Following on from the previous journal article where we learnt about the production of cotton, we’re now going to take a look at the next step in the process, how the spun cotton gets its famous blue colour.
There are a few different methods which can be used to dye the yarn for denim production. The most common techniques are rope dyeing and slasher dyeing, each have their own advantages and disadvantages and have a large impact on the price of the denim and the manner in which it will fade.
The method of rope dyeing was invented in 1915 and is almost unique to denim. Slasher dyeing was introduced in the 1970’s and is much less labour intensive than rope dyeing.
Soaking the yarn in dye baths is a shared requirement for both techniques, the major difference is the way in which the yarns are arranged and fed through the dye baths.
Rope dyeing involves twisting about 380 to 420 individual strands of yarn in to ‘ropes’ before they’re sent through the dyeing range.
Slasher dyeing involves laying each individual strand next to each other to form a sheet.
Before either of these dyeing processes commence, the yarn must first go through a pre-wash stage where it is treated with caustic soda and a wetting agent to remove any impurities and natural oils in the cotton which could cause inconsistencies with the dyeing. This stage is crucial for how the dye will permeate the cotton as well as making sure the colour is as consistent as possible.
Next, comes the preparation of the dye. Up until the 19th century, natural indigo was extracted from the leaves of the Indigofera tinctoria plant, but the production of the plant could no longer keep up with the demands of the textile industry and the search for synthetic indigo started.
In 1865, Adolf von Baeyer, a German chemist began working on the synthesis of indigo and in 1897 synthetic indigo was launched. In 1905, Baeyer won the Nobel prize in Chemistry for his work on organic dyes including indigo.
Indigo, in both its natural and synthetic form is water insoluble until it undergoes a chemical change. By adding additives to the hot water and indigo mixture, the indigo molecules will slowly change. After some time, the indigo will be soluble with water and will be able to attach to the cotton fibers.
The application of colour can now begin. The process of dyeing denim depends on oxygen as this is what creates the indigo colour. When the yarn emerges from the first dye bath it is green, as the yarn dries and is exposed to oxygen, the colour changes to blue.
During the rope dyeing process, the yarns are immersed in dye baths for around 20 to 30 seconds which is then followed by 60 to 180 seconds of oxidization. This process will then be repeated up to 12 times.
Due to the short bursts of exposure to the dye, the core of each strand will remain undyed, this is because the dye doesn’t have enough time to permeate through to the center of the yarn. As the denim is worn and washed, the undyed yarn core will slowly reveal itself producing a faded effect on the fabric.
The slasher dyeing process follows a similar dye bath and oxidization process as the rope dyeing process. This technique, however, takes considerably less time as each individual thread is exposed to the dye, meaning that the submersion time and oxidization time is reduced. Although this process takes less time, it is more challenging to produce an even coverage with the dye and colour inconsistencies are a lot more noticeable within the finished product.
Sulphur can also be mixed with indigo dye to produce different colours. By reducing the amount of indigo and adding sulphur to the dye vat, shades of black can be created. We offer a black denim option on all our frames as well as blue and undyed options too! Take a look at them here.
Jack has carried out his own experiments with indigo dye, take a look at the video below to see the wonderful sunglasses he made!
Come back next time to see how the dyed indigo yarns are transformed in to fabric.
Image Credits: Image1:https://japanblue-jeans.com/blog/2022/06/09/what-is-rope-dyeing/?___store=japanblue_en image2: https://www.hancott.com/denim/hancott.html image3: https://goldenrussetfarm.com/dyeing-fabric-with-fresh-indigo/ image4: https://en.wikipedia.org/wiki/Adolf_von_Baeyer image5: https://www.inqova.com/post/a-walkthrough-of-how-denim-is-made image6: https://robindenim.com/2017/11/fabric-called-denim-indigo-dyeing/ image7: https://japanblue-jeans.com/blog/2022/06/09/what-is-rope-dyeing/?___store=japanblue_en image8: https://onedenimmills.com/one-denim-full-process/