In March 2017 a phenomenon, well-known to coin collectors, caused nationwide media interest in Germany: The coin cabinet of the Staatliche Kunstsammlungen Dresden had to close its exhibition practically overnight because of sudden impurities, colloquially known as “milk spots” on coins. The story was covered in issue 05/2017 of “Münzen & Sammeln” (Coins & collecting) magazine. A summary of the article can be found here.
According to the coin cabinet, more and more serious changes appeared on the surface of the non-preserved silver coins and medals since the end of 2016. Instead of a naturally grown patina, the rare collectors items had a white-milky covering. According to the coin cabinet, about 100 coins are affected: “The affected objects were immediately taken out of the showcases and are treated in a special restoration workshop,” a Dresden State Art Collections spokesperson said. The experts have been working on the removal of the white coverings since then: “It can already be said that the surface changes can be remedied by restoration measures and that the original state can be restored,” says Dr. Stephan Adam, press secretary of the Staatliche Kunstsammlung Dresden.
The experience of the experts in the Dresden State Art Collection is probably no surprise for collectors of silver coins. The opaque, murky patches that can appear on silver coins have been considered annoying for years. Production companies such as the Perth Mint have been trying to get to the bottom of the cause of the contamination. Problems with the water quality during the cleaning process of the coins or impurities of the embossing tools were regarded as possible additions, but this theory has been rejected in the meantime. The Perth Mint now assumes that microscopic air particles in the product systems cause the change in the surface of the coins. However, the deposits cannot be removed without damage to the coin.