you can find many different ways to clean your ancient coins on the internet and most sellers will send instructions with your order.most of them will tell you to soak them in distilled water or olive oil from anywhere from minutes to days to even months,ive even seen sellers selling "pre-soaked" coins.one even went so far as to say his were pre-soaked for a full year! LOL                                                   

 ive tried all these meathods the distilled water and olive oil do work but take a real long time to yield any results,i dont have the patience for that,some wont show any progress no matter how long you soak them.it is these stubborn coins, that this guide will be most helpfull. what i did was to go to wall-mart or any hardware store pick up a bottle of  LIME AWAY or C.L.R. (calcium,lime,rust) I used a shot glass to put 1 coin in 1 glass with about 1/2 oz of cleaner . as soon as the coin is in the solution it will start to fizz like an alka seltzer tablet,only let it soak for a moment(15-30 seconds or till fizzing stops) remove the coin and wipe with an old rag or towel use a little elbow greese and watch as centuries of crud dissolves in your hand.only a little will come off at a time so you may need to repeat.once you can see the actual coin or details coming through i suggest then switching to a more non corrosive more conventional meathod like distilled water or olive oil

hope this has been helpfull it works for me,i would not leave them soaking overnight you might wake up to a shot of mud. try it on one that looks like it may not become a prize coin and see for yourself.