DU 401: How Denim Is Made
Summary
Most people see jeans appear in a store with no visibility into the supply chain that made them. The journey from cotton field to finished bolt is long, technically complex, and — for most of its history — deliberately opaque. This article walks the full lifecycle: cotton plant to spinning mill, dyeing vat to loom, finishing range to bolt. Understanding that journey is one of the most important things a serious denim buyer or maker can do — and the more you understand about how denim is made, the harder it becomes to be indifferent about how it should be.
Q&A
Why is staple length the most important characteristic of cotton for denim?
Staple length — the average length of individual cotton fibers — determines the quality of yarn that can be spun, which determines the quality of the fabric and the character of its fades. Short-staple cotton produces rougher yarn prone to pilling. Extra-long staple (ELS) cotton, at 1.38 inches and above, produces finer, stronger yarn with a natural luster that takes indigo more evenly, fades more cleanly, and has a softness shorter-staple yarn cannot match. The differences in absolute length are small; the downstream effects on the finished jean are enormous.
Why do premium Japanese denim makers prize Zimbabwean cotton?
Zimbabwe produces extra-long staple cotton considered by many to be the finest in the world for denim purposes. The semi-arid climate produces strong, long fibers, and crucially, the cotton is hand-picked rather than machine-harvested. Mechanical harvesters shear fibers and reduce staple length. Hand-picking preserves the full fiber length and removes flawed portions at the source. The result is strong, soft, and slightly irregular — the irregularity being a feature, not a flaw, since it produces the natural slub character that gives high-end raw denim its distinctive surface texture.
What makes rope dyeing produce better fades than slasher dyeing?
Rope dyeing bundles warp yarns into a rope and feeds them through 6–8 dye boxes with oxidation periods between each dip, building multiple concentric layers of indigo around a white core. Slasher dyeing arranges yarns flat in a sheet — faster and cheaper, but producing deeper penetration and less surface concentration. More penetration means less ring dyeing, which means a whiter core is harder to achieve, which means flatter, less dramatic fades. Rope dyeing produces the most concentrated surface indigo with the whitest possible core — the structural requirement for high-contrast fades.
What is the difference between right-hand twill, left-hand twill, and broken twill?
All three are 3×1 twill weaves, but the direction of the diagonal differs. Right-hand twill (RHT) runs lower-left to upper-right — the Levi's standard and global default, producing well-defined horizontal fades. Left-hand twill (LHT), used by Lee, runs the opposite direction, producing a softer hand and more vertical fade character through the interaction of Z-twist yarn with the opposing weave direction. Broken twill, Wrangler's 1964 innovation, alternates direction at regular intervals to prevent the leg twist that develops in both RHT and LHT denim through washing.
What is sanforization and why does it matter?
Sanforization is a mechanical pre-shrinking process applied to denim at the mill using steam, heat, and compression, forcing the yarns to relax before the fabric is cut into jeans. Without it, a washed fabric can shrink by as much as 10%. Sanforized denim allows precise sizing and eliminates guesswork. Unsanforized (shrink-to-fit or loomstate) denim has its devotees — the first wash is a deliberate, managed event rather than just laundering — but for most applications, sanforized is the right choice.
Test Your Knowledge
Click each answer to reveal whether it's correct.
1. What is staple length and why does it matter for denim?
A. The length of the finished bolt — longer bolts produce more pairs per run
❌ Incorrect. Staple length refers to the individual cotton fiber, not the bolt. It is measured in fractions of an inch, not meters.
B. The average length of individual cotton fibers — longer fibers produce finer, stronger yarn that takes dye more evenly and fades more cleanly
✓ Correct. Staple length is the single most important characteristic of cotton for denim purposes. Extra-long staple (ELS) cotton at 1.38 inches and above produces finer, stronger yarn with natural luster that takes indigo more evenly, fades in a cleaner, more defined way, and has a softness shorter-staple yarn cannot match. Small differences in fiber length produce enormous downstream effects on the finished jean.
C. The length of warp yarn in a dye vat — longer vats produce deeper indigo saturation
❌ Incorrect. Staple length is a property of raw cotton fiber, measured before ginning, spinning, or dyeing. It has nothing to do with dye vat length.
D. The width of selvedge fabric — wider selvedge produces more pairs per bolt
❌ Incorrect. Fabric width is a property of the weave, not staple length. Staple length refers to the individual cotton fiber measured in fractions of an inch.
2. What makes ring spinning produce better fades than open-end spinning?
A. Ring spinning uses more indigo per yard of yarn
❌ Incorrect. The amount of indigo applied is determined by the dyeing process, not the spinning method. Ring spinning's advantage is in the character of the yarn it produces.
B. Ring spinning produces natural variations in yarn thickness — slubs — that give fabric surface texture and drive uneven, high-contrast fades
✓ Correct. Ring spinning drafts and twists fiber simultaneously through a rotating ring and traveler mechanism. It's slower and more expensive than open-end spinning, but produces yarn with natural variations in thickness — slubs — that give ring-spun fabric its characteristic surface texture and drive the uneven, high-contrast fades raw denim enthusiasts prize. Open-end spinning produces a more uniform yarn that fades more flatly.
C. Ring spinning produces a tighter weave that holds indigo on the surface longer
❌ Incorrect. Weave tightness is determined by the loom construction, not the spinning method. Ring spinning's advantage is the natural variation in yarn thickness it produces — the slubs that create surface texture and uneven fade potential.
D. Ring spinning is done by hand, producing a more artisanal result
❌ Incorrect. Ring spinning is a machine process — it uses a rotating ring and traveler mechanism. It's not done by hand. The difference from open-end spinning is mechanical, not artisanal.
3. Why is indigo applied to the warp yarn before weaving rather than to the finished cloth?
A. Indigo cannot penetrate woven fabric — it must be applied before the threads are interlocked
❌ Incorrect. Indigo can be applied to finished cloth — it's just not how denim is made. The reason for warp-only dyeing is structural: it creates a fabric that is blue on the face and white on the interior, which makes fading possible.
B. Dyeing only the warp yarns creates a fabric that is blue on the face and white in the core — making fading possible as the surface indigo wears away to reveal the white weft underneath
✓ Correct. The warp yarns are dyed with indigo; the weft yarns are left white. When woven in a 3×1 twill — three warp yarns showing for every one weft — the face of the cloth is predominantly blue while the interior is white. This structure is what makes fading possible: every pair of raw denim is, from the moment it leaves the loom, in the process of revealing the white cotton underneath. The fades aren't something you create. They're something you uncover.
C. Pre-dyeing the yarn is faster and cheaper than dyeing finished cloth
❌ Incorrect. Yarn-dyeing is actually slower and more complex than piece-dyeing finished cloth. The reason for doing it is structural — to produce the warp-face color distribution that makes denim's distinctive fade character possible.
D. Indigo can only bond with unspun cotton fiber, not with woven cloth
❌ Incorrect. Indigo doesn't bond with cotton fiber at all — it physically traps on the surface regardless of fiber state. The reason for warp-dyeing is structural, not chemical.
4. What happened to Vidalia Mills and what does it illustrate?
A. It was acquired by a Japanese mill group, demonstrating the strength of American selvedge production
❌ Incorrect. Vidalia shuttered in late 2024 and its assets were auctioned in 2025. It was not acquired.
B. It shuttered in 2024 after failing to sustain a fully traceable, sustainable American selvedge supply chain — illustrating that good intentions are not a business model
✓ Correct. Vidalia committed to 100% e3 certified cotton, acquired 46 vintage Draper X3 looms from the shuttered Cone White Oak plant, and set out to prove American farm-to-bolt selvedge denim was possible. It shuttered in late 2024, undone by the true cost of sustainable farming, operational complexity, capital shortfalls, and the difficulty of maintaining vintage machinery without institutional knowledge. The vision was right. The execution was brutal. Good intentions, it turns out, are not a business model.
C. It successfully produced American selvedge denim and was eventually sold to Levi's
❌ Incorrect. Vidalia did not succeed and was not sold to Levi's. It shuttered in late 2024 and its assets were auctioned in 2025.
D. It proved that American-made selvedge denim cannot compete on quality with Japanese production
❌ Incorrect. Vidalia's failure was not about quality — the vision and approach were sound. It failed due to the economics of sustainable farming, operational complexity, capital shortfalls, and vintage machinery maintenance challenges.
← Previous: DU 306 | ↑ Denim University | Next: DU 402 →
By Eric Steffen
Founder / Maker
FITTED Underground
The Supply Chain Nobody Talks About
There's a question I get in the workshop more than almost any other. Someone picks up a bolt of fabric, runs their thumb across the face, feels the weight of it, and asks: where does this come from?
The honest answer is that most people in the denim industry couldn't tell you with any precision. The supply chain between a cotton field and a finished bolt of fabric is long, technically complex, and — for most of its history — deliberately opaque. Mills buy from merchants. Merchants buy from ginners. Ginners buy from farmers. By the time a brand puts its label on a finished jean, the cotton that went into it may have passed through a dozen transactions across multiple countries, with no single party able to account for the whole journey.
That opacity isn't inevitable. It's a choice. Understanding what actually happens between seed and selvedge is one of the most important things a serious denim buyer — or maker — can do.
That opacity isn't inevitable. It's a choice. And understanding what actually happens between seed and selvedge is one of the most important things a serious denim buyer — or maker — can do.
This article walks the full lifecycle: from the cotton plant in the field, through spinning and dyeing, through the loom, through finishing, and into the bolt of fabric that eventually becomes a pair of jeans.
Where It Begins: The Cotton Plant
Cotton is a warm-climate crop. It needs long, hot summers, moderate rainfall, and well-drained soil. The major growing regions today are the United States (primarily Texas, California, and the Southeast), India, China, Australia, and West and Southern Africa. Each region produces cotton with slightly different characteristics, shaped by climate, soil, and how the crop is harvested.
The United States grows mostly Upland cotton — the workhorse of the global textile industry. About nine out of ten cotton plants in the world are Upland varieties. It's reliable, mechanically harvestable, and produced at enormous scale. Australian cotton is known for its cleanliness and consistency, and is widely used in high-quality denim that values a heavier, more textured hand. Then there are the specialty cottons — grown in smaller quantities, often hand-picked, and sought after by mills willing to pay a premium for what they produce. This is where the most interesting denim stories begin.
Not All Cotton Is the Same: Staple Length
The single most important characteristic of cotton for denim purposes is staple length — the average length of the individual fiber within a sample. It's measured in fractions of an inch, and the differences between grades, while small in absolute terms, have enormous downstream effects on the yarn, the fabric, and the finished jean.
Short-staple cotton (under about 1.1 inches) is the most common. It spins into serviceable yarn, but the shorter fibers produce a rougher surface and are more prone to pilling. Medium-staple Upland cotton, the global standard, typically falls between 1.1 and 1.25 inches. Long-staple cotton begins to show meaningful improvements in smoothness, strength, and how the fiber accepts dye. Extra-long staple (ELS) cotton, at 1.38 inches and above, is in a different category entirely.
ELS cotton produces a finer, stronger yarn with a natural luster. It takes indigo more evenly and deeply, fades in a cleaner, more defined way, and has a softness that shorter-staple yarn cannot match. Supima cotton — grown primarily in California and Arizona from Pima stock — is the best-known American ELS variety. Egyptian Giza cotton is another. But for denim specifically, the most coveted ELS cotton in the world comes from a more unlikely source.
The Cotton the Japanese Obsessed Over: Africa
When Japanese denim makers in the 1980s and 1990s set out to recreate vintage American denim, they eventually traced the question of raw material back to its source. What made the best vintage Levi's feel the way they felt? Part of the answer was the loom. Part of it was the indigo. And part of it, they discovered, was the cotton.
Zimbabwe produces extra-long staple cotton considered by many to be the finest in the world for denim purposes. The semi-arid climate produces strong, long fibers, and crucially, the cotton is hand-picked rather than machine-harvested. Mechanical harvesters apply significant force to the plant, which shears the fibers and reduces staple length. Hand-picking preserves the full fiber length and removes flawed portions of the crop at the source. The result is a raw material that is strong, soft, and slightly irregular — the irregularity being a feature, not a flaw, since it produces the natural slub character that gives high-end raw denim its distinctive surface texture. Full Count and Momotaro Jeans are among the Japanese brands that have built their identities around Zimbabwe cotton.
West African cotton — particularly from Burkina Faso — has also earned a following among premium denim makers. I have several pieces woven from Burkina Faso cotton in my own collection and it's exceptional. One telling detail: the selvedge ID is often woven in red and green — the colors of the Burkina Faso flag. It's a small gesture, but the kind of specificity that signals a maker paying attention to the full story of what they're producing.
Growing Cotton More Responsibly
Conventional cotton production is resource-intensive. Growing a single pound of cotton requires an estimated 1,300 gallons of water. The crop relies heavily on pesticides and synthetic fertilizers, and continuous cultivation in the same regions causes persistent soil depletion.
Organic cotton addresses some of this — no synthetic pesticides, soil rotation, and significantly reduced water usage in well-managed programs. It costs more to grow and certify, but the farming practices are better for the land and the communities that work it.
The most ambitious American attempt to build a fully traceable, sustainable selvedge denim supply chain was Vidalia Mills in Vidalia, Louisiana. Founded around 2018 in a former Fruit of the Loom facility on the banks of the Mississippi, Vidalia committed to 100% e3 certified cotton — a BASF program that traces cotton from seed to farmer to gin to mill, with documented metrics on water use, pesticide reduction, and soil management. They acquired 46 vintage Draper X3 shuttle looms from the shuttered Cone White Oak plant in Greensboro, North Carolina — the same looms that had woven the red-line selvedge used in Levi's 501s. For a moment, it looked like proof that American farm-to-bolt selvedge denim was possible. Vidalia shuttered in late 2024 and its assets were auctioned in 2025, undone by the true cost of sustainable farming, operational complexity, capital shortfalls, and the difficulty of maintaining vintage machinery without the institutional knowledge to support it. The vision was right. The execution was brutal. Good intentions, it turns out, are not a business model.
Hemp is increasingly discussed as a longer-term alternative fiber, and for good reason. It requires roughly 95% less water than conventional cotton, is naturally pest-resistant, and produces fibers three to four times stronger than cotton that soften substantially with wear and washing. The tradeoffs are real: hemp in its raw state is stiffer and coarser than cotton, the supply chain is still largely China-based, and cultivation infrastructure in the US is still developing following decades of legal restrictions. Cotton-hemp blends are the most practical near-term path — combining hemp's sustainability advantages with cotton's established softness. For FITTED Underground, the direction is clear even if the timeline isn't.
From Fiber to Thread: Spinning
Once cotton is harvested, ginned, and cleaned, it arrives at the spinning mill as a compressed bale of raw fiber. The bales are broken apart, cleaned further, and fed through a carding machine that aligns the fibers into a loose rope-like structure called a sliver. From there, two paths diverge.
Ring spinning is the traditional method. Fiber is drafted and twisted simultaneously through a rotating ring and traveler mechanism. It's slower and more expensive, but produces a yarn with natural variations in thickness — the slight irregularities known as slubs — that give ring-spun fabric its characteristic surface texture and drive the uneven, high-contrast fades that raw denim enthusiasts prize.
Open-end spinning uses centrifugal force to spin fibers directly off a rotating drum. It's faster, cheaper, and produces a more uniform yarn. For mass-market denim, that uniformity is a feature. For raw denim made for fading, it produces flatter, less differentiated results over time.
The direction the yarn is twisted — Z-twist (clockwise) or S-twist (counterclockwise) — has downstream effects worth understanding. Most denim warp yarn is Z-twist. When Z-twist yarn is woven into a right-hand twill, the twist and weave directions align, producing a denser, flatter surface with well-defined horizontal fades. When the same Z-twist yarn is woven into a left-hand twill, the opposing directions gently open the fiber ends, producing a softer hand and more vertical fade character. It's the kind of detail experienced makers feel in the cloth before they can explain it technically.
Dyeing the Warp: Color Before the Loom
This is the step that makes denim unique among woven textiles. With most colored fabrics, dye is applied to finished cloth. With denim, color is applied to the warp yarn before it ever reaches the loom.
The warp yarns — the vertical threads — are dyed with indigo. The weft yarns — the horizontal threads — are left white. When the fabric is woven with more warp than weft showing on the face (exactly what a 3×1 twill produces), the result is a fabric that appears deep blue on the outside and white on the inside. This isn't a surface treatment. It's built into the structure of the cloth.
Indigo doesn't chemically bond with cotton fibers. It ring-dyes only the outer layers of the yarn, sitting on the surface without penetrating to the core. This is what makes fading possible: every pair of raw denim jeans is, from the moment it leaves the loom, in the process of revealing the white cotton underneath. The fades aren't something you create. They're something you uncover.
Achieving good color depth requires multiple dips — typically six to twelve passes through the vat, with oxidation time between each. The yarn emerges from the bath green and turns blue as the indigo reacts with oxygen in the air.
Rope dyeing is the premium method. Warp yarns are gathered into bundles and fed through the dye vats together, producing natural variation in dye penetration that distributes across the fabric width during beaming — creating subtle tonal complexity and superior fade quality. It's slower and more capital-intensive, but the results justify it. It's the method used by premium Japanese mills, including Collect Mills, our fabric supplier in Kojima.
Slasher dyeing arranges yarns in a flat sheet in the same order they'll be woven. It's faster and cheaper, but more susceptible to side-to-side color variation and produces less depth. It's the dominant method in mass-market production.
The fades aren't something you create. They're something you uncover — built into the structure of the cloth from the moment it leaves the loom.
Beyond indigo: sulfur dyes produce blacks, browns, and dark greens — most black denim is sulfur-dyed, and unlike indigo, sulfur fully penetrates the fiber, which is why black denim lightens overall rather than revealing a white core. Reactive dyes bond chemically with cotton and produce vivid, saturated colors used in fashion denim where fade potential isn't the goal. Natural and botanical dyes — persimmon, coffee, woad, natural indigo from the Indigofera plant — are used by smaller producers seeking to reduce chemical load. The cobalt and blue-black shades you sometimes see in premium denim are typically achieved through overdyeing: layering indigo with sulfur black to create a darker, cooler blue with greater depth.
The Loom: Weaving Denim
Denim is a 3×1 twill. Three warp yarns pass over every one weft yarn before the next weft pass begins, and this sequence shifts by one position with each row. The result is the diagonal rib pattern — the twill line — that runs across the face of every pair of jeans ever made.
This structure is what puts indigo on the outside and white on the inside. Three indigo warp yarns surface for every one white weft yarn, so the face of the cloth is predominantly blue. Flip it inside out and white dominates. It's not a trick of the dye. It's woven in.
We've covered the two loom types in depth in DU 101 and DU 102. Briefly: the shuttle loom uses a continuous weft thread that returns at the fabric edge on each pass, creating the self-finished selvedge, producing fabric approximately 30 inches wide. The projectile or rapier loom inserts cut lengths of weft at high speed, producing raw edges and fabric approximately 60 inches wide at far greater output.
Weave direction creates meaningful differences in the finished cloth. Right-hand twill (RHT) — diagonal running lower-left to upper-right — is the Levi's standard and the global default. Left-hand twill (LHT) runs the opposite direction, was used by Lee, and produces a softer hand and more vertical fades through the same Z-twist opposition described above. Broken twill, Wrangler's 1964 innovation, alternates direction at regular intervals to prevent the leg twist that develops in both RHT and LHT denim through washing.
Finishing: Singeing, Sizing, and Sanforization
The fabric that comes off the loom is called greige goods. It's stiff, slightly hairy, and dimensionally unstable — wash it and it would shrink unpredictably. Before it can become a garment, it needs to be finished.
Singeing passes the fabric rapidly over open gas flames, burning off the fine surface fibers without igniting the cloth. The result is a cleaner, smoother face and reduced pilling in wear.
Sizing is a starch-based coating applied to warp yarns before weaving to reduce breakage on the loom. Some sizing remains in the greige fabric after weaving, contributing to the characteristic stiffness of raw denim fresh from the bolt.
Sanforization — covered in depth in DU 103 — pre-shrinks the fabric at the mill using steam, heat, and mechanical compression, forcing the yarns to relax before the denim is cut into jeans. Without it, a washed fabric can shrink by as much as 10%. The vast majority of denim, including most raw denim, is sanforized. Unsanforized denim — shrink-to-fit or loomstate — has its devotees, but for most applications, sanforized fabric is the right choice: it allows precise sizing and eliminates guesswork.
Into the Roll: The Bolt
Once finished and inspected, denim is wound into rolls for shipment. Selvedge denim runs approximately 30 inches wide and 55 meters per bolt. Wide goods run approximately 60 inches wide at similar lengths for small-batch production, though industrial rolls for large manufacturers can weigh hundreds or thousands of pounds.
At the maker scale, a 55-meter bolt of 30-inch selvedge denim yields roughly 15 to 18 pairs of jeans depending on fit and cut efficiency. Each bolt represents the accumulated labor of everyone who touched that cotton from the field to the finishing range. At that scale, waste is not abstract — it's tangible.
What This Means for Us
I started making jeans in 2014 with almost no knowledge of where the fabric came from. I knew what selvedge was. I knew what weight I preferred. Beyond that, the supply chain was a black box.
Over the past decade, learning about it has become inseparable from learning to make jeans well. You cannot make decisions about quality, sustainability, or value honestly if you don't understand what you're sourcing and why.
For FITTED Underground, the fabric relationship that has mattered most is our partnership with Collect Mills in Kojima, Japan — the town where domestic Japanese denim production began, within the Japan Blue Group's network of mills that have been refining this craft for decades. Choosing a supplier like Collect Mills isn't just a quality decision. It's a decision about whose knowledge and values you're aligning with.
But I'm also aware of how much further there is to go. The direction we're moving toward — more organic cotton, recycled fibers where they make sense, and possibly hemp blends as that supply chain matures — isn't driven by marketing. It's driven by paying attention. The more you understand about how denim is made, the harder it becomes to be indifferent about how it should be made.
That's what this level of Denim University is about. Not rules. Not hierarchy. Just a clearer picture of what you're holding in your hands.
What Comes Next
DU 401 traces the journey from cotton field to finished bolt. The next article moves to the workshop: how raw fabric becomes a pattern, how a pattern becomes a jean, and what the decisions made at the cutting table mean for fit, efficiency, and craft. That's DU 402: The Art of Pattern Making.
← Previous: DU 306 — How to Repair Raw Denim | Next: DU 402 — The Art of Pattern Making →
Core Curriculum
Complete the core curriculum by reading these essential classes.
- DU 101 — What is Raw Denim?
- DU 102 — What is Selvedge Denim?
- DU 104 — Denim Weight Explained
- DU 201 — How to Buy Your First Pair of Raw Denim Jeans
- DU 204 — How Much to Spend on Raw Denim
- How It Works — FITTED Underground
Eric Steffen is the founder of FITTED Underground, a custom jeans and raw denim workshop at 108 Bayard Street in Williamsburg, Brooklyn. He has been making jeans by hand since 2014. Denim University is his attempt to share everything he's learned — about the history, the craft, and the culture behind the world's most enduring garment.
