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Message:: [QUOTE="beef1020, post: 2296375, member: 24544"]I think a better approach here is to think about what you mean by cleaning a coin.  Are you trying to remove foreign material from the coin's surface, like dirt, dust, grime, etc.  Or are you trying to alter the surface of the coin, i.e remove oxidation or otherwise change the surface material of the coin. In my view, the first is almost always acceptable and I would go so far as to say prudent in the case of copper coins.  Dirt, oil, and other grime left on a copper coin will over time cause corrosion, so removal is an appropriate part of conservation.  On the other hand, altering the surface of a coin is almost never a good idea in my opinion.   Toning, regardless of how ugly it looks, should be left as is.  Details should never be strengthened, corrosion should never be smoothed.  Anything that alters the metal should be avoided. The trick is knowing the techniques that will safely remove foreign material while not reacting with the underlying metal.  This process will be different depending on the metal used in the coin.  If you are going down this road, do your research and test on coins with no numismatic value first.[/QUOTE]