Shining Pressed Pennies

So I decided to go ahead and finally shine my pressed pennies from my last trip the other day. I put them in a mug of coke, left them for a couple of hours & came back to find that they were not nice and shiny… They had turned black?!?!
That’s never happened before!
It was only the US pennies which turned, my UK ones were marvelously shiny!
So my question is: how do I fix this?!
I got some of them as small surprise gifts for each of my friends & family (the character on each sums up their personality) so I’m quite anxious to get them looking lovely.

Oh no! I’ve never heard of that, all I can suggest is trying another cleaning tip like ketchup to see if that clears the black off. I hope it works!

I’m not a chemist, but I’m wondering if it is because US pennies are mostly made of zinc with a copper coating, and when you press them, the zinc becomes exposed and then was oxidized quickly due to the presence of the acid in the Coke. I’m not sure what you would do to fix it – maybe cleaning with something like Brasso?

I used Brasso on my pennies for our upcoming trip. It works very well, but it is time-consuming. You have to scrub each penny. But they are shiny!

I used 1/4 cup of white vinegar and 1 teaspoon of salt to 2 cups of water, let them sit for about 15 mins or so, make sure they do not overlap, and I flipped them at the halfway time, after I used a paste of baking soda and water with a toothbrush to shine up my pennies, don’t know how this will affect already pressed pennies, but it is worth a shot