True indigo yarns are dyed with natural or synthetic indigo dyes, but can also be imitated with sulphur dyes, VAT dyes or direct dyes. The methods to identify genuine imitation indigo yarn are as follows:
(1) A drop of concentrated nitric acid is lightly applied to the dye sample. If the yarn is indigo, it will immediately show a white stain with green edges.
After the dye sample is ignited on the flame, it immediately leaves the flame and is placed in the white porcelain plate. Due to the heat sublimation of indigo, blue marks are left on the white porcelain plate, while other dyes have no such phenomenon.
(3) Cut the dye sample into pieces and put it in glass test tube, then heat it on the alcohol lamp and dry it. Indigo is vaporized by heat, red gas escapes, and blue sediment is deposited after cooling in glass tubes.
The dyed samples can be recycled with dimethylformamide (DMF) solution. In the case of real indigo yarn, the dye on the sample can be taken completely and become colorless, while the dye of other imitations cannot be fully extracted.