@tunniaoc nailed it (first). The glue protects the underlying copper from toning so it stays bright and shiny.
Anyone else noting the odd similarity between this coin and the one posted here?
http://goccf.com/t/351190
"If you climb a good tree, you get a push."
-----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed."
-----King Adz