^ that was my first instinct and so i have googled 'cleaning old coins and this is the first hit:
Cleaning Coins
Coin Cleaning Rules | Cleaning Different Coin Types | Cleaning Coins Process | Drying Coins
The first rule of coin cleaning is: If you don't know its value, don't clean it, or if you think it is valuable, don't clean it, or if you know it is valuable, don't clean it. In other words, do not clean your discovered coins; leave them untouched and stored in proper holders. The reason for this is that coin dealers and collectors are interested in purchasing coins in their original condition and natural state of preservation.