After prolonged exposure to molecules in the air, coins have a tendency to become coated in a white or green layer of oxidation. Removing this oxidation requires acidic solutions, such as lemon juice, vinegar or sour milk. While there are more powerful liquid cleaners used by experienced coin collectors, the following procedure might be all you need to restore the shine to your oxidised coins.
Fill a small bowl halfway with clear vinegar.
Place the bowl in your microwave and warm the vinegar for four to five seconds.
Add around 15-30 ml (a few teaspoons) of salt to the vinegar and stir with a spoon until the salt is completely dissolved. Vinegar and salt combine chemically to create hydrochloric acid.
Drop your oxidised coins into the solution. Swirl the mixture around the coins every 30 minutes or so and flip the coins every hour.
Remove the coins after four or five hours and place them on a wad of kitchen roll for drying. Repeat the procedure as many times as you like with a new batch of vinegar/salt solution.
Use lemon juice as an alternative. In both cases, the acid eats away the oxidation from the coins, leaving a sheen behind.
Do not leave your coins in the vinegar/salt bath for more than 48 hours or the acid produced will begin eating away at the metal.