Casein is a semi-synthetic plastic present in skimmed milk. A rennet is used to precipitate it out. The casein is mixed with water, dyes or pigments and drawn out into shapes for machining. It is then cured by immersing it into a solution of formaldehyde. In the 1920s and 1930s it was known commercially as 'Erinoid' and was used to make furniture handles, buttons and other decorative items.
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