This Encyclopedia entry is a must-know term. Learn more in our Denim vocabulary.
Dyeing is the process of adding colour to yarn. It’s done by soaking the yarn in a liquid that contains a dyestuff.
For denim, the most common dyestuff is indigo.
Denim is blue on the front and white on the back because only the warp yarns are dyed while the weft yarns are left naturally undyed or bleached.
How denim is dyed
For industrial-scale indigo dyeing, yarns are bundled in ropes or spread out as sheets and run through several dyebaths in what’s known as ‘continuous dyeing ranges.’
These two methods are called ‘rope dyeing’ and ‘slasher dyeing,’ respectively.
While dyeing these days has been mechanised in large industrialised production lines, it was traditionally done by hand.
Hank dyeing: The predecessor to 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.
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.
An obvious downside is that production output is much lower, which makes it more expensive. Also, the many dips minimise the ring effect.
Disclaimer: 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, it is the application that dictates the ring dye effect, not the naturalness of the indigo.
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. It’s more labour-intensive compared to slasher dyeing, 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.
Slasher dyeing explained
Slasher dyeing was introduced in the 1970s. Rather than having the yarns bundled in ropes, they’re laid out as a carpet, also called a sheet, which is warped onto the beam.
Although that is the main difference between the two dyeing processes, dyeing yarns 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. Once dyed, the beam of yarn is ready to go onto the weave.
Another advantage is that, because the 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
Below is a comparison of the advantages of the two dyeing techniques that are primarily used for denim production.
- Rope dyeing gives a good ring dye effect, which is crucial to get a good fade.
- Production capacity of rope dyeing range is higher.
- It consumes less reducing agent.
- It produces less yarn waste.
- A lower concentration of dye is needed.
- It’s easier to control high depth shades as you have less unevenness, which can cause shade variation.
- The tension in the yarn in rope dyeing forces a more even penetration of indigo in the cross-section of the yarn. This gives a better fastness and better depth in the quality, plus a better consistency and a richer cast.
- You avoid centre to side dye shade variation, which is a common problem with slasher dyeing.
- Slasher dyeing puts less tension on the yarn, which makes the process suited for finer yarn counts.
- The process makes rebeaming redundant as you don’t need to open up ropes before the sizing process.
- With sheet dyeing, you have more flexibility in terms of colours that can be achieved as the sheet of yarn is level throughout the dyeing range.
- Slasher dyeing also has lower production minimums.
- And it (usually) uses less energy.