Okay, I know that there is this modern trend towards not cleaning a coin EVER EVER. I understand that a great toned coin IS very attractive so I understand the sentiment that one would prefer to leave a nice tone on the coin.
In ancients its all about preserving the patina...in modern coins a patina would mean the coin is pretty much ruined...by patina I mean a thin or thick layer of corrosion on copper coins that is often a pleasing green or desert sandy. There is a reason for this mainly having to do with the effect the corrosion has on the coin, pitting...so its better to keep the corrosion on the coin and keep it from further corroding (cleaning, storage, waxing, etc..). But the coin is more than a thousand years old so its not ruined...it has a pleasing patina...
But if we see a green spot growing on a Franz II Kreutzer we would panic...we dont WANT the green crust on it. I dont at least.
Now if its nice as is, dont do anything. But I have spent some time in the trenches cleaning VERY dirty ancients in lots of 50 to 100 coins. I also buy old medals and coins from anywhere between the beginning of Imperial Rome on up in all conditions.
When you have 200 ancient coins to clean, some of them encased in stone hard dirt, you try every trick in the book hoping that you can get that coin out of the crust well preserved. I know there are others here who clean ancients and can relate, what is the chance on most lots of 10 to get a well preserved coin, by that I mean a coin like these or better:
For me its pretty rare, if I am luck 1 or 2 out of 10? but when you get a nice coin cleaned up, preserving that patina...its exciting.
But trust me when I say I have used electricity, vinegar, lemon juice, heating and cooling, tape and glue, picks of all variety, different cleaning agents, you name it. Mostly on coins that are simply encrusted with more than a thousand years of crust.
I get coins and medals from 16 to 1800's and they are dull and grimy. They have been sitting in an old drawer. Many are in almost perfect condition but have somehow gathered filth that sticks. They are often hundreds of years old...it happens.
here is one I posted in another thread, sadly I dont have a before shot:
When I got the coin above, it looked like it had been sitting in an ashtray that had dried spilt soda in it. It was sticky, black with a layer of something nasty.
It looked like a nice coin but I could tell it was just filthy so I dropped it into some Distilled Water like I would a dirty ancient. I let it sit in the water for days, this is something I have done with a lot of coins that come to me with black splotched of crust...you would be amazed what a nice soak in distilled water can do. If it has a lot of black dirt clinging to the details, just let it soak for longer. Take it out every once in awhile and gently firmly pat it with a very soft cloth...you will see the change. If you are brave, after the filth is soft you can pick it out of the pits.
With the coin above as well as other bronze and copper coins I will go one step further. After a quick soak in distilled water to loosen up the crud, I let it soak for a time in olive oil. I have soaked coins (roman crusted) in olive oil for a year with no significant change in the color of the metal. I soaked the coin above for several weeks, took it out, did a quick soak in hot water with a mild soap....its like a new OLD coin. It did not lose its toning. I like a toned coin for the most part more than one with a brilliant uncalculated shine.
Before I started cleaning this coin, I didnt even know the fields behind the lion had horizontal lines...they were all filled in with crud. I think its 100X more pleasing than it was before, no artificial toning, not stripped...perfect.
I have done this with other coins, always with a pleasing result because neither distilled water or olive oil can effect a coin fast enough to ruin it like dipping. You can easily gauge when enough is enough because it is a matter of days and weeks instead of seconds of a dip.
This is a medal I used water and oil on, it looks outstanding now as before it had a dull haze of grime with dirt collecting around the letters in the inscription:
I could hardly see detail on this coin when I got it, it had a coat of black grime and a heavy dark patina, IMO it looks much better now, I only use water on the silver but it still has tone, just not so thick:
Now I know people just as a reflex just say dont do ANYTHING, but its better to let people know what is safe to do and what is last resort or probably best undid unless you have experience.
If I were to sell any of these I would be truthful and say exactly what was done to them. I would call it very light, minimal cleaning. The only thing I did was remove grime, thin heavy tone, and in some cases stop a couple cases of bronze disease. The coins are better for it, more attractive, and they have potentially harmful residue that has collected on them in the course of a few hundred years removed.
If you dont WANT to clean a dirty coin, more power to you, but I know a lot of collectors want to and there is no reason why one shouldn't. Hell, I have soaked handfuls of world coins simply filthy from circulation and many of them have come out looking as good as new. I only do Distilled water for white metals and water and/or oil for brass, bronze, copper.
like these common world coins that were filthy but now rather attractive (one even had a bit of a rainbow tone under the grime):
In conclusion, a good soak in distilled water does wonders and no harm, sometimes a coin is in poor condition and if you can do some very minor things to improve it, why shouldn't you? Certainly all coins do not need anything done to them....but many do. I am positive that before many coins are sold they get a minor touch up...
The way I was told distilled water works is that water bonds well. When you have mineral water it is bonded with minerals, distilled water is closer to pure water thus it works by softening and bonding with the grim on the coin, often times a grimy coin will come out, with a pat dry you can see the difference...it might take days but there is almost always an improvement unless what you are trying to get of is actually corrosion.
Lets just be honest about it, I dont like grime on coins...I am not altering the coin, it just gets a bit of a bath...MANY coins need it...I think there should be a new movement calling for people to give their dirty coins a bath.
To be honest though, it matters how you get the coin as this stuff is often done before you get your hands on it and you cant tell because its just the difference between a nice toned coin and a nice toned coin with dirt in its legends and fields.
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