In most fields of art, antiques and collectibles, the concept of “cleaning” or “restoration” is common. After all, old objects tend to lose their original appearance and functionality over time. Antique cars break down, paintings accumulate dirt and dust, silver pieces tarnish, etc. However, numismatists are sensitive to cleaning or restoring coins. If asked whether cleaning coins is acceptable, an expert’s most likely response would be “it depends.”
The biggest difference between acceptable and unacceptable cleaning: the degree of restoration. A commonly approved form of cleaning is “dipping,” or removing a microscopic layer of toning (or tarnish) off a coin. Dipping in moderation does not alter a coin’s luster or surfaces—it merely lightens the coin and removes potentially unattractive coloration. This is considered restoring a coin to how it once looked, rather than altering or artificially modifying a coin’s appearance.
On the opposite end of the spectrum, an unacceptable form of cleaning would be polishing using harsh abrasives. Doing so imparts an unnatural glossy appearance to a coin; this is considered completely artificial and inconsistent with the coin’s original appearance. To a novice, polished coins look “brand new,” but experienced collectors know otherwise. Restoring a coin back to how it once looked is acceptible, but imparting a bizarre atypical appearance will damage a coin’s value.
In between gentle dipping and harsh polishing is a gray area. Techniques like soft brushing and somewhat more potent chemicals are sometimes deemed acceptable—but not always. For instance, many vintage US gold coins have been scrubbed lightly with baking soda. The coins end up looking rather bright, but not beyond what’s considered passable. Lightly scrubbed coins are rejected by some collectors, but others don’t mind the look. The grading services take a similar approach; some pieces get rejected as “improperly cleaned” while other specimens are graded normally.
Complicating the debate further, collectors have become increasingly drawn to original, uncleaned coins. Once viewed as dark and dirty, coins with original patina and “crust” are now becoming extremely desirable. Ironically, coins that once traded at a discount for being unsightly and “filthy” are now worth a premium for being “fresh” and untouched by restorers. This concept sound strange, but actually this phenomenon exists in other collectible fields too (like cars and antique furniture).
While cleaning and restoration is a complex issue, it can be boiled down to one phrase: not too much. Harshly and excessive cleaning is considered a huge negative, especially if it looks unnatural. Gentle cleaning is passable, but this can be a matter of individual tastes. More and more, collectors are gravitating towards coins that are uncleaned and untouched. What used to be considered dirty and unattractive is now viewed as original and fresh.