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I know you can clean silver coins by placing them on aluminium in boiling water and some soda crystals, this will remove the tarnish,
would this be a desirable thing to do to a hammered coin or a cardinal sin? :-\

Cheers
Salty

It's not always necessary Karl but with some coins you can really improve the way they look by toning back to the desired effect, to me it isn't a cardinal sin and I do what i think is right to make the coin more attractive.

You can also spit on them Kark, lol , or dip them in water,  then place in foil and fold over,  hold between finger and thumb  till it heats up and smells of rotten eggs,  ohhh what a pong,   this will loosen the black vertigra,
 when the smell subsides unwrap it and just genly rub over  the top with your finger  to highlight the detail and leave the black background there.

Personally the most I will do to clean a hammered coin is to wash the mud off it as I like the patination that has accumulated over the centuries, but on occasion I have used electrolysis on coins that are pitted and this method, in my opinion, gives better tresults than the tin foil technique. 

as a rule i don't clean any more than just wash the dirt off . ive seen lovely looking resulted with cleaning but worry about long term damage . i always feel it just cant hurt to just leave them just as they come out of the water . if you have a batted poor condition coin that's different and a clean can make a more pleasing looking find but i would not do ANYTHING on your good coins  :)