How to identify true and imitation indigo yarn

True indigo yarns are dyed with natural or synthetic indigo dyes, but can also be imitated with sulfide dyes, VAT dyes or direct dyes. The methods of identifying true imitation indigo yarn are as follows:

① A drop of concentrated nitric acid solution is lightly coated on the dyed sample. If it is indigo yarn, it will immediately show white spots and its edge is green.

② After the dyeing sample is lit on the flame, immediately leave the flame and put in the white porcelain plate. Because the indigo leaves blue traces on the white porcelain plate after being heated and sublimated, there is no such phenomenon for other dyes.

③ The dyed sample was cut into pieces and placed in a glass test tube, then heated and dried on an alcohol lamp. Indigo vaporizes when heated, with red gas escaping and blue precipitate as it cools in a glass test tube.

④ The dyed samples can be treated with DMF solution cycle. In the case of true indigo yarn, the dye on the sample can be removed completely and become colorless, while the dye of other imitation can not be removed completely.