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Mercury, the Culprit Who Turns Paintings Black

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Mercury released from the red pigment in Adoration of the Magi (Peter Paul Rubens, 1624) has formed black stains in some places. *Image source: KONINKLIJK MUSEUM VOOR SCHONE KUNSTEN, ANTWERP/GIRAUDON/THE BRIDGEMAN ART LIBRARY

 

 For several centuries, the pigment vermilion has been a favorite of artists, however, it is notorious for turning black due to its degradation over time. The source of the lack coloring found in art pieces from ancient Roman frescoes to the baroque paintings by Peter Paul Rubens, still remains unclear. Today, researchers have found that the culprit to turn paintings black is elemental mercury, which is formed under the exposure to light and chlorine ions in the air. The discovery is the first step to developing a reasonable approach to prevent further degradation of historic masterpieces.

Vermillion, also known as cinnabar, is a kind of mineral made of the elements mercury and sulfur. In the year of 2005, Katrien Keune, a chemist from the University of Amsterdam, and her co-workers found that the compound can be broken down via a battery of chemical reactions initiated by light and chlorine ions, which are particularly abundant in the air near sea.

Keune and her group noted that the ultimate product of this reaction was pure mercury, which is often used in old-fashioned thermometers. Accumulated mercury can reside in microscopic pores and crevices in paintings and is considered to be responsible for the blackening effect.

However, this theory is difficult to prove—metallic mercury is so volatile that it becomes very hard to determine whether it has formed in a given reaction, notes Koen Janssens, a chemist from the University of Antwerp in Belgium and a co-author of the new study. “It disappears before you can analyze it.”

In order to prove that the altered cinnabar was metallic mercury, lead author Karolien De Wael and her group designed the following experiment—they placed the pigment on a platinum surface and dipped it into water that contained chloride ions. When laser light was cast on the mineral, it began to turn black (see inset in below picture). By applying electric voltage, the mercury ions were released from the pigment to the water. Furthermore, the voltage required was precisely that needed for elemental mercury to release ions, says De Wael. These latest findings were published in this week’s Angewandte Chemie.

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The alteration of vermillion in Adoration of the Magi is similar to the one that researchers produced artificially in the lab (see inset).

Keune says that her group has convincingly demonstrated that the black material is elemental mercury, and will be of “extreme value” for determining how to store and exhibit works of art more properly.

Janssens agrees with this point and believes that the exact cause of vermillion’s blackening may help reservators to prevent it more efficiently. It is not practical to store paintings in the dark, but it is possible to keep chlorine from reaching the pigment. For example, we can do this by making sure the applied protective layer of varnish does its job appropriately.

Source: Nature News: Paintings turning black? Blame mercury

Image source: Nature

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