People attach them to paper, attach coins to another coin. We see this a lot here. The clear glue is Elmer's glue. The harder types are acrylic/epoxy/super glue. (The list can go on and on) These may just peel off the coin with a fingernail? or have to be dissolved, with something like lacquer thinner. Sometimes it is permanent. But it is not a mint error, so it can be spent if it is not a variety coin.