Expert: PAPAJACK - 9/24/2009

Question
I have an old collection of bronze medals and find some have black spots. I understand the spots are caused by sulphur. How can I clean the medals and prevent the spots from recurring?

Many thanks,
Gene

Answer
This is a common issue and often ruined by improper handling or cleaning. If you are selling them do not clean them.
Let me state as a Coin Collector that coins are worth much less after having been cleaned. There is usually no reason to clean a coin. Unless the substance on the coin is destroying the coin (like acid, PVC damage etc.) it is not necessary to clean it. Any coin that has ever been cleaned will sell for less.

True coin collectors don't recommend cleaning coins at all.
When a coin is cleaned the original surface is removed thus removing the luster that the purest collectors want to see.
The cleaned surface allows other contaminates to attack the surface. Also the cleaning may deposit other particles on the surface.
Also note that coins that are naturally toned (especially silver coins) bring higher prices than cleaned coins. Sometimes double the usual amount. Also if the coins have an unusual blue, olive, or rainbow colored toning they sometimes can be sold for many times their usual price.

This is a good question though and other than a liquid detergent and distilled water cleaning and pat dry I haven’t heard of a restorative process that is cost effective.

You can try other PH natural inert solvents at your own risk like lacquer thinner, acetone etc…

I have not tried it but some people have said letting it soak in non corrosive oil (olive oil, gun oil etc.) wiping and washing or even chipping away at crusty dirt with a tooth pick or orange stick.

Solutions that are too acid or basic will change the surface of the coin since copper is very reactive.

Thank You and Good Luck

PapaJack