I had a silver-looking coin (not valuable, don't worry) that was heavily covered in black. I thought it could be silver, so I cleaned it with the electrolysis method (salt + sodium bicarbonate solution; copper on the positive pole and the coin on the negative). This didn't really work, though it seemed to make the coin less black, so I realized the coin was not silver.
I found out it is a copper-nickel coin. Since then I tried immersion in vinegar + salt solution, which made it a bit cleaner, and scrubbing with sodium bicarbonate, which made it cleaner still. But I noticed that, though some parts are silvery, most of the coin has a slight redness as of copper. Is this the copper from the coin, or did I somehow manage to deposit copper on it? How can I make the coin silvery again?