Coins can generally be cleaned in a variety of ways. They can be cleaned with a soft toothbrush dipped in a mild soap and water. They can also be soaked in a cup of white vinegar with a tablespoon of salt dissolved in it, for example. Muriatic acid can be used to clean coins that are covered with a grimy crust. You have to be careful cleaning coins, especially when you're using strong chemicals such as acids. You can easily wipe the details off a coin and turn it into a worthless piece of metal.
Choose a well-ventilated area to work in. Muriatic acid fumes can cause lung damage. They are both toxic and corrosive (see resources). Protect yourself with long-sleeved shirt and pants, closed shoes or boots, rubber gloves and safety goggles.
Dilute the muriatic acid with water in a small plastic bowl. Pour the water in the bowl first. Then pour the acid in slowly so it doesn't splash. Use one part acid to 20 parts water. Carefully stir the solution with a plastic spoon.
Place the coins in the diluted muriatic acid to clean them. After the bubbling stops -- it can take a half minute to as long as a couple minutes -- remove the coins from the bowl. Rinse them well with clean water.
Mix up a solution of baking soda and water in a clean plastic bowl. Use one part soda to three parts water. Place the coins in the solution for 24 hours.
Remove the clean coins from the soda water and rinse them with clean water. Then, dry them with a soft cloth.