Archaeon, copper, bronze and brass metal items are one of the hardest things to get clean. Like buckleboy says, if you remove too much the fine detail can be lost. Of course it depends what the gunk is on the coin, and how much. Bronze coins in general are wrecked by today's chemical / lime mix spread on farmland and most have lost their features.
Here are some ideas (no guarantee's)! Always rinse things off with distilled water when you are done, dry until it is really dry, and then seal with microcrystal wax or something like that. The tools that you should have are toothpicks, soft brushes, a toothbrush with shortened (cut off) bristles, and things like that..
1) One method that I use for these coins is to use Olive Oil. Put the object in the oil overnight or longer, then use a toothpick carefully to get dirt out between the letters, and then repeat. And repeat...and repeat...and, well you get the picture. This process can take days, weeks, or even months. You must be careful with anything you do to the coin, if it is valuable of course. Try it first on coins that you know are not worth anything first.
I am not saying that this is the most wonderful way to clean copper and such (I just said its is very hard to clean these things), and olive oil is a bit on the acid side so there is always a chance that it 'could' damage the patina.
One improvement can be made in the above description. If you have or can get an old coffee maker, you know one of those things that have a 'keep warm' metal plate on the bottom, then you can put your object and oil in say a very small glass container and put it on the plate. Turn on the maker of course. That will improve the process, but still it can take a long time. The advantage of this method is that the object will in the most cases never be much worse off than when you started. Many many people use this method.
The method can also be used with silver coins, add a little lemon juice to the oil, then using the 'keep warm' plate use the procedure above.
Hey, I know it takes forever, but you WILL be amazed at the final product. The key in this method is removing the gunk by a tiny layer by layer, and not damaging the patina...
2) Buy some acid free vaseline, and rub some on the coin. Let it sit for some time and then carefully rub or with a soft brush brush it off. And, just like all the methods, repeat.
3) This is another method that I have heard of : If you wish to stop further deterioration clean the coin up as much as possible. Place coin in Bensotriazole solution for 10hrs, bake dry and glaze with liquid plastic / wax to seal. I am not sure if this method removes or changes the patina.....
4) Some people use ammonia thinned out to 10% to clean the coins, then rinse with distilled water.
Be careful with bronze disease (bright green spots). You must remove that mechanically, if you don't it will continue to eat up a coin or artifact. Then the professionals treat it with BENZOTRIAZOLE (Benzotriazole in Ethanole) to stop it then, but I don't know how to tell you do do that. I do know that they use about a 6-7% solution for a hour to a day to stop the disease. Then dry it carefully and wax it.
REMEMBER try all methods on not valuable coins or objects before your valuable item. OR JUST LEAVE THE OBJECT IN THE CONDITION IT WAS WHEN YOU FOUND IT. This does not apply if the object has bronze disease!
