I can be brief about this subject. If you don't know exactly how to do this then don't do it.
You can easily damage the coins. However if you want to try it, first take a look at the Royal Dutch Mint
or the website of "het Geldmuseum"
You can find all the information you need about cleaning coins.(only in Dutch)
On Tarcisio's website you can find an article about
cleaning coins through electrolysis
At this moment I have in my collection coins from a lot of countries, but from a
lot of other countries I don't have any coins. Here is a list
with countries from which I have coins in my collection. I am very interested
to trade coins with a collector who offers coins from countries which are not in the list.
Besides coins from countries which are not in my collection, I also would like
to add specific coins to complete specific parts of my collection. Here you can
find a list
with these coins complete with catalog number and period. Since I don't
collect coins by year the specific year is not important, unless there are
issued in only one year. I'm only interested in circulation coins.
Coins from various countries are sometimes dated according to different era then the Christian era commonly used in the western world.
Here you can find a table with some used era and a short explanation about that era. This might be a help to determine some coins.
Over the World various numeric symbols are used. Here you'll find an overview of most of them which can help to identify coins.
Here you can find some short movies about coin collecting, storing systems, cleaning of coins, various collecting theme's etc. (comment only in Dutch)