Why Is It Bad To Clean Coins?

Conserved?

The olive oil, distilled water, acetone and like methods. I won't use dipping either because that term can also be confusing.

When coins are struck the planchet moving over the die, etches microscopic lines in the dies. The lines are transferred to the coin as "flow lines" and are what gives a coin luster. When coins are dipped in a chemical such as a weak acid to remove toning or tarnish or corrosion or whatever you wish to call it, the acid attacks those lines. The claim is usually that one or two dips won't really effect the flow lines that much. But no one really knows how many times the coin has been dipped before. It could have been a number of times because once the original toning is striped away the metal is re-activated and the coin will probably tone/tarnish even faster then it did the first time and probably in a very splotchy uneven manner as well, which will only make someone else want to dip it again. Sooner or later the flow lines will be blunted enough to impair the luster. When that happens, it's over--R.I.P

This is the whole irony of dipping. It's done to expose a coin's luster again and make them pretty and eventually it is what will destroy the luster--Forever.

When it comes to cleaning coins, there is almost no problem that can be solved without creating a new problem. Even removing dirt will leave the area where the dirt was with a brighter color than surrounding area.

There are proper ways to "clean" or conserve a coin. When done right, it removes none of the metal, causes no direct harm to the coin, and removes only the agents that the "cleaner" intends to remove. That said, I wouldn't aim at removing dirt off the coin unless the coin is already corroding. Crusty, original surface coins are almost always better than their ugly, "white" counterparts. If a coin can be guessed as cleaned, it's value is usually significantly decreased, if not decimated.

It also depends on your future plans for coins. If, for instance, you just want a nice, shinny looking coin collection, not worried about future values, clean away. Way, way back when I was a kid it was normal, even necessary to clean all coins. If you showed anyone a coin collection the first thing you would hear is why don't you clean them, they are all dirty. And clean them we did. Used to use things like baking soda/water, gasoline, kerosine, battery acid and anything else that would get that dirt off.
As already noted lots have changed since the good old days. Antique Roadshow usually explains removing a layer of material deminishes the value due to loss of originality. An example is an old car, 57 Chevy, if all hoses on the engine were replaced with wrong color hoses you've lost originality. If radial tires put on, not only lost originality, but now unsafe.
One problem with cleaning coins is if the so called dirt is in reality a substance formed from the coins metal with an outside source, the removal of that substance removes some of the coins material. The darkening of a Copper coin is in reality the normal Oxydation process forming CuO, Copper Oxide. Further exposure to the air with moisture and Carbon Dioxide will form Copper Carbonate, a greenish Patina or layer. Removing this removes some of the Copper and it will start all over again. Continuous removal will slowly show up as massive wear making the coin less desirable to collectors.

Very briefly it is because while it is POSSIBLE to sometimes clean a coin in such a way as to improve it, it is not easy to determine the best method to use. And it is VERY VERY easy to damage a coin by cleaning it improperly. So the knee jerk reaction is to always tell newbies just not to clean them. Because every coin has to b evaluated on its own merits an you could easily write at least a pamphlet on the advice and methods for cleaning each individual coin because what works on one will not work on another.

thq, you would be surprised by how nice that black, corroded, 1795 cent would look after a year or two riding around in our pants pocket with your other change eve without any preliminary cleaning. I've carried black corroded 1798 and 1802 cents for a couple years and come out with nice brown fairly smooth coins. They were pretty much junk when I started and fairly attractive collectible coins when I finished.