One of the most frightful experiences a serious collector or dealer will encounter is discovering a collection of rare coins that have been improperly cleaned. Many people, upon finding a neat old coin collection, that was put away years ago by a friend or relative, will look at it, and think “my goodness these coins are dirty and black”. It is instinctive to want to make the coins look as “good” as possible, especially if a sale is considered. So, they go and visit the silver or copper polish under the sink. This of course is a BIG mistake. I once had the unpleasant experience of looking at a collection that was obviously formed by a person who focused on obtaining rare, and very high quality examples of Canadian Victorian 25 Cents (1870 to 1901). Although most of these items we either brand new or close to it, in terms of wear, they were all harshly cleaned. After a lengthy discussion it was discovered that the original owner had cleaned the coins with a silver polish to make them look better. Value before cleaning was likely $20,000+, after cleaning, less than $500.
Older coins are almost always made of metal; platinum, gold, silver, and copper being the most common used. The first two of these are the least reactive to the environment, and even over hundreds of years are likely to show little tarnishing or toning. Silver and copper however, react quite quickly and change color when exposed to air and moisture. Silver coins will first tone to nice rainbow colors, and then progressively turn black. Copper coins start out a bright orange red, and turn brown. People seriously interested in coins, like other collectors, want their items in their original state. By cleaning a coin with a silver or brass polish what you are really doing is removing the original layer on the surface, and exposing the deeper un-oxidized metal. By doing this you are taking away part of the original coin. One of my old friends sums this up as follows…it is like taking away all of the layers of skin on a person, they may still be lovely on the inside, but will certainly look less appealing on the outside. Does this mean that coins are never cleaned by professionals? No. The difference however is in the approach. There are occasions when a coin must be cleaned to preserve it. Professional restorers however will make a great effort not to damage the original “skin” of the coin when cleaning. An effort is made only to remove contaminants on the surface, using milder compounds. They are not using silver polish or brass-o or other harsh chemicals that will erode the surface of the coin. So should you clean those old silver or copper coins to make them shine again? Only if you want them to be virtually worthless!