I bought a couple of 'project' coins. The first one, the one this thread is about, was quite cheap and in poor shape, and even though I normally go for coins in much better condition and in good detail, I decided to go for it as I think it has some potential. It is an anonymous Byzantine follis (class D from the reign of Constantine IX, 1042 -1045 AD). These coins are hard to find in good shape. I was attracted by the fact that the Christ side seems to have really good detail hiding under the dirt. At least I thought it was dirt when I first saw the seller photos. In hand, the brown layer is quite thin and it is almost like a second patina on top of the green patina underneath. I thought that maybe I can end up with a pretty nice coin if I clean it properly, but again, I could be completely wrong... So, how do I go about it? Not quite sure where to start... I read many threads in cointalk from members that do this kind of thing regularly, so I am thinking of soaking it in distilled water for a few days, then deep freeze for a few hours and then gently rub it with a soft toothbrush. Is this a good start? What shall I do next?
Cleaning byzantines is a totally different thing than cleaning LRBs. Metal is softer and patina is generally less stable. For your coin, i recommend the dry cleaning. Gently pass a dull toothpick through the deeper areas, careful not to scratch the coin. Then brush it. Do not wet it.
I see the coin as nice now so would suggest the 'First do no harm' approach. Dry brushing with soft tools (toothpick, tooth brush) might help but you have excellent face detail now. Take care. I wish mine had that face!
If this is okay with everyone concerned, I'd like to second @dougsmit. Hippocratic Oath: 'first, do no harm.' ...Right (cf. John Cleese, "Fawlty Towers"): the .pgs of my example are hiding somewhere. Never mind. This one of yours is at least as good. Otherwise, @catadc is dead on, especially for the obverse. Since you're very evidently on my page about cleaning, especially in real time, it would be redundant to say, 'proceed with caution.'