cleaning glue from coins/charms

[hikeinmts]

I recently purchased an old card of 30 different Korean charms. They were glued to the card. All but 4 or 5 came unglued
on the trip from the States to here in Korea.
Question: How do I clean the glue off the back of the charm without harming the charm, or with the minimal amount
of harm? No info on the card about what the charms are made of, but I would think that they are either brass or copper.
They are heavy, but I don't think they are iron.
Thank you for any help with this "sticky" situation.

[Iron Patch]

If they are 100% metal boiling might do the trick.