How Much Will Cleaning A Coin Lower Its Grade?

Sounds like a hypothetical question that has a firm basis in reality.
EVERY coin that has been cleaned, needs to be examined and assessed on it's individual merits, to find to extent of damage caused by the cleaning.

Expect to loose anywhere from 5% to 50% of the value by the use of non abrasive cleaning.

Some ancient coins HAVE to be cleaned, after being dug up, just to enable identification!

It may or may not change the grade, as mentioned in previous posts, it will lower the value no matter what, even a lightly cleaned coin being cleaned more is not a good thing. Conservation on the other hand can be benificial for a coins health, but leave that to an expert, I've been handling coins for a few decades now, both as a collector and as a dealer, and I have a lot of experience, but beyond basic stuff like acetone baths, I don't even mess with coins myself, it's a pretty specialized field and only a few people are really good at it and know what they are doing.

For conservation of a rare coin see http://www.ncscoin.com/ they are the only company that is doing conservation/cleaning on a level that is acceptable to collectors, it's not exactly cheap, but you get what you pay for, and even NCS can't conserve every coin they receive, each one will get a different method of treatment.

Cleaning will not lower the value on ultra rarities, for example the lowest grade 1913 nickel would be worth the same cleaned or uncleaned. The same with the 1870 S three dollar gold coin. The only other time where cleaning will not affect value is for low grade bullion coins which are traded for their metal content and not their appearance. For example a low grade cleaned common date franklin will go for the same price as one that hasn't been cleaned. But besides those two exceptions cleaning a coin will nearly always result in a substantial decrease in value.

Suppose you had a cleaned Morgan, that looks like someone took silver polish to. Could you, by carring it in your pocket, be able to remove all evidence of the cleaning? I realize that this would lower the grade, but I can't stand the shiny thing as it is...

I have carried two silver dollars as pocket pieces for several years, one Morgan and one peace. they both are bright white and have a cleaned look to them even tho they have not been cleaned just spending years in my pockets.

I was foolish enough during my first days buying silver to think that cleaning was fine. I can see no damage so it is all good right? Wrong, use a 10X loupe and enjoy the 1000's of micro scratches you just ruined the coin with....

I notice nice graded coins where the seller who knows his coins claims it has been cleaned before being slabbed, but due to decent grade and real nice color toning or key date it still brought big money and the cleaning was irrelevant.....

So there are always exceptions to the general rule IMO....

If I buy a Morgan in a belt buckle, I would buy as scrap silver an not as a Morgan silver dollar. As to say it no longer has any numismatic value. However, often these belt buckles were never worn and just set collecting dust forgotten in a drawer for 35 years (protected an in-cased acquiring no wear) until someone found an realized it had value. They can be removed in some cases an turn out to have no damage an still list as BU perhaps, or so I have heard.....