Without the photo of the back of this button, we cannot classify it.
Casein identification guide: casein is a thermoset semi-synthetic polymer made from milk, hardened in formaldehyde
2. Easily hand-carved, used by skilled craftsmen and sculptors, mostly European, to create unique quality pieces.
source: Button Materials A-Z: Identification Guide
by Jocelyn Howells
The above is based on the National Button Society's Classification System.
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