I use vinegar to clean copper. It's why ketchup works to clean blocks, the acidic content of the vinegar. You can also use muriatic acid, but it's NASTY stuff. Not recommended unless you are used to handling dangerous goods.
And yes, I know about using gloves to handle copper. We do copper cladding and flashings sometimes here at work. I still think a clearcoat would be a good idea as copper tarnishes anyways when exposed to air.
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