I bought a little book today, from a charity shop, that has proved very informative - but has to be taken with a pinch of salt in places! It is a 1970 issue of "Teach yourself Numismatics" by C.C. Chamberlain. One pearl of wisdom I thought I'd pass on:- "Cleaning Coins. Where a copper coin has a fine patina or a silver coin is richly 'toned', they are best left alone. A badly corroded coin, however, can be improved by brushing with sawdust soaked in diluted sulphuric or citric acid. Ammonia will remove green oxide." So now you know!