nigre
/ˈnʌɪɡə/(also niger) Soap-making
noun
A dark-coloured solution of soap and impurities (colouring material, alkali, etc.) which settles out from the pure soap during the final stage of the manufacturing process.
Origin
Mid 19th century; earliest use found in Andrew Ure (1778–1857), chemist. Apparently from classical Latin nigr-, niger black.