The first commercial batch of synthetic indigo came to market in 1897. Its discovery would be crowned by a Nobel Prize for its contribution to the advancement of modern chemistry. Now, 125 years later, it owes its high level of recognition to its ties to the denim industry. In less than a decade, the modern synthetic dyestuff had replaced its natural equivalent, and its dominance continues, despite the specific processes needed to make the pigment bond to cotton fibres. 

The history of synthetic indigo is closely linked to that of modern chemistry, its high point being the Nobel Prize for chemistry bestowed to Adolf von Baeyer in 1905. The German scientist had begun investigating the chemical structure of indigo in 1865, and would successfully synthesise it in 1883. Throughout the 19th century intense research would lead to several key breakthroughs in chemistry and dyestuffs. A British scientist, Andrew Perkins, “invented” mauveine in 1856. He would go on to identify aniline as a precursor to intense colouring agents.

Synthetic indigo remains special in that it replicates the identical molecular structure of natural indigo. “This is worth acknowledging, because it is in contrast to all other synthetically developed colourants since then,” says BluConnection co-founder Peter Zinser.

Badische Anilin & Soda Fabrik (BASF), the first company to commercialise synthetic indigo, had been founded in Ludwigshafen in 1865 by Friedrich Engelhorn. He was looking to develop a new business in textile dyes made from tar, a by-product of his gasworks. The company’s historical records indicate that the introduction of Indigo Pure was the result of 17 years of intense research and a sizable investment of 18 million Gold Marks.

Like many early chemicals companies, BASF’s various divisions would come to be reorganised and rearranged. The conglomerate’s historical textile dyestuff activities became a part of DyStar in 2000. A leading supplier of synthetic indigo, DyStar was formed in 1995, merging the textile dyestuff businesses of Bayer, Hoechst and Mitsubishi. It went through a period of varied ownership and fortune, and its main shareholders are now China-based Longshen and India-based Kiri Industries.

The origins of Swiss chemicals company Archroma, another leading supplier of indigo, also go back to the early years of synthetic dyes. Its roots trace back to Sandoz, founded in Basel in 1886 to produce textile dyes. Sandoz and Hoechst merged their specialty chemicals activities to form Clariant in 1995. In 2013, US-based SK Capital Partners acquired what was then Clariant’s textile chemicals, paper specialties and emulsions businesses to create a new entity it named Archroma.

BluConnection, founded in 2010, specialises exclusively in indigo and denim. It launched its business with a 30% pre-reduced indigo, DenimBlu30, manufactured in its zero-discharge production site in Singapore. Its founders include Peter Zinser, Alexander Bock, Andreas Mendel and Georg Schnitzer, considered one of the developers of pre-reduced indigo.

A challenging dyeing process

The challenge with indigo, be it natural or synthetic, is that it is insoluble in water, which is a specific property of pigments, and, once reduced, and ready for dyeing, it is highly sensitive to oxygen. The most common reducing agent today is sodium hydrosulfite, which converts indigo into its “leuco” form. This operation, conducted before and during the dyeing process, is what generates high levels of salt in wastewater. Pre-reduced indigo, such as BluConnection’s, is hydrogenated, and considered a greener solution.

The other specific characteristic of indigo dyeing is the near magical process that makes yarns progressively go from yellow to green to blue. Chemistry is important to stabilise and optimise the process, but it is basically oxygen in the air that brings out the coveted and unique indigo blue. Here again, indigo is special, says Mr Zinser, in that it does not require a chemical oxidising agent that other types of dyes need to express their colour. It can therefore be considered a better, lower impact process. Successive dyebaths, and dips, will lead to darker hues. The dyebaths are most often reused, sometimes for years, and topped up with more indigo as required.

Reducing the reducing agent

A key area where the impact of indigo dyeing could be improved upon is thus in the chemistry needed to reduce the pigment, which would contribute to decreasing the global use of hydrosulfite. It is the chemical that generates salt in effluent. Here, as in all chemicals, the dose makes the ‘poison’.

Hydrosulfite is used to reduce indigo and to remove oxygen from the dye vat, but it will continue its action in wastewater treatment plants, interfering with their ability to break down pollutants. “Excess hydrosulfite can only be treated with large amounts of insufflated air, with great expenditure of energy and corresponding CO2 emissions. Even if it were possible to oxidise all the hydrosulfite, it would turn into salt, making it impossible to use for agricultural purposes and making river water uninhabitable for freshwater species,” says Luigi Caccia, general manager of Italian mill Pure Denim. In water-stressed denim producing areas, it is, he says, “the most damaging effect of denim dyeing”.
This is why pre-reduced indigo is considered one of the most important breakthroughs in indigo dyeing. That was in 1993, when it was first introduced by BASF. It basically shifted the job of reducing indigo to the dye suppliers instead of the dyehouses. “This allowed a substantial reduction in salts and bases, making it a more sustainable product,” Paolo Gnutti, head of PG Denim, tells Inside Denim.

Pre-reduced indigo decreases the amounts of hydrosulfite required by between 50% and 70%, says Mr Zinser. It also contributes to “substantial water savings and better safety and hygiene for workers in dyehouses”.

The global market share of pre-reduced indigo is estimated to be roughly 30%. Mr Zinser points out that it is useful to break down its adoption into two markets, China and the rest of the world. He says that outside of China the share of pre-reduced indigo is higher, in the order of 60%. He believes that it can even exceed 80% in some denim producing countries. “Many of our customers, often leading denim mills, use 100% pre-reduced indigo,” he says.

Italian denim mill Berto says that 98% of its fabrics are dyed with pre-reduced indigo, and that it is transitioning the remaining 2% of powder indigo to liquid. It is currently making new investments in its dyeing process, which will lead to further improvements, the company says.

With or without salt 

Indigo pigment can be reduced using an electrochemical process that does away with all hydrosulfite at this stage of the process. This technique, initially developed by Swiss scientist David Crettenand, is licensed by Swiss company Sedo Engineering with its Smart Indigo machine. A unit is installed at Italian factory Pure Denim, which helped develop the first prototype and has been using the machine since 2015, transforming indigo powder into a pre-reduced liquid (30% concentration), explains Luigi Caccia. Indigo powder and caustic soda are fed into the device and pass through an electric field to form a liquid leuco indigo solution. Sixteen Smart Indigo units are in operation, many in Asia, with new machines booked by mills in Mexico and Egypt.

DyStar says its Indigo Vat 40% Solution can lead to a 60% to 70% reduction of sodium hydrosulfite usage. It has been on the market for some 15 years. The company also introduced a few years back a biodegradable organic reducing agent which “does the job without the negative effects of the salt”, says Günther Widler, DyStar’s head of technology for denim. The company is now working on filtering out excess indigo to release cleaner wastewater, which he says could be reused, ultimately leading to a closed loop use of water and dyestuffs in factories.

Addressing aniline 

The latest development in synthetic indigo processing is from Archroma. It launched a near aniline-free pre-reduced liquid indigo in 2018. Aniline is present, but remains below limits of detection according to industry standards, the company takes care to mention. It has calculated that in the first three years of commercialisation of its Denisol Pure Indigo, its partners have been able to remove some 18 tonnes of aniline from their processes. Market demand has led the company to start to increase capacity, says Umberto Devita, Archroma’s global manager for indigo.

Pure Denim uses an activated charcoal filter to remove aniline from wastewater. “But it is impossible to remove all aniline; aniline is a fact,” says Mr Caccia. Here again, pre-reduced indigo is considered a better solution. Indigo powder will have a higher level of aniline impurity, which can reach 6,000 ppm, says Mr Zinser. The lower concentrations in pre-reduced indigo will make it easier to produce jeans that do not exceed permitted levels. BluConnection has developed a purification process which filters out aniline from its pre-reduced indigo, allowing a finished denim product to stay within the accepted 20 to 50 ppm limits. “Aniline does not however disappear. We, as do other chemicals companies, reduce its presence so that our customers do not have to,” he says.

Rethinking dyeing

Innovation in the dyeing process itself is another way the industry is reducing water usage and pollution levels in effluent. Foam dyeing is one such near waterless process. Introduced by Spanish mill Royo as Dry Indigo in 2018, it uses 65% less energy and 90% fewer chemicals than standard methods. As no water is necessary, there is no wastewater to dispose of, the company says. Spray dyeing is another water-saving dyeing technique.

CleanKore, a company based in Ohio, has developed a patented process that tweaks existing dyeing techniques to make indigo bond only to the outer surface of a yarn, leaving an undyed white core which can make some operations unnecessary, such as potassium permanganate, and can optimise processing in laundries. Achieving this requires neither new equipment nor new chemicals, Ryan Ripley, business development manager, tells Inside Denim. “We alter various elements of a dye range to achieve the white core.” CleanKore says this white core makes it possible to achieve brighter fading of indigo blue as well as of sulphur black. Mr Ripley says the initial result is “as close as possible to conventional dyeing”.

The company has partnered with Archroma and can thus rely on the Swiss company’s global network of mill and wash technicians to assist dyehouses in implementing the system. Arvind, in India, and Kipas, in Turkey, are early adopters, along with Tak Sang, in China, and Atlantic Mills, in Thailand. Industrial production runs of CleanKore dyed fabrics have begun, which means the first products could be in stores during 2022. 

Natural and biotech indigo

Denim dyehouses are often wary of natural indigo. In pigment form, it is indistinguishable from synthetic indigo, which opens the door to fraud. Its use in the industry has remained niche, despite its aura among consumers. Many point out that it is not possible to grow sufficient quantities of indigo plants to meet global demand for synthetic dyes, which is said to represent some 70,000 tonnes a year. “Natural indigo is produced in low quantities, with high costs and technical limitations, and is only suitable for very high-quality products,” says Mr Gnutti.

Stony Creek Colors is striving to change the industry’s outlook on natural indigo. It has recently expanded production to tropical regions to secure year-round supplies. The company has completed two years of validation of IndiGold, a pre-reduced plant-based indigo with a concentration of 20%. “It will be available in a soluble, liquid form for commercial use by denim mills during 2022,” says Sarah Bellos, company founder and CEO. “Modern genetic and genomic tools are rapidly improving our yield per tonne of crop biomass,” she states, noting that Stony Creek’s natural indigo will have a climate positive impact as it can play a role in regenerative agriculture.

Mills seem to place more faith in the promises of biotech dyes. “My personal opinion is that biotech indigo would be better than natural indigo,” says Luigi Caccia, in reference to best use of farmland and the risk of deforestation. Pure Denim is currently working closely with a European company to bring a biotech indigo to market.

BluConnection is also closely monitoring progress in biotech indigo. “Despite our interest and openness to new tech and to cooperation in this field, we have been unable to obtain a sample of bio indigo worth mentioning so far,” says Mr Zinser. He questions the ability of this new process to reach industrial scale in the near future.

Sarah Bellos also questions the efficiency of biotechnology. “From a cost perspective, I find it hard to believe that fermentation will ever beat the productivity of agriculture. Stony Creek Colors’ indigo precursors are grown directly inside plants using solar energy and extracted with water. It is a renewable and scalable solution and our costs will come down as crop and market adoption expand,” she says.

Huue founders Tammy Hsu and Michelle Zhu believe otherwise. The biotech start-up (formerly known as Tinctorium) is making progress on its drop-in biobased indigo. “Our process uses microbes to create a bio-identical indigo that will perform like any other indigo,” says Michelle Zhu, Huue CEO and co-founder.

The company is progressively increasing the size of its fermentation vats. The next step would be to find an industrial partner with expertise in fermentation to further scale up. The price of the future bio-manufactured dyestuff is as yet unknown. “Like any innovative product, our biotech indigo will start as a premium solution and we know we can bring costs down over time,” says Ms Zhu. But, in the current context and with growing pressure from consumers, “we cannot wait for a solution to check  all the boxes, we need to begin progress now.” A bio-manufactured indigo, without toxic chemicals, and without the inconsistency of natural indigo, could provide a real solution to a major apparel industry hot spot, she says.

In the course of its 125 years of existence, much has been done to make synthetic indigo dyeing as sustainable as possible. One could just as well say that not much was needed to achieve this. The high-performance pigment has seen improvements in reducing agents, which have had a clear impact on water usage and effluent.

New indigos in development, be they biotech or natural, are modelled on current industry standards, and will need to align with acceptable market prices, even if they do have the bonus of being aniline-free. Producers of synthetic dyes do not view them as competitors, having developed what they consider to be the most efficient, industry- and market-proven solution. They do however suffer from pervasive chemicals-bashing and from the tainted image of denim as part of the global fashion industry. Lack of transparency and reliable data on denim dyeing impacts does not help to put a positive twist on synthetic indigo, which many in the industry believe to be one of the more sustainable dyes.

The history of synthetic indigo

1883 Adolf von Baeyer first synthesises indigo. 
1897 German chemicals company Badische Anilin & Soda Fabrik (BASF) begins production of synthetic indigo on an industrial scale. 
1905 The Nobel Prize in Chemistry is awarded to Johann Friedrich Wilhelm Adolf von Baeyer. 
1906 BASF introduces hydrosulfite in powder form, simplifying vat dye and indigo processing. 
1932 Japanese chemicals company Mitsui launches commercial production of synthetic indigo, the result of research begun in 1913.
1993 BASF introduces 20% pre-reduced indigo, obtained via catalytic hydrogenation, leading to a substantial reduction in salts and bases. 
1995 Dystar, a company combining the textile dye businesses of Bayer, Hoechst and Mitsubishi, is founded. 
2000 DyStar integrates BASF’s textile dye unit, and introduces 40% pre-reduced indigo. 
2004 Smart Indigo is first commercialised using an electrochemical reduction method patented by Swiss chemical engineer David Crettenand. 
2010 BluConnection is founded by Alexander Bock, Andreas Mendel, Peter Zinser and Georg Schnitzer. It launches DenimBlu30, the first 30% pre-reduced indigo from its zero discharge production site in Singapore.
2013 Archroma is formed by SK Capital from the Textile Chemicals, Paper Specialties and Emulsions businesses of Clariant. 
2017 “River blue”, a documentary by Mark Angelo, draws attention to the impacts of indigo dyeing on freshwater resources.  
2018 Archroma introduces Denisol Pure Indigo, an aniline-free indigo (in which the presence of aniline is below measurable levels).  Spanish mill Tejidos Royo introduces Dry Indigo, a foam dyeing method derived from a process developed with Texas Tech University and machinery maker Gaston. 
Tinctorium is founded, later renamed Huue, to develop a biotech plant-based indigo. 
2021 Stony Creek Colors launches IndiGold, a plant-based pre-reduced indigo.

Archroma’s Pure Indigo Icon2 combines its aniline-free indigo with CleanKore technology to allow the production of aniline-free and potassium permanganate-free denim. 
Top photo: Archroma 

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