How Indigo dyeing went from Natural to Synthetic

How Indigo dyeing went from Natural to Synthetic

This is the centenary year of the Champaran Satyagraha, when Mahatma Gandhi highlighted the woeful plight of Indian farmers who had been forced to cultivate indigo by the British. It also happens to be a time when synthetic rather than natural indigo is in vogue.

 

History

Indigo dye is an organic compound with a distinctive blue color. Historically, indigo was a natural dye extracted from the leaves of certain plants, and this process was important economically because blue dyes were once rare. A large percentage of indigo dye produced today, several thousand tones each year, is synthetic. It is the blue often associated with denim cloth and blue jeans.

The synthesis of N-(2-carboxyphenyl) glycine from the easy to obtain aniline provided a new and economically attractive route. BASF developed a commercially feasible manufacturing process that was in use by 1897, at which time 19,000 tons of indigo were being produced from plant sources. By 1914 this had dropped to 1,000 tons by 1914 and continued to contract. By 2011 50,000 tons of synthetic indigo were being produced worldwide

It is a natural dye used since ancient times to produce beautiful blues that range from sky blue to deep, almost black blue. Frequently, people are nervous about dyeing with indigo. It is a tricky dye to work with, mostly because it is not water soluble and will not bind to fabric unless the dye vat is deprived of oxygen.

Demand for indigo dramatically increased during the industrial revolution, in part due to the popularity of Levi Strauss’s blue denim jeans. The natural extraction process was expensive and could not produce the mass quantities required for the burgeoning garment industry. So chemists began searching for synthetic methods of producing the dye. In 1883 Adolf von Baeyer (of Baeyer aspirin fame) researched indigo’s chemical structure. He found that he could treat omega-bromoacetanilide with an alkali (a substance that is high in pH) to produce oxindole. Later, based on this observation, K. Heumann identified a synthesis pathway to produce indigo. Within 14 years their work resulted in the first commercial production of the synthetic dye. In 1905 Baeyer was awarded the Nobel Prize for his discovery.


The indigo does not bind well to synthetics. It basically doesn’t bind at all. Wool, cotton, linen and silk are all fairly easy to dye with indigo. No mordant (a presoak solution that improves color retention with some natural dyes) is required when you dye with indigo. Unlike most natural dyes, the length of time the items spends in the vat does not dictate the richness of the color. An item should only be submerged for a few minutes at a time.

Mechanized textile production has led to a big boom in clothing and accessories by big international brands. But rather than succumb to the competition, Indian designers are still creating the indigenous dye in the traditional way, and generating jobs for village-based artisans. Synthetic indigo is used extensively, especially for dyeing denim.

“Few people use natural indigo to dye denim,” says designer Pallavi Mohan. Denim is made in huge quantities in India, Pakistan and Italy where a lot of indigo dues are not natural. However any chemical used ethically and judiciously, is better for the environment.”

 

The Future

Manufacturers who use indigo in dying operations are also seeking to improve their use of the dye. For example, Burlington’s Denim Division introduced a technology in 1994 they call “Stone Free,” which allows indigo dye in the fabric to break down 50% faster in the stonewash cycle. Compared to traditional methods of stonewashing fabric dyed with indigo, their new process uses few, if any, pumice stones which help give the fabric its faded look. Therefore, pumice stone handling and storage costs are reduced, along with time required to separate pumice from garments after stonewashing. It also uses much less bleach. Therefore, this new process not only reduces garment damage, but also reduces waste produced by the stones and bleach.

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