Denim fabric is the soul of jeans. To understand how to pick the perfect pair or why jeans fade, you must first learn how denim is made.
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Raw Material
The quality of cotton determines the quality of jeans.
Cotton is the most common textile fiber and the most important raw material for making denim. For superior-quality jeans, the primary requisite is premium yarn, which depends on the quality of the cotton.
Suppose the denim is made of 100% pure cotton; the first step is to procure the cotton.
In developed countries, cotton is harvested with picking machines. In developing regions, such as Africa and Southeast Asia, it is often picked by hand.
The quality of cotton varies significantly with origin, boasting diverse characteristics like fiber length, fineness, maturity, strength, color, and feel.
Generally, high-quality denim fabrics are made from American, Egyptian, or Zimbabwean cotton, and as mentioned in previous conversational threads, Xinjiang cotton is also a solid choice.
Regardless of the origin, the cotton must undergo processes like harvesting, cleaning, carding, and sorting to prepare for the next steps.
Of course, cotton is not the only determinant of denim quality.
Spinning
Spinning also affects the style of the fabric and how it fades.
Spinning involves converting shorter cotton fibers into long yarn using a twisting method, making them suitable for weaving.
Before the Industrial Revolution, spinning was done by hand.
Today, common spinning methods are ring spinning and open-end spinning. Generally, ring-spun yarn has fewer hairy fibers, higher strength, and superior quality. Open-end spun yarn is cheaper: it’s produced quicker, with more short fibers, resulting in a fuzzier yarn that cannot achieve a high count or twist.
Open-end spun yarn is uniform and ideal for producing less expensive jeans. Yarn produced by ring spinning is softer and less uniform, which gives a stronger slubby effect and does not absorb indigo dye as much, thus making the fading more pronounced.
For stretch denim fabrics, elastane fibers are added during spinning, with the ring-spinning method yielding the best results. The elastane becomes the core of the yarn, surrounded by cotton fibers, maintaining the softness and fading characteristics of cotton while adding elasticity.
Indigo Dyeing
The spun yarn is woven into denim fabric, but denim is dyed before weaving, known as yarn-dyed fabric. Typically, only the warp yarns are dyed with indigo, whereas the weft yarns remain uncolored, giving denim its characteristic blue front and white back.
Indigo dyeing involves three key elements: indigo dye, caustic soda, and sodium hydrosulfite. In an alkaline environment (caustic soda), indigo is reduced to a water-soluble leuco form (using sodium hydrosulfite) with an affinity for the fibers. The yarn is dyed and then oxidized to an insoluble blue form, fixing the color.
There are various dyeing methods, with rope dyeing and slasher (sheet) dyeing being most common.
Rope dyeing is widely used in the industry, where warp yarns are twisted into ropes and immersed in indigo vats. When removed, the yarn turns yellow-green but becomes blue through oxidation. The process involves re-beaming and sizing the dyed yarns.
Slasher dyeing, on the other hand, lays white yarns flat in dye vats, repeatedly immersing them in each vat with subsequent oxidation to achieve shades like indigo or sulfur black. The yarns are pre-dried and sized, resulting in evenly dyed warp yarns ready for weaving.
Rope dyeing is suited for deeper indigo shades due to longer oxidation, while slasher dyeing, with its integrated dyeing and sizing, is better for small batches and high-count yarns, though neither process inherently ranks higher in quality.
For denim aficionados, rope dyeing offers a high-contrast fading effect because the indigo doesn’t thoroughly penetrate the yarn core.
The Fading Principle of Indigo Denim:
Indigo dye doesn’t seep into the core of the yarn. With washing and abrasion, the dye on the surface rubs off, revealing the white core.
Weaving
The indigo-dyed warp (vertical) yarns interlace with the white weft (horizontal) yarns through a specific weaving structure to form denim.
Looms require arranging the warp yarns into two layers, creating a shed for the weft insertion device—shuttles, projectile, rapier, air or water jet—to pass through, allowing interweaving with the weft.
Selvedge denim is produced on shuttle looms (traditional looms) and is recognized by the colored edge, often referred to as red selvedge or red ear, an indicator of a narrower fabric width and characteristic edge.
Selvedge denim doesn’t imply superior quality but commands a higher price due to its labor-intensive, costlier production, not its craft or quality. Post-1940s, the inefficient narrow shuttle looms were phased out for mass production.
However, Selvedge is cherished by denim purists for its vintage appeal, not quality. Levi’s pioneered the iconic denim trait of using red selvedge, sparking widespread emulation.
Post-Weaving Adjustments
Upon weaving, denim can be cut and sewn into jeans as raw, dry, or unwashed denim—also known as “raw denim.” This refers to the original, untreated state of denim.
However, raw denim can shrink unpredictably after washing. Thus, brands, both niche and mass-produced, usually opt for sanforized denim, which is pre-shrunk.
Denim undergoing spinning, dyeing, and weaving is under continuous tension and stretches. If the fibers don’t contract before being made into jeans, they’ll shrink upon relaxation, meaning washing.
Sanforization pre-shrinks fabric through processes involving water, steam, heat, and pressure, reducing shrinkage from 5-10% to 1-3% and stabilizing dimensions for easier size selection regardless of later washes.
Sanforized and one-wash denim straddle the line between raw and washed denim—different states of raw,—yet have little effect on “denim nurturance.”
Untreated denim can look fuzzy and lacks dimensional stability, so apart from raw denim, it usually undergoes finishing processes that improve fabric style and feel.
Finishing involves functional processes to correct or prevent size stability issues and creative processes to enhance appearance and feel.
Denim lovers may prize the original raw appearance, but most consumers prefer a soft, comfortable fit and the unique look of washed jeans.
After finishing, the fabric is smoother, brighter, more stable in size, and ready for garment production.
As the market desires new colors and softer textures, denim manufacturers constantly explore new technologies to enhance the fabric’s tactile and visual appeal.
