Learn About Natural and Synthetic Indigo and the Differences Between Rope Dyeing and Slasher Dyeing

Indigo is the blue in blue jeans. The dyestuff has been popular around the world for several millennia. It’s been the colour of royals, which made it desirable to the proletariat. It’s one of the most colourfast natural dyes. And it remains beautiful as it fades to a myriad of bright, blue hues.

While most other colours become dull with wash and wear, this is when indigo truly comes to life. This is without a doubt the most important reason why we continue to wear indigo-dyed garments today. With denim jeans being the most popular indigo-dyed garment, my five-episode series about how denim is made naturally discusses indigo dyeing.

I’m approaching denim’s production process chronologically in this series. In the first episode, I discuss cotton and its benefits. In the second episode, I uncover how the cotton is spun into yarn. In this episode, I explore indigo dyeing. I outline indigo’s history; I discuss natural and synthetic indigo, and I explain the two indigo dyeing techniques that are primarily used for denim production, rope dyeing and slasher dyeing.

What Is Indigo?

The indigo colour originates from India. The name ‘indigo’ comes from the Greek word ‘indiko�?n’—which became ‘indicum’ in Latin—and the original meaning was simply “a substance from India.�?

Archaeologists have traced the use of indigo back 5,000 years, which makes it one of the oldest dyestuffs still in use today. The oldest preserved indigo-dyed textile fragments in existence were unearthed in pyramids built during Ancient Egypt’s Fifth Dynasty. That means they’re as much as 4,500 years old.

In South Asia, the indigo pigment has traditionally been extracted from dried leaves from the indigofera tinctoria plant—also known as ‘true indigo.’ This is what we call ‘natural indigo’ today. The dye is extracted through fermentation; a series of biochemical reactions produce an indigo sludge that can be dried into blocks and then ground into powder. It requires a great deal of expertise, time and work, explains Henry Wong, marketing and product development director at Artistic Fabric Mills.

The idea of natural indigo is very romantic to us denimheads,�? Wong argues. “We are attracted to the fact that natural indigo can be very time and labour intensive, which elevates it to the level of the artisanal.�?

Europe had its version of natural indigo made from woad, a plant with similar properties to indigofera tinctoria. The first woad-dyed textiles appeared in Europe in the 8th century BC; in other words in the early Iron Age. For more than a thousand years, woad dominated in Europe. But true indigo binds better to less absorbent fibres such as cotton, which made it the favoured alternative.

The exotic true indigo was seen as a serious threat. In what indigo historian Jenny Balfour-Paul calls the “woad war,�? woad growers, merchants, and even entire nations fought against the invasion of true indigo as they (with good reason) feared to lose their livelihoods. As late as the 18th century, using true indigo remained punishable by death in Germany and France. Nevertheless, true indigo eventually surpassed woad.

The final death blow to woad was the introduction of synthesised indigo. The basic chemical structure of synthetic indigo was discovered in 1878 by the German chemist, Adolf von Baeyer. It was, in fact, the first synthesised dyestuff ever made, which won von Baeyer the Nobel Prize in Chemistry in 1905. Working with Germany’s Badische Anilin- & Soda-Fabrik (BASF), he spent three decades, and more money than the company’s entire capital value, refining synthetic indigo.

The result, introduced as Indigo Pure in 1897, was a phenomenal success, despite initial scepticism. By 1914, 95% of all natural indigo production had disappeared. Today, almost all blue denim is dyed with synthetic indigo.

Indigo Dyeing Explained

Regardless of whether indigo is extracted naturally from grounded plant leaves or synthesised from petroleum products, it starts as a powder—traditionally, at least. To get the colour pigments onto the spun yarn, which is usually made from 100% cotton, the powder must be turned into liquid. That’s where it gets complicated.

Indigo is a vat dye. That means it’s water-insoluble, which is why a reducing agent—usually sodium hydrosulfite—is needed to solubilise the dyestuff. The instant the yarn gets in contact with the atmospheric oxygen, after it’s been dipped in the dyeing vat, the oxidation process binds the indigo molecules to the fibres of the yarn. But why do indigo-dyed fabrics fade?

Why Indigo-Dyed Fabrics Fade

The reason for indigo’s unique fading property is found in the vat dyeing process, which results in the ‘ring effect’ that gives denim its disposition for high-contrast fades. Technically explained, the indigo molecules are too big for the lymphatic channels of the cotton fibre, which means the dyestuff only binds externally.

With traditional hand-dyeing techniques, such as the traditional Japanese ‘aizome’—which uses fermented natural indigo—you can achieve higher dye permeation of the yarn. The result is fabric that fades slower. The yarn is dipped numerous times until the desired colour is reached.

Even with several dips in industrial indigo dyeing, the indigo pigments stay on the surface of the yarn. As the colour slowly wears and washes off, the undyed core appears.

This is true for both natural and synthetic indigo when used in modern-day indigo dyeing. So what are the differences between the two types of indigo?

The Differences Between Natural Indigo and Synthetic Indigo

Not surprisingly, the most significant difference between the two is that synthetic indigo is a lot cheaper than natural indigo.

The price of synthetic indigo usually varies between $1 to $5 per 100 grammes whereas the price of natural indigo ranges from $20 to $40 per 100 grammes. With different qualities and grades, prices may vary even more.

To get a better idea of what this means at the cost of an average pair of jeans, you need to multiply those numbers by the amount of indigo needed to dye the fabric blue. According to Muzammil Usmani, who’s the product development manager of Indigo Textile, you need roughly 25 grammes of indigo to reach a shade depth of 4%, which is common for 100% indigo dyeing, on a 12 oz. denim.

Muzammil did some pretty hardcore calculations, taking into account the weight, width and shade depth. I’m going to spare you those. But you’ll have between 15 to 30 grammes of indigo dye deposited in your jeans. Just to put things into perspective. In addition to actual price per unit, you also need to consider how much indigo you need.

Natural Indigo Is Inconsistent In Colour

An explanation for the big price difference is found in the method of dye extraction and production, which makes natural indigo much less colour-stable and thus even more costly to use.

The natural dyestuff contains impurities, and not even the best producers can guarantee the level of consistency that modern denim manufacturers demand. Denim mills which use natural indigo will be challenged with obtaining shade consistency and production feasibility, which drives up costs.

You cannot promise the same fabric shade in a running fabric for a number of seasons,�? says Muzammil Usmani. “With every new crop, the shade and hue is different.�?

Regarding visual differences, denim woven from yarn dyed with natural indigo has more colour variation, a distinctive green cast—which is the tone of the fabric—and it fades slowly. Contrarily, denim that is woven from yarn dyed with synthetic indigo has a more uniform colour, a red cast—at least when it is not mixed with sulfur—and it fades faster with higher contrast.

Usmani argues that there’s no replacement for natural indigo when it comes to the colours in creates. But, in today’s market, it’s impossible to meet the demand. Henry Wong agrees:

There is no feasible way for the natural indigo supply chain to support consumers’ voracious appetite for jeans.�?

Natural Indigo Is Less Sustainable in Denim Production

In a day and age where sustainability is a top priority for most denim makers, natural indigo doesn’t make much sense.

Textile chemistry engineer, Miguel Sánchez, who’s the head of global business development at Archroma, argues that “natural�? doesn’t mean “good for the environment.�? As he explains, natural dyes have several restrictions in terms of performance, quality consistency and application compared to the synthetic dyes.

First of all, for the supply chain to keep up with demand, the natural indigo growing industry would take up a considerable amount of the world’s arable land. It takes 13 acres to produce the amount of natural indigo required to dye 1 acre worth of cotton. Land that should be, and is, used to grow food.

One dye set requires almost 100 times the amount of natural indigo compared synthetic indigo to be able to achieve the same depth,�? says Amrin Sachathep, director at Atlantic Mills.

On top of that, natural Indigo has lower build-up properties and lower fixation rates compared to synthetic indigo. This means you need more dye. The lower fixation rate also means you need more water to wash off unfixed dye in the rinsing step.

And, if natural indigo is dyed with a conventional chemical reducing agent, the impact of the byproduct overshadows any advantage that might be claimed from the dye’s natural origin. The alternative of natural fermentation from organic waste can only be considered for craft works, Sánchez argues.

The bottom line is that, even though synthesised indigo is made from petroleum products, it’s the most eco-friendly of the two. At least when used in denim production. Now, there’s an even better alternative.

Pre-Reduced Indigo: The Eco-Friendly Alternative

Both natural and synthetic indigo traditionally comes in powder. Synthetic powder indigo is cheap and readily available. But, as I alluded to earlier, things are changing.

Many makers now use ‘pre-reduced indigo,’ which cuts the use of reducing agent chemicals significantly for the mill. It’s an important step in the direction towards a more sustainable future for denim.

As Muzammil Usmani explains, producers are shifting towards pre-reduced indigo as it’s more consistent, there’s less hustle with it and, above all, it’s shade brilliance is superior. He anticipates that powder indigo will eventually end up like natural indigo; a romanticised part of the past. Henry Wong agrees and elaborates:

In my part of the world (Wong works in Pakistan), the indigo powder vs. paste debate is now a bit dated. The major players have fortunately moved beyond that last generation of indigo dyestuff, which required extensive use of sodium hydrosulfite to reduce the indigo into its dyeable form.�?

Pre-reduced indigo is a more environmentally-friendly alternative because the solubilisation of the dye, which is done by the dye maker, requires no or far lower concentrations of the reducing agent, explains Miguel Sánchez. Another way to use less indigo is to replace it with another colour.

Alternatives to 100% Indigo Dyeing

Not all denim is dyed with indigo only. Denim makers have three ways to dye: with 100% indigo, a with mix of indigo and sulfur, and with 100% sulfur. The latter is what’s used for black and colour denim.

In the 1970s, when denim makers scaled for mass production, sulfur was introduced to replace (some of) the indigo baths to cut costs. These days, sulfur is often used to add so-called tops and bottoms on indigo-dyed yarn, which is one way to create the cast. However, you can control the cast without sulfur simply by changing the chemistry of the dye bath.

Dyeing with 100% indigo produces a characteristic red cast like the one known from the iconic denim that Cone made for Levi’s in the 50s and 60s.

Now, let’s move onto how the different types of indigo, and their alternatives, are used to dye our jeans blue.

How Denim Is Dyed With Indigo

To understand how denim is dyed, we need to look at the two continuous dyeing ranges, ‘rope dyeing’ and ‘slasher dyeing,’ and how they compare.

When we talk about conventional denim, it’s important to keep in mind that only the warp yarn is dyed. That’s why denim is mainly blue on the outside and mainly white on the inside. But I talk much more about that in episode 4, which discusses weaving.

While dyeing these days has been mechanised in large industrialised production lines, it was traditionally done by hand.

Hank Dyeing: The Predecessor to Modern Continuous Dyeing

Before modern continuous dyeing ranges, the yarn was dyed with ‘hank dyeing,’ also known as ‘skein dyeing.’ Hank dyeing is one of the oldest yarn dyeing techniques, and it’s traditionally done by hand.

The technique is simple but time-consuming. Bundles of yarn, known as ‘skeins,’ are first rinsed to open the fibres. Then, the skeins are dipped in the dyeing vat, usually numerous times. Next, they’re washed and then dipped again. The process is repeated until the desired colour intensity is achieved. At this point, the yarn is steamed to fix the dye.

Henry Wong notes that the many dips often minimise the ring effect. Because hank dyeing is usually done with natural indigo, misconceptions that natural indigo dyed denim does not fade with contrast has gained a foothold. However, Wong emphasises that it’s the application that dictates the ring dye effect, not the naturalness of the indigo.

The benefit of hank dyeing is that it yields a rich and deep colour. The technique also puts let stress on the yarn compared to continuous dyeing ranges. But, obviously, production output is much lower, which makes it more expensive. There days, hank dyeing is used only for very small scale, artisanal production setups.

Rope Dyeing Explained

Rope dyeing is one of the two predominant dyeing techniques used in denim production these days. The method was invented in 1915, making it the older option. It’s also the most labour-intensive, and the process is almost unique to denim. But is it the better option?

Rope dyeing starts with warping the yarn onto a beam. The ropes of 380 to 420 individual yarns are bundled together before they go through the dyeing range. 24 ropes is the most common amount used in one dyeing range. But modern machinery can handle as many as 48 ropes.

Before the yarn hits the indigo dye, it goes through a couple of important rinse boxes; it’s treated with caustic soda and a wetting agent to remove natural oils in the cotton and impurities that cause inconsistencies in the dyeing. The rinsing is crucial for how the dye will permeate the yarn as well as the consistency of the colour.

After rinsing, the yarn is immersed in the indigo dye boxes for 20 to 30 seconds. This step is followed by 60 to 180 seconds of oxidation, also known as ‘skying.’ Modern dyeing ranges can have up to 12 indigo dye boxes, each of which is followed by an oxidation range. The standard dyeing range has six dye boxes, which translated six dips.

Following the actual dyeing, the yarn is rinsed, usually in three washing boxes. In the last box, a softener is added to ease the opening of the ropes. This also neutralises any chemicals used throughout the dyeing range. The yarn is then dried, and the ropes are open and rebeamed.

Then comes the sizing process; the yarn is encapsulated with a protective starch coating to reduces yarn abrasion and breakage in weaving. The starch also makes the yarn less “hairy,�? which prevents it from entangling. It’s this starch that gives raw denim its stiffness—in combination with the tightness of the weave—as it’s not entirely removed therefore the final garment is sold in retail.

In addition, some modern rope dyeing ranges have steamers at both ends with extra treatment boxes to ensure that any dyeing material can be mixed with indigo to meet the ever-changing demands of the market, Muzammil Usmani explains.

Slasher Dyeing Explained

Slasher dyeing was introduced in the 1970s. Rather than having the yarn bundled in ropes, it’s laid out as a carpet, also called a sheet, as it’s warped onto the beam. Other than that, the two dyeing processes are more or less identical. Still, the two dyeing techniques are far from the same.

Dyeing yarn as a sheet has both benefits and challenges. As the yarns do not have to be bundled and rebeamed before the sizing process, slasher dyeing requires less manual labour compared to rope dyeing. The main advantage of the slasher dyeing is that once dyed the beam of yarn is ready to go onto the weave.

Because the individual yarns are dyed separately, they only need 10 to 15 seconds immersion in each indigo bath, and the oxidation time can be reduced to 30 to 60 seconds. That’s because a larger part of the surface of each yarn is exposed, which causes quicker oxidation.

Another distinction of slasher dyeing compared to rope dyeing is that the dyeing vats need to be more colour-consistent to ensure that the entire sheet is dyed evenly. With rope dyeing, you have natural inconsistency in the amount of indigo that the yarn is exposed to, which creates a planned unevenness in the fabric when the yarn is laid out.

There is, in fact, a third kind of continuous dyeing range called loop dyeing. It’s almost the same as slasher dyeing; the difference is that loop dyeing has only one dye bath, which means it requires less space and, to get deeper shade depths, the sheet is dipped 3 to 4 times in the same dye bath.

Comparison of Rope Dyeing and Slasher Dyeing

At the end of the day, Simon Giuliani of Candiani Denim argues that it’s the spinning and weaving methods that really determine how the denim fades.

That being said, let’s round off with a comparison of the two indigo dyeing techniques that are primarily used for denim production.

The next episode of the series discusses how denim is woven. You can read it here.

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