The tarnished cap will not change the effects of the copper. If you'd like to get it clean just because you do (I do it too) use some citric acid. Just mix a little with warm water and clean the cap with it. That should do the trick, you may want to let it sit if it doesn't come right off.
--- In email@example.com, "last2blast" wrote:
> My pot still has a stainless steel pot that it easy to clean with just soap and water, but the inside of my copper cap is tarnished (dark gray). This copper cap is also showing signs of bright green tarnish near the seal. I clean the inside of the cap with a plastic scrubber and dish soap, but it does not remove that dark tarnish. I could mix up some cleaner out of vinegar and ammonia if that copper should be cleaned down to shiny metal.
> Should that tarnish be removed from inside of my copper cap?