If you're only trying to remove the dark tarnish from the native copper (and leave it shiny like a new penny), one of the best things to use is a product called "Copper Dip" (it used to be called "Copper Brite"). It transforms the native copper almost instantly and with no scrubbing (all you have to do is rinse it off). It's a dry granular acid mix you hydrate yourself -- I make it in a one gallon plastic milk container. You can get it in very large amounts from lapidary supply houses. One of the places that carries it in more affordable amounts is Keweenaw Gem and Gift in Houghton, Michigan (in the UP). They sell it in amounts that can make up to one gallon for around $8.50. You can order it over the phone at 800-554-8447 and they'll mail it to you.