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Casein identification guide: casein is a thermoset semi-synthetic polymer made from milk, hardened in formaldehyde.
8. Takes a high polish, but the back may be less polished, helping to differentiate from Bakelite, which is tumble-polished on all sides.
This button has two OME’s — white metal and blue satin glass.
source: Button Materials A-Z: Identification Guide
by Jocelyn Howells