Once It Has Been Cleaned, Can It Ever Be Restored?

You can't undo the damage - it's too late. The Second Law of Thermodynamics says "You can't dis-clean a coin". Yes, I'm paraphrasing, but it's still true. And applying some kind of "artificial toning" to try to cover up the fact that a coin has been cleaned is, in the eyes of most collectors, an even more serious crime than cleaning them in the first place was.

You have three honourable options:

- sell it (making sure your buyer knows the coin has been cleaned) and using the money to buy a better replacement. This passes on the problem of "what to do with a cleaned coin" to someone else.

- let the coin re-tone naturally. It will probably never look "natural" again and given my previous statement about deceptive artificial toning, you wouldn't really want it to, but it will slowly look "better".

- use the coin as a pocket-piece. Good honest wear destroys old cleaning damage in a way that everybody is happy to accept. Of course, if your coins were beautiful AU or MS pieces before you mistreated them, you will end up with F or VF coins after sufficient pocket-piece time, with resultant loss in value. Ideally, since a cleaned coin has already lost value, you'd want to keep it as a pocket-piece only until the devaluation caused by wear matches the original devaluation caused by cleaning. Personally, deliberately damaging a coin in this way goes against all my collector instincts, but it is the only way to restore the lost honour of a cleaned coin.

I occationally purchase cleaned and/or polished coins to see if any restorations are possible. Over many years of experimenting, I've found that as a rule, most coins just can not be restored to originality. However, note I say MOST. I've had some coins that I took to several dealers and they could not tell they were ever cleaned. This usually was due to the coins themselves as to just how much damage was done.
For example I purchased a 1995 Lincoln Cent Double Die with a big finger print on the obverse. Since it was already in a MS state, numerous dippings in Acetone, Laquer Thinners, Mineral spirits and even jewlery cleaners from Walmart got rid of that print. That was one of the coins I showed to many dealers and they could not tell. Some similar results came from other slightly cleaned early Lincoln Cents.
Coins that had been polished seldom ever returned to anything close to normal. Nickels especially.
I fooled a few dealers with coins that had been harshly cleaned and after treating with gun bluing solutions, they ALMOST, looked OK. But after a closer examination, usually got caught.
One of my problems with attempting to return cleaned coins to normal coins is regardless of how good I do, most dealers around me now know I am doing these experiments so any coins I show them, become suspect.
This really makes for a good experiment. So if you can find a cleaned coin or more, try to do something to make it look normal.

I once read in a coin magazine that likely in the 1800s a very large percentage of silver coins had been cleaned at least once to remove tarnish. It has been over a hundred years now and those coins have re-toned and it takes a real good eye to see that it had been cleaned.

If it was a circulated coin and someone wiped the tarnish off 150 years ago and the coin hasn't been touched since, how would you ever know?

Of course proofs and uncirculated coins wouldn't apply here.

Interesting ... thanks guys, that actually makes me feel a bit better about cleaning those babies 40 years ago (now I only have 60 more years until I can unload them) ... hmmmmm, uh-oh.

CHORE => now I have to go look-up verdigris

thanks again

Not really true. As noted a cleaned coin becomes a cleaned coin. There are so many stories of people that USED to clean coins and all sorts of old artifacts not realizing they were removing some of what was there originally. In the good old days, cleaning coins was almost as common as taking a shower or bath yourself. Many, many people used to spit on their fingers and wipe of dirt on coins to see better what the dates were or some other details. Other situatioins warranted cleaning coins. My Dad had a gas station when I was a kid and almost all his change looked like they were greased. So at the end of the day he would throw all his change in a can of gasoline to get that grease off. People have always cleaned coins to some extent and not necessarily for fooling others. The coins I try to restore to some normallicy are never sold, given away, etc so no one really is effected.
One of my best results comes from placing coins on a piece of wood on a kitchen window for several months.

So I guess hard cleaning to "restore" a coin to its "original" state is a big NO!

But what about general surface cleaning?

I have a lot of coins that just have surface grime on it, just greasy dirt, that just rubbing your finger over will remove it, but then pack it into the crevices of the coin.

Would wiping over these with say a damp microfibre cloth be acceptable or soapy water and a very soft toothbrush. Does that count as cleaning?

Removal of "dirt", as in actual, physical, literal dirt, can be done with a wash in water. Acetone can remove thinks like glue, stickytape residue and the green goo that forms on coins stored in the nastier kinds of plastics. Try to avoid any kind of rubbing or scrubbing, especially where "real dirt" is concerned. Soil can contain tiny but sharp pieces of rock and sand grains, that can cause scratches if you rub it across the metal surface.

All of this qualifies as "acceptable" cleaning, as you're removing surface contaminiants that aren't chemically bonded to the coin. Even so, you might still end up with a "cleaned-looking" coin, especially if the dirt (or whatever) has been there a while: the coin underneath the dirt might still be shiny and new, while the rest of the coin's surface exposed to air for decades has oxidized or tarnished.