I have this Justinian half-follis that I think might be a candidate for cleaning. It has a heavy black patina that resembles dried tar, and it kind of looks like it might be hiding detail underneath. In a few places it is thin enough where I can see some brown -colored coin surface. Does it look like it can be cleaned? If so, what should my approach be? Thanks guys! (doing this from my phone so I couldn't put them side-by-side, and my phone is refusing to crop them for some reason - click the photos to enlarge them)
On that one where the patina is ugly, I would use a soda ash and bi-carbonate soda with distilled water. What I find it gradually dissolves the patina in this case the black caked on deposit, if left long enough 2-4 weeks, after emptying the black water regularly the coin will start to look brown.
I've used sodium hydroxide (lye) with usually good success on coins like this, but you have to be very careful and could potentially ruin a coin. If you clean it too much, you might also lose detail.
If you think the black gunk could be some organic material, try some toluene on it and see if it discolors the solvent. If not, move on to some soapy water and use a toothbrush on it.
When I see that black stuff, I throw in the towel on cleaning, I'm never able to improve it. It does tease that there is something underneath, but I've never been able to remove it without doing more harm than good.
I agree with this advice. If this is a real black patina caused by copper corrosion, like copper oxide, then organic solvents will not remove it. You could also try acetone first to see if this is paint. In the old days people used to paint coins to "improve" them. It could be really old varnish or it could be plain old dirt. Kentucky's advised approach should remove any of those without hurting real patina or the coin. If they don't get it off then I would leave it.
The coin is already clean. There is no caked-on dirt or encrustation. The problem is that you don't like the patina. It's very thick and probably obscuring some additional detail. So your only real option is to remove the patina. Just about everyone in this forum will implore you to NOT remove the patina. They are probably correct, but my response is more nuanced: how devastated would you be if you ruined the coin? I think there's a good chance that you will improve the appearance of the coin a little if you used electrolysis to remove the patina and stripped the coin down to bare metal. But be aware of a few caveats: 1) you will end up with a bright, shiny coin that many collectors will find unattractive. (However, the coin will darken over time; be patient and resist the temptation to apply an artificial patina.); 2) the surface will be rougher, with a lot of teeny-tiny bumps and pits. I have used electrolysis to clean hundreds of coins over the years. It's rare, but not impossible, to ruin a coin by doing this. It's common, however, to uncover surface problems that were obscured by the patina. Based on my experience and the type of patina on your coin, I think that electrolysis would probably improve the obverse appearance and reveal a little more detail. I don't think it would do much for the reverse and probably leave it looking rougher and more pitted. Just keep in mind, however, that there is a very small, but non-zero chance that you will uncover something horrible when you remove the patina. It has happened to me, which is why I never use electrolysis on a coin that I absolutely could not bear to lose. Let the hate mail begin . . .
I'm a huge fan of black patina, but that's too thick. I'm afraid that removing it may reveal damage, or pitting etc... that would be worse looking. As stated above, results of cleaning this coin may not be favourable.
Just my opinion, but not removing it leaves you with a buttugly coin. If this were a rare treasure, don't touch it...perhaps sell it to some of the people who seem to like this kind of thing.
No! The other option is to sell/give the coin to someone who likes it as is and buy yourself one that does not offend you. If you think it is a loser now, you should be happy with the few cents it brings.
If you want to check it for paint, a quick dip in acetone will let you know. I had a Mamaea that had a suspicious green patina. I put it into a jar of acetone which instantly turned bright green. It didn't look better, or worse, just different. You are obviously not happy with the way it looks now. Either sell it or dip it, or put it at the back of your collection.
I think this entire thread has good lessons for us. The primary one is to avoid buying coins you don't like the way they are. If it were easy to improve the appearance of a coin, the previous owner(s) would have tried already. I have been collecting for almost 50 years and I see far more "black patina" coins now than I used to. I think many of them have applied surfaces which conceal previous problems and the person who applied it thought the coin would look better with the black than without. Yes, there are some coins with natural black patina, but it should be thin and glossy, not thick and dull like applied "gunk" (as someone called it). Here is a lesson, for those who are paying attention. If the coin is so inexpensive that you like it with black gunk, fine. If it is not likable as is, don't buy it. As usual, Doug is right.
I recently bought a lot of, I think, 5 early 4th century nummi, all of which had a nice even, black patina. Imagine my disappointment when I received the coins and the patina rubbed off after a light brushing with a silver-bristled brush. I think the previous owner just covered them with black shoe polish or something similar. Sometimes coins with a silver wash or a 20/1 copper/silver ratio will build up a thick black patina like the coin in the OP. It can occur naturally. I don't know what the silver content of the OP coin is, though.
