I had the pleasure of attending a mini conservation seminar given by Susan Maltby. I learned a good deal from her and probably have copies of every column she has ever written for Coin World.
I do not claim to have her knowledge concerning general conservation; however, as one of the founders of the Numismatic Conservation Service I believe my comments on coin conservation carry some weight. Although I no longer work at NCS, our results with coins were more often than not outstanding. I still conserve coins every day (at no cost to our customers) at the Independent Coin Graders grading service in Tampa.
First, Dr. Maltby’s overview of cleaning methods is very informative as usual. One distinction I should add is the difference between run-of-the-mill circulated coins, ancients, metal detector finds, and rare, expensive, Proof or high-grade coins. With this letter, I wish to warn your readers that they should never wipe the surface of a coin in the latter group (especially hazy modern Proofs) lightly with anything to remove “early stage” silver sulfide.
As Dr. Maltby mentions, galvanic cleaning does not restore a coin either and may ruin its eye appeal. Abrasive cleaning may be suitable in some cases when it is done by a professional using proper technique and proprietary ingredients.
In my experience, nonabrasive, completely neutralized, chemical cleaning has proven to be the best method of conservation. When done properly, there is usually no evidence that a coin has been conserved.