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a class of black dyes consisting of Nigrosine Spirit Soluble, Nigrosine Water Soluble, and Nigrosine Base B (oil-soluble). Nigrosine Spirit Soluble is prepared by heating a mixture of aniline, nitrobenzene, and cast-iron chips. It is a mixture of quinonimine dyes and is used to color plastics and varnishes. Nigrosine Water Soluble is prepared by sulfonation of Nigrosine Spirit Soluble and is mainly used in dyeing leather goods. Boiling Nigrosine Spirit Soluble with caustic soda yields Nigrosine Base B, which is used in the preparation of shoe polish and as the ink in typewriter ribbons.