Indigo dye is an organic compound with a distinctive blue color. Historically, indigo was a natural dye extracted from plants, and this process was important economically because blue dyes were once rare. Nearly all indigo dye produced today several thousand tons each year is synthetic. It is the blue of blue jeans.
USES
Natural indigo
Plant sources.A variety of plants have provided indigo throughout history, but most natural indigo was obtained from those in the genus Indigofera, which are native to the tropics. The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known as Indigofera sumatrana). A common alternative used in the relatively colder subtropical locations such as Japan's Ryukyu Islands and Taiwan is Strobilanthes cusia . In Central and South America the two species Indigofera suffruticosa ) and Indigofera arrecta (Natal indigo) were the most important. In temperate climates indigo can also be obtained from woad (Isatis tinctoria) and dyer's knotweed (Polygonum tinctorum), although the Indigofera species yield more dye.
Extraction
The precursor to indigo is indican, a colorless, water-soluble derivative of the amino acid tryptophan. Indican readily hydrolyzes to release D-glucose and indoxyl.Oxidation by exposure to air converts indoxyl to indigo. Indican was obtained from the processing of the plant's leaves, which contain as much as 0.2 – 0.8 % of this compound. The leaves were soaked in water and fermented in order to convert the glycoside indican present in the plant to the blue dye indigotin. The precipitate from the fermented leaf solution was mixed with a strong base such as lye, pressed into cakes, dried, and powdered. The powder was then mixed with various other substances to produce different shades of blue and purple.
Cultivation
The demand for indigo in the 19th century is indicated by the fact that in 1897, 7000 square kilometers were dedicated to the cultivation of indican-producing plants, mainly in India. By comparison, the country of Luxembourg consists of 2,586 square kilometers.
In literature, the play Nildarpan by Dinabandhu Mitra is based on the indigo slavery and forceful cultivation of indigo in India. It played an essential part in the Bengali indigo revolt of 1858 called Nilbidraha.
Direct printing
Two different methods for the direct application of indigo were developed in England in the 18th century and remained in use well into the 19th century. The first method, known as pencil blue because it was most often applied by pencil or brush, could be used to achieve dark hues. Arsenic trisulfide and a thickener were added to the indigo vat. The arsenic compound delayed the oxidation of the indigo long enough to paint the dye onto fabrics.
The second method was known as china blue due to its resemblance to Chinese blue-and-white porcelain. Instead of using an indigo solution directly, the process involved printing the insoluble form of indigo onto the fabric. The indigo was then reduced in a sequence of baths of iron sulfate, with air-oxidation between each immersion. The china blue process could make sharp designs, but it could not produce the dark hues possible with the pencil blue method.
Around 1880 the glucose process was developed. It finally enabled the direct printing of indigo onto fabric and could produce inexpensive dark indigo prints unattainable with the china blue method.
Since 2004 freeze-dried indigo, or instant indigo, has become available. In this method the indigo has already been reduced, and then freeze-dried into a crystal. The crystals are added to warm water to create the dye pot. As in a standard indigo dye pot, care has to be taken to avoid mixing in oxygen. Freeze-dried indigo is simple to use, and the crystals can be stored indefinitely as long as they are not exposed to moisture.
