Where does the white coating come from?
The Coin Cabinet in Dresden’s Royal Palace has to be temporarily closed. A whitish coating was found on 100 silver coins on display.
Jens Dornheim has to speak of damage: “It’s hard, but there’s no other way to put it.” The restorer at the Coin Cabinet of the Dresden State Art Collections has found a whitish coating on 100 silver coins on display. Initial analyses have shown that a silver chloride coating has formed on the surfaces of various unpreserved silver coins covered with historical sufid patina. “The coating is extremely thin and difficult or impossible for visitors to see,” says Dornheim. The restorer discovered it in December during one of his weekly tours of the exhibition. It makes the silver of various alloys appear like the much duller pewter.
The coin cabinet has now been closed as a precaution and all 1,400 silver coins have been removed from the display cases. The exhibition shows a total of 3,300 objects on 350 square meters. The Dresden Coin Cabinet’s collection includes 300,000 objects from antiquity to the present day. It is one of the largest universal collections of European significance. Saxon coins and medals alone account for 30,000.
In Dresden, people are still at a complete loss as to where the white coating comes from. After more than ten years of closure, the Coin Cabinet moved into the rebuilt Residenzschloss in 2015 and opened its new permanent exhibition there on June 7, 2015. “Of course, all the display case materials were tested beforehand,” says Jens Dornheim. Coins and medals made from other materials are not damaged. Even exhibits made of the much less noble and more reactive lead do not show any deposits.
Jens Dornheim was able to clean most of the coins with silver chloride plating with water. The coating could only be removed from two pieces. “We have not observed any surface changes on the undersides, but they are particularly strong on rougher surfaces,” says Dornheim and therefore assumes that an “air problem” leads to the silver chloride deposits. But exposure to light could also be a decisive factor, although all display cases are illuminated UV-free. The conservator does not want to commit himself to a single cause at the moment; too many different factors could play a role in the presence of hydrochloric acid and its reaction with the visible silver surfaces.
The causes are being investigated
One thing is certain: no coin cabinet in Europe has ever seen anything comparable. After discovering the silver chloride, the Dresden restorer asked specialist colleagues from various coin cabinets for advice, but neither they were aware of the phenomenon nor could anything be found about it in the specialist literature, says Dornheim.
The search for the reasons for the chemical reaction of the silver coins has now begun. An organic cause has already been ruled out. Further research is underway together with the Technical University of Dresden and the Dresden University of Fine Arts. When there are results, Dresden will communicate them to all experts, promises Jens Dornheim.
The good news
However, visitors will not have to do without the Saxon Coin Treasures while the cabinet is closed. The art collections intend to integrate temporary coin exhibitions into their other exhibitions over the next few weeks.