beneath it. ‘At the very top there
should have been mercury,’ explains Da Pieve, ‘but mercury itself cannot be detected
by this technique unless you do it at very low temperatures.’
Boon is impressed by the experimental demonstration, but he remains unconvinced
that the multi-stage theoretical model is applicable in all cases. ‘As far as I can tell
they say that the chloride is there in much larger amounts,’ he says, ‘and maybe that
explains why the mechanism that they are proposing might work in their case [of the
Monastery of Pedralbes].’ In some paintings kept in controlled environments,
however, he still believes the chlorine is likely to be only a catalyst. ‘Some really good
chemical work is now necessary,’ he says.
Da Pieve hopes that a fuller understanding of the processes leading to vermilion
degradation may, in time, lead to art galleries being able to prevent it or slow it down
by, for example, excluding certain frequencies of light from museum and art galleries'
lights. ‘I guess they already use lights of specific frequencies,’ she says, ‘but it could
help to choose those frequencies in a better way.’
Tim Wogan
CHEMISTRY WORLD NEWS, 29 OCTOBER 2013
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