I’ve mentioned that a great way to clean your copper parts is to put them in HCl for a little while. Strips the oxide down in seconds leaving the bare crystal. Well…
Don’t do that. Just don’t. Any residual chloride will cause a runaway, nearly irreversible chemical reaction with oxygen over weeks/months/years. It’s different from regular patina in that it results in yucky white and green spots that flake off as opposed to providing a protective barrier. This is also known as bronze disease.
As an alternative, we just did some experiments today and citric acid (~0.05 - 1 M) works very well too, though it does take a bit longer and needs heat + agitation. Theoretically, though, the shine should last a lot longer.
I’ll try to remember to post some pictures comparing the two methods a couple of months after washing.