How To Clean Coins?

It is generally best to not clean coins as most methods of cleaning can damage a coin. Any type of rubbing will create light thin scratches called hairlines so rubbing is to be completely avoided. If a coin is heavily toned or tarnished, the only way to remove it is to dip it in a chemical solution, of which there are a few to choose from. The problem with using said chemicals is that they strip the toning which is chemically bonded to the silver so some silver is removed in the process. It can help a coin in a few cases and in skilled hands, but in most cases the resulting coin has a very dull appearance and unnatural color. Coins that have been "dipped" sell for considerably less than an original coin, that is why the connotation of a cleaned coin is a bad thing.

As for dirt, grime, and other stuff, a soaking in distilled water followed by drying on a towel is ok(just no rubbing). You can also utilize organic solvents such as acetone or xylene. The solvents will remove oils, finger grease, fresh fingerprints(but not old prints), green slimy PVC residue from old soft 2x2 flips, and other organic gunk. These solvents do not interact at all with coinage metal and will only remove surface contamination. For proper usage, just use the search function at the top of the page as there have been many posts that have addressed the topic. Obviously, these solvents are flammable so you should take great care using them- open ventilated work area and away from any ignition source.

With all of that said, if you are unsure of what you are doing, it is best to just stick with a simple soak in water. It is perfectly natural for copper to turn brown and silver to tone a number of different colors, it is all part of the oxidation process that forms a patina on the metal surface. The vast majority of old coins should not look like they were just struck yesterday with full bright color, that is usually an unnatural appearance. There are a few coins with that level of preservation but only because owners through the generations have taken great care in preserving the coin.