I personally would not do anything more to it that a good toothbrushing [with soft bristles] with dish soap & hot water, rub with soft cloth, repeat until hopefully you can at least get a date.
After that I would soak it in olive oil for months, and give it another toothbrushing and see if any progress was made.
There are really no good ways to clean an old dug copper without trashing it, and trying to make it into something it is not.
Post a pic when you can [scans work just fine too if you got a scanner].