After seeing a neat YouTube video (everything starts this way, doesn't it?) showing a chemist removing the copper plating from a zinc-core penny to make a solid zinc penny, I decided to try the experiment myself. Upon further research, I saw that Theodore Gray the element collector also had a solid zinc penny, but he had used cyanide to remove the copper plating. Since I didn't feel like exposing myself to extremely toxic salts, I decided to go with the YouTube method.
The reaction uses calcium hydroxide and elemental sulfur to oxidize away the penny's copper plating but not the underlying zinc. If I remember correctly (I did this experiment some time ago), the reaction smells awful, so it is best performed outside. I didn't have any calcium hydroxide, so I substituted in sodium hydroxide drain cleaner and used gardening sulfur as my source of sulfur. After that, I simply followed the video's directions.
The pennies come out of the solution blackened with copper oxide, so I tried to remove it with a scrubbing pad. That got rid of the copper oxide, but it also scratched the zinc pennies, making them less shiny than they otherwise might have been. I would recommend going with the YouTube video's recommended cleaning method using ceramic cooktop cleaner. I suppose the dullness could also be because of my substitutions, but the reaction still worked well using sodium hydroxide, so I doubt that was the case. Nonetheless, I was really impressed that a reaction could remove only the copper on a penny while leaving the zinc untouched. After polishing the pennies with a Dremel wheel, I was left with ten solid zinc pennies.
Platings provide opportunities to observe the subtle differences in colors of transition metals. While nearly all transition metals are some color of gray, some have different hues. I had some pennies with a layer of zinc or nickel plated over the copper, so I put them together with the solid zinc penny for a nice comparison. Nickel definitely has a golden hue compared to zinc, which I find interesting.