Gold and silver coins yes that would be okay... if they are dug up then you will probably have to clean them a little, usually just to identify them, but you must do it with care. Base metal cupro-nickel, nickel-brass even more care, they are harder and thus the brush won't damage them as much as silver but they are more reactive that the gold/silver coins... it'd be pretty pointless to clean cupro-nickel or nickelbrass anyhow cos those coins are usually only worth anything in absolute top grades Copper well there is no safe way to clean copper/bronze coins that i know of so you'll just have to be careful. If we're talking UK coins then anything hammered clean with care and it'll probably still be worth something cos most hammered coins have been dug up at one point or other and the whole cleaning issue is totally different with those... obviously you don't vinegar them or anything but if cleaned carefully with distilled water and a soft brush they will probably still go for something nearer the catalogue value. Roman coins etc, clean away... most Roman coins on the market have been cleaned anyhow at some point or other, try to minimise the damage though. Dug up Gold and silver Milled coins before 1816 are probably worth cleaning cos they'll still fetch a few quid. (nothing like the book price but they be worth something), gold and silver 1816-1920 are always worth their bullion, most of them will propbably be low grade and not worth too much anyhow so cleaning them won't knock that much off. Any milled copper or any coins after 1920 probably aren't really worth the hastle. They're not worth all that much anyhow.