Indigo blue, also known as indigo dye, is an organic compound with the chemical formula C16H10N2O2 that appears as a dark-blue powder with a coppery luster. The chemical was first extracted as a natural dye from plants in India. It is the oldest dye to be used in textile dyeing and printing. Almost all of the indigo dye produced are synthetic.
Indigo dye is made from the leaves of Polygonum Tinctorium. The indigo leaves are fermented for 2 to 3 months into a black clump with 2 - 10 % of indigo pigment. Wood ash, lime and bran are then mixed with the indigo clump and subjected to further water mixing, heated to 30~40 ℃, and then dried to form insoluble blue indigo.
Indigo is subjected to sulfonation with concentrated sulfuric acid, followed by dilution and then neutralization with soda ash. Sodium chloride is then added for salting out, before the mixture is filtered, washed and dried to get the finished product.
Pure product can be used for the manufacturing of food dyes or processed into organic pigments. It can be used as food coloring agent in beverages, wine, candy, pastry coloring, dyeing cherry and green plum.
The main use of indigo is used as a dye for cotton yarns which are used for the production of denim clothing for blue jeans. It is also used for the dyeing of wool or silk.