Denim jeans primarily get their distinctive blue color from indigo dye, which is applied to the cotton yarns before the fabric is woven.
The most common method for dyeing denim on an industrial-scale uses a continuous process in large machines known as 'continuous dyeing ranges.' Unlike dyes that fully penetrate the fiber, indigo dye is applied mainly to the surface of the yarn. This partial dyeing is key to the iconic fading characteristic of denim.
There are two primary methods used in these continuous dyeing ranges to prepare the yarn:
- Rope Dyeing: In this technique, the cotton yarns are bundled in ropes. These ropes are then systematically guided through a series of indigo dyebaths. This method often leads to better color consistency and a deeper indigo shade.
- Slasher Dyeing: Alternatively, the yarns are spread out as sheets. This process is often integrated with the sizing process that prepares the yarn for weaving. While typically faster, slasher dyeing can sometimes result in less even dye penetration compared to rope dyeing.
Regardless of whether the yarn is prepared as ropes or sheets, the core dyeing process involves running the yarn through several dyebaths. The yarn is quickly dipped into the indigo solution, then lifted out and exposed to air. This air exposure allows the indigo dye to oxidize and become insoluble, fixing onto the yarn surface. This dip-and-oxidize cycle is repeated multiple times (often 6 to 10 times) within the 'continuous dyeing ranges'.
This multi-dip process is crucial. By limiting the time the yarn spends in the dye bath and allowing oxidation between dips, the dye builds up on the exterior of the cotton fibers without fully penetrating the core. This leaves the interior of the yarn largely undyed. When the denim fabric is later worn and abraded, the surface-dyed fibers wear away, revealing the undyed white core underneath – this is what creates the unique fading patterns and high-low color variation that denim is famous for.
