Indigo dye is an important dyestuff with a distinctive blue color (see indigo). The natural dye comes from several species of plant, but nearly all indigo produced today is synthetic. Among other uses, it is used in the production of denim cloth for blue jeans.

Sources and uses

A variety of plants have provided indigo throughout history, but most natural indigo is obtained from those in the genus Indigo era, which are native to the tropics. Was the only source of the dye until about 1900? Within a short time, however, synthetic indigo had almost completely superseded natural indigo and today nearly all indigo produced is synthetic. Over one billion pairs of jeans around the world are dyed blue with azure. For many years indigo was used to produce deep navy blue colors on wool. Indigo does not bond strongly to the fiber, and wear and repeated washing may slowly remove the dye.

History

India is believed to be the oldest center of indigo dyeing in the Old World. It was a primary supplier of indigo to Europe as early as the Greco-Roman era. The association of India with indigo is reflect in the Greek word for the dye, which was indicant. The Romans used the term indium, which passed into Italian dialect and eventually into English as the word indigo. Indigo is among the oldest dyes to be used for textile dyeing and printing. Many Asian countries, such as India, China, and Japan, have second-hand indigo as a dye for centuries. The dye was also known to ancient civilizations in Mesopotamia, Egypt, Greece, Rome, Britain, Peru, and Africa. In Mesopotamia, a Neo-Babylonian cuneiform tablet of the 7th century gives a recipe for the dyeing of wool, where lapis-colored wool (quantum) is produced by repetitive fascination and airing of the cloth. Most probably, indigo was imported from India.

In 1865 the German chemist Johann Friedrich Wilhelm Adolf von Baeyer began operational with indigo. His work culminated in the first synthesis of indigo in 1880 and the statement of its chemical structure three years later. BASF developed a commercially feasible manufacturing process that was in use by 1897, and by 1913 natural indigo had been almost completely replaced by synthetic indigo. In 2002, 17,000 tons of synthetic indigo were produced worldwide.

Developments in dyeing technology

Indigo is a challenging dye to use because it is not soluble in water; to be dissolved, it must undergo a chemical change. When a submerged fabric is removed from the dye bath, the indigo quickly combines with oxygen in the air and reverts to its inexplicable form. When it first became widely available in Europe in the sixteenth century, European dyers and printers struggled with indigo because of this distinctive property.

A reindustrialize process for dyeing with indigo, used in Europe, was to dissolve the indigo in stale urine. Urine reduces the water-insoluble indigo to a soluble substance Synthetic urea to replace urine became obtainable in the 1800s.

Another preindustrial method, used in Japan, was to dissolve the indigo in a heated vat in which a culture of thermophilic, anaerobic microorganisms was maintained. Some species of such bacteria generate hydrogen as a metabolic product, which can convert inexplicable indigo into soluble indigo white.

Around 1880 the glucose method was developed. It finally enabled the direct printing of indigo onto fabric and could produce reasonably priced dark indigo prints impossible with the china blue method.

Chemical properties

The naturally occurring substance is indicant, which is colorless and soluble in water. Indicant can easily be hydrolyzed to glucose and indoxyl. Mild oxidation, such as by contact to air, converts indoxyl to indigo.

The manufacturing process developed in the late 1800s is still in use throughout the world. In this process, indoxyl is synthesized by the fusion of sodium phenylglycinate in a combination of sodium hydroxide and sodamide.

Indigo treated with sulfuric acid produces a blue-green color. It became available in the mid-1700s. It was made from excretions of a common Mediterranean Sea snail. In 1909 its structure was shown to be 6, 6′-dibromoindigo. It has never been shaped unnaturally on a profitable basis.