lemon juice or vinegar with salt will EAT THE COPPER. hydrogen peroxide and/or 1 minute sessions of electrolysis will be more gentle on the details. lemon juice adds flavor to tuna, save the vinegar for some salad
Ace, thank you for your comments. Based on them I pulled the coin out of the lemon/salt mixture. I don't think I did much damage - nature hurt this coin over 150-200 years waaayyy more than I did.
I tried the hydrogen peroxide, which caused bubbling but overnight it didn't seem to make much of a contribution.
Electrolysis, however was incredibly effective. My son and I watched a video on how to do it and then gave it a try (I'm comfortable with this stuff since I have a physics undergrad degree).
Amazing results (relatively speaking - the coin is still badly degraded).
After applying some oil to make it shine here's what I'm able to see:


I'll never get a year off of the coin, but I can clearly see the stars around the outline of the bust facing left.
Sadly the back is completely destroyed by nature:
My best guess - based on the "soft point" of the crown on her forehead and the general shape of the bust - is that this is a Large Cent from the 1820's. For comparison (thanks to this website for the pics:
http://www.largecents.net/collection/photoindex.html)
Thank you for letting me hijack this thread and for the comments about lemon juice, vinegar, hydrogen peroxide and electrolysis.
- Brian