Probably way to late for Ben but lots of information here...http://metaldetectingworld.com/clea...n_coin.shtml
I would not recommrnd most of the techniques for collectable coins but, if you have nothing to lose with badly damaged coins, could be of use.
I would be interested in comments.
Me? I am rushing off to see if I can find some Aqua Regia (prepared by mixing three parts of concentrated Hydrochloric acid (HCl) with one part concentrated Nitric acid (HNO3) followed by dilution to about 1:10 -NOTE: Aqua Regia is the only acid that will dissolve gold and platinum). Nitric acid is very corrosive -will even dissolve glass!
Just the thing to soak my gold coins in; NOT.