I bought this roman Q. Minucius Rufus republican denarius a few weeks ago, it has some great detail but also a lot of "dirt" on the surface. I tried putting it in water but that didn't do much. Should I try other ways of cleaning or leave it as it is?
Try industrial grade acetone, not nail polish remover. Do not rub or scrub the coin either. Nice looking.
I wouldn't try acetone on that coin. Might turn the encrusted material an ugly color and not remove a thing.
If the distilled water didn't do much, than you might be stuck with how it looks. I think it looks fine the way it is. Nothing wrong with some crust.
I'm not an acient coin person, but I think it looks great as is. Possible to use something that it will not do what you desire and lessen the beauty.
In addition to the water, a little mild soap (Dawn or equivalent) and a scrub with a toothbrush might do wonders. I have also had some good luck with lemon juice.
Black gunk on silver is typically horn silver (AgCl, silver chloride). It can be chemically treated but be aware that it is a form of silver corrosion, not a simple earthen deposit. Since the silver component came from the coin itself, the surface revealed after treatment may be rough.
The coin looks attractive as it is. I would not touch it (unless you think the deposits are somehow damaging).
Normally you don't clean coins, unless you are a professional at restoring. Because 99%+ of the time, you will do more harm than good. In regards to ancients there is a little more leniency about cleaning. Letting it soak in distilled water can't hurt it? And I have heard people soaking their coins in olive oil but I am not suggesting that. In this case, this coin is beautiful the way it is and I wouldn't want to risk ruining it, I am in the do nothing group.
I don't know if you can get this product that I use in the u.k. It's called EDTA, short for Ethylene-diamine tetra-acetic acid. You use 50 grams in a litre of pure water, and leave it to soak. It really shifts the dirt off silver coins. I generally check the soaking coin after half an hour, then use a cotton bud to wipe off the dirt. It seems safe enough to leave it as I have done for a day, with no damage done. In David Villanueva's excellent book "Cleaning Coins & Artefacts", he also mentions Sodium thiosulphate for cleaning silver, but I've not tried this one. All the best with your lovely coin. Oh, and I honestly think it looks okay as it is, it shows it has age which you'll loose if it's all shiny!
Sodium Thiosulfate. Just use a little, like a teaspoon with half a cup of distilled water. Watch it for 5 mins, maybe another 5 mins, take out, gently wipe with a cotton shirt or something, see how looks. If you treat long enough it will lighten the horn silver, but what results will show some porosity. Usually it’s better than horn silver though, in my opinion
