Harper's new monthly magazine 239, 1870
To distinguish between Berlin blue and indigo or aniline blue, as dyes on cloth, it is only necessary to moisten a small portion of the colored cloth with a few drops of a solution of fluoride of potassium in water, and to direct upon it a current of steam. The Berlin blue by this process is removed, while indigo and aniline blues remain undisturbed.
The same method can be used in distinguishing nut-gall ink from that of indigo carmine, the former being destroyed in the process, while the latter assumes a red color.
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