Looking to use something that is usually around the house.
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@Zengirl2, I think for the best results you'll want to use acetone, which is easy to find online and in a lot of drug stores and art stores (and maybe even home improvement stores). Acetone is ideal because it is miscible with water and with oil, so it cleans away a variety of kinds of grime. It also dries the surface of the copper since it will dissolve with water and then evaporate. Having a dry surface is probably important to reduce the rate of oxidation of the copper (which I assume, based on electronegativity, interferes with its reactivity with sulfur). Isopropanol has pretty similar properties, but the commonly available rubbing alcohol in drug stores is 70% isopropanol and 30% water. So, that mixture is less effective both at removing any oils, and at leaving a dry surface. You could try both acetone and rubbing alcohol and see if you observe any differences in their effects on the copper.
Just a note that we'd looked at some single-use acetone nail polish removing pads, because people wouldn't necessarily need a whole bottle -- but at one pad per day per copper piece, that'd get used pretty fast.