Treatment: Cleaning coins

You should never clean coins using cleaning products such as silver or copper polish, special cleaning pads or a silver cloth. They will not only make the coins shine like your silverware, but will also leave abrasions – and copper and bronze coins will lose their original colour. Generally speaking, this will strongly reduce the value of the coins. Silver coins become tarnished over time, mainly due to sulphur compounds and humidity in the air. This toning or discolouration – also called ‘patina’ – often creates an attractive effect, especially in older coins, that is very popular with collectors.

In the case of tin or zinc coins, it is especially important to take care that they are not affected by what is know as tin or zinc pest. A process of oxidation causes a reaction in these coins, which become covered in a white powdery substance. There is a danger of this process spreading to other coins, so any afflicted coins should therefore be immediately removed from the collection. People sometimes apply a layer of varnish to the coin, but this is definitely not enough to halt the process.

The best way to avoid problems is to touch the surface of a coin as little as possible. Instead, try to get used to always holding a coin by its edge, between your thumb and index finger. Fingers are naturally a little oily and leave marks behind, especially on the field of coins that have been struck using polished dies. The acids in fingerprints can permanently stain the coin. For the same reason, you should never attach a sticker or label onto coins.

Nowadays, coins are often kept in cardboard coin holders lined with cellophane which are folded around the coin. These are self-adhesive or can be stapled closed. Information such as reference number, quality, price and suchlike can then be written on the card. However, the problem with this is that it is no longer possible to hold the coin in your hand, resulting in particularly the edge – the ‘third side’! – no longer being entirely visible and making it harder to assess the quality. The same problem applies to slabbed coins. Because these coins are encapsulated in a sealed protective casing made of rigid plastic, the edge can no longer be examined – which is a great shame!

If you intend to use plastic album folios or plastic bags, you should always make sure that they are made from ‘non-leaching’ plastic since they can otherwise damage your coins over time. Bronze coins in particular develop a green coating if they are kept in plastic that is not plasticiser-free and acid-free.

For more information about cleaning coins, see the Encyclopedie van munten en papiergeld