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After attempting to clean the coins in soft drink, most of the dirt was washed off but after trying to clean the remainder of the dirt off was quite hard as being in dirt and mud for several weeks, the dirt has dried onto most of the 20 cent coins and is rusted onto the face of the coin. For even more scientific results I believe you should leave the coins in the soft drink for another 15 minutes and then the coin may be 100% clean.
