Great questions! The reaction doesn't harm the metal. You could allow it to tarnish again, and then do this cleaning method again, and the coin would remain the same. Any kind of scrubbing you do (like with toothpaste or baking soda) will definitely remove any dirt still on the coin, but that will definitely wear the surface. Cleaning coins with numismatic (coin collector) value is definitely frowned upon. This coin was only worth a couple bucks to a collector. It was worth it to me to clean it for the demonstration. I also like cleaning up other old copper coins and items (brass and bronze work well with this method also).

For copper, it's dissolved salt and vinegar.

For silver, it's dissolved baking soda and aluminum.

Those are 2 of my 800 YouTube videos. Most of the rest are shadowbanned (almost no views) because I talk about real things, like silver as sound money, the government is corrupt, end the fed, and so on.

Those 2 videos alone account for 1/3 of ALL views on ALL my videos!

Anyway, have fun cleaning your old (non-valuable) coins, rings, chains, etc :)