Blackening of the red pigment vermillion

Date: 8 oktober 2013

Introduction: Scientists at the University of Antwerp make a breakthrough and solve a key problem in art history.

For centuries on end, vermillion red remained one of the most popular pigments and was used by our greatest painters. It is a widely known fact, however, that over time the red degrades and turns black. Researchers at the University of Antwerp have finally been able to pinpoint the cause of this degradation: mercury. Their research has been published in the prestigious journal Angewandte Chemie.

Ancient Roman frescos in Pompeii and the masterpieces of Rubens, Peter Brueghel and many other celebrated artists: all have deteriorated as the vermillion pigment used in the paint turns black. Scientists had already ascertained that this is caused by exposure to light, chlorine and moisture but the substance which triggered the chemical reaction remained a mystery. Antwerp researchers have recently made a breakthrough, however.

“Several hypotheses had already been suggested”, says project manager Professor Karolien De Wael (University of Antwerp). “Conducting a number of electro-chemical experiments has now enabled us to establish exactly what happens to the vermillion.”

The scientists applied pigment to an electrode and exposed it to light and chlorine ions. An electric voltage was then passed through the electrode and the current measured.

"When the pigment was exposed to light and chlorine ions we were able to detect an increased current at a particular voltage”, explains De Wael. “What’s more, the voltage required turned out to be exactly that needed for metallic mercury. In effect, then, you could say that paintings such as The Adoration of the Magi by Peter Paul Rubens, which belongs to the collection of Antwerp’s Royal Museum of Fine Arts, are weeping mercury tears. We used paint samples from this masterpiece for our research.”

Volatile mercury

“It wasn’t easy to prove that metallic mercury is being produced because it tends to be very volatile”, admits Professor Koen Janssens (UAntwerp). “It forms but before you have the chance to analyse it, it evaporates or reacts again.” The mercury accumulates in the ‘pores’ and in the many crevices in the paint, ultimately leading to the discoloration of the vermillion.

The results of the Antwerp study have not gone unnoticed. The latest issue of prestigious journal Angewandte Chemie* includes an article about the research, which makes sense since this discovery may facilitate better conservation of historic masterpieces. “This discoloration is usually influenced by light and chlorine”, explains Willemien Anaf, who is a PhD student. “Of course, it is impossible to store paintings in the dark but museums face an important challenge in keeping chlorine away from the paint. Protective layers of varnish play an important role here.”

The News pages of leading scientific journal Nature have also reported on the study’s findings.More information?

Prof. Dr. Karolien De Wael, University of Antwerp, karolien.dewael@uantwerpen.be, +32 491 52 39 08Prof. Dr. Koen Janssens, University of Antwerp, koen.janssens@uantwerpen.be

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