Please note that i'm not responsible for coins you damage using these techniques below.I'm only sharing with you what i know in cleaning ancient coins.
If you bought some uncleaned ancient coins and you're thinking of cleaning them i would tell you " Don't!".My moto has always been "If you can read it don't clean it!".If you can read what's on the coin or able to make up the face or what ever is on a coin don't try to clean it because you are taking the risk of ruining it. Ancient coins with time interacts with the elements around them and creates a thin discolorating layer all over the coin called "patina".The patina's color can be green ,light gray ,light brown sometimes dark brown and even black.The particular different coloration are connected to the ground where the coin/coins have been found in.Basically the patina is a protection created by nature and is highly desirable to collectors and the patina actually gives a coin its value.
There are three types of Patinas.The first is the genuine one created by the interaction of the metal of the coin with the surroundings elements changing the coin's color.The second patina is "sand patina" which is sand that gets stuck to the coin after going through hundreds and thousands of years of seasons changes that carries with it the heat the cold and humidity.With all these changes and with time the sand around the coin stick to the coin like crazy glue and its not easy to remove.But that kind of patina is also very desirable by collectors . Sand patina comes usually in two colors the ordinary sand color that you find on the beach and the red sand color that ressambles the soil found in your garden.And the third patina is the fake patina created by chemicals like acids,i even heard that if you feed coins to a goat the acid in her stomach are strong enough to create a artificial patina.So...If you're new to this hobby you better be carefull in bying coins that you think look real because they may not be.Only time and handling hundreds of coins and reading about them can help you spot the genuine coin from the fake.As a beguiner in this field it's always better to buy your coins from known dealers and avoid for now bying from the net.Also i think a good place to learn about ancient coins is a museum.Most museums usually have few genuine coins on display.So why not give them a visit and see what real ancient coins look like ?:)
If you feel a coin needs a bit more cleaning use water and soap then dry your coin afterward by putting it in a oven for a few minutes, use the heat of the oven to dry it not the flames. And if soap doesn't do the job then i suggest dipping your coin/coins in olive oil for a while.The oil gets underneath the dirt and sort of pry them of of the coin.But one has to keep a eye on the coins and remove them from time to time to get a nice tooth brush brushing before dropping them back in the oil.And olive oil needs to be changed when it gets green.There's one thing i dont like about olive oil and that is if coins sit in it for a long period of time they tend to become dark in color a dark patina developps onto them.
Another is Electrolysis i've tried it on bronze coins and i've ruined all of them.When you connect this thing bubbles from the coin start to come up to the surface and you see pieces of the dirt falling off to the bottom of the container you are using.After few minutes the metal of the coins becomes sort of full of tiny pits.It gets changed on a moleculaire level.So i dont recommand it.Unless the amperage can be reduced to a minimum maybe then the dirt will be only affected by the current and fall off and not the metal of the coin.
I recently tried rough PLASTIC scouring pads and found out that they are good in removing light to medium dirt.The rubbing should be done under running warm water and in circular moves.Like hold the coin in the left hand and the scouring pad in the right and under running water put your right thumb on the pad on the coin then do half circular turns while applying a constant degree of force.Try 3 to 4 turns then stop check your coin then repeat it again until the dirt is cleared and the coin become sort of readable.But be carefull apply ONLY circular moves straight rubbing gets you through the dirt through the patina then the metal in no time.