IDENTIFICATION OF DYES ON OLD TEXTILES
THE COLORS OF THE LAKES formed by the two natural black dyes logwood and tannin are shown in Figure 4. The two types of black can be distinguished unambiguously by treatment with stannous chloride. Logwood blacks change to violet, and the solution is colored an intense magenta. Tannin blacks lose their color when boiled with stannous chloride; the color is not restored on treatment with ammonia. They also lose their color when treated with 10% sulfuric acid.
Lakes of black natural dyes.