Why Wool Felts and Cotton Doesn't: Natural Fiber Behavior Explained

The fiber determines what you can make.

Wool stretches and springs back. Cotton doesn't. Silk drapes. Linen holds its shape. These aren't aesthetic preferences - they're physical properties built into the fiber structure at the molecular level.

Understanding what each fiber actually does - how it behaves under tension, how it accepts dye, how it wears over time - changes what's possible in textile work. You can't make wool behave like linen, or cotton perform like silk. The material imposes constraints and offers opportunities.

Here's what the major natural fibers are, what makes them different, and why those differences matter.

Protein Fibers vs. Cellulose Fibers: The Fundamental Division

Natural fibers divide into two categories based on their chemical structure.

Protein fibers come from animals. Wool, silk, alpaca, mohair, cashmere. Their structure is keratin (wool) or fibroin (silk) - long chains of amino acids. They have scales, crimp, and elasticity built in.

Cellulose fibers come from plants. Cotton, linen, hemp, ramie, bamboo. Their structure is cellulose - long chains of glucose molecules. They're smooth, straight, and relatively inelastic.

This chemical difference determines almost everything about how the fibers behave:

• Protein fibers felt. Cellulose fibers don't.
• Protein fibers take natural dyes easily. Cellulose fibers resist them.
• Protein fibers have warmth without weight. Cellulose fibers are cooler and heavier.
• Protein fibers stretch and recover. Cellulose fibers don't bounce back.

Everything else - the specific characteristics of wool versus silk, or cotton versus linen - builds on this fundamental protein/cellulose split.

Wool: The Elastic Insulator

Wool is sheep hair. The fiber is covered in overlapping scales and has natural crimp - the waviness you see in raw fleece.

Those scales are why wool felts. When wool gets wet and agitated, the scales catch on each other and lock together. The fiber mass shrinks and densifies. This is useful if you want felt. It's a disaster if you wanted a sweater to stay its original size.

The crimp creates elasticity. Wool can stretch 30-50% of its length and spring back. This makes wool forgiving on the loom - tension irregularities don't necessarily ruin the cloth. It also makes wool comfortable to wear - the fabric gives with body movement.

Wool's structure traps air. Dead air space is insulation. This is why wool is warm relative to its weight, and why it stays warm even when damp. The crimp creates pockets where air gets trapped.

For dyeing: Wool is the easiest fiber to dye with natural dyes. The protein structure accepts mordants readily, and color uptake is strong. Most natural dye instructions assume wool as the fiber.

For weaving: Wool is forgiving. Its elasticity tolerates tension variations. The crimp helps yarns grip each other in the cloth structure. It's a good fiber for learning on a rigid heddle loom because it doesn't punish mistakes as severely as other fibers.

For felting: Wool is the primary felting fiber. The scales and crimp make it possible. Not all wool felts equally well - some breeds produce wool with more pronounced scales and felt more aggressively. Merino felts readily. Some coarse wools resist felting.

Drawbacks: Wool pills (those little balls of fiber that form on the surface). It's vulnerable to moth damage. Some people find it itchy, though that varies by wool type - fine merino doesn't itch the way coarse wool does. And felting, while useful intentionally, is a constant risk with wool textiles.

Silk: Strength and Luster

Silk is the protein fiber that's nothing like wool.

It's extruded by silkworms - specifically Bombyx mori, the domesticated silk moth. The fiber is fibroin, a different protein than keratin. It's smooth (no scales), continuous (individual filaments can be hundreds of meters long), and triangular in cross-section.

That triangular cross-section creates silk's characteristic luster. Light refracts off the angled surfaces, producing shine. The smoothness contributes - there are no scales to scatter light.

Silk is strong. Pound for pound, silk is stronger than steel in tensile strength. This is why silk thread was used historically in surgical sutures and why silk fabrics can be woven so fine yet remain durable.

Silk also drapes. The smooth fiber surface and the flexibility of the long filaments mean silk fabric falls and moves fluidly. This is partly why silk is associated with luxury - the way it moves on the body is distinctive.

For dyeing: Silk takes natural dyes as readily as wool. The protein structure accepts mordants. Colors on silk often appear more intense than on wool because the smooth, lustrous surface reflects more light.

For weaving: Silk is less forgiving than wool. It has no elasticity - silk yarns don't stretch and recover. Tension needs to be even and consistent. But the long filament length means less hairiness - silk yarns are smooth, and the resulting fabric is smooth.

For felting: Silk doesn't felt. No scales, no crimp, no felting. This makes silk stable - it won't accidentally shrink - but also means silk can't be used for most felting techniques.

Drawbacks: Silk is expensive. The cultivation and processing are labor-intensive. Silk also degrades in sunlight faster than other fibers - UV breaks down the protein structure. And silk is vulnerable to perspiration - the acids in sweat can weaken the fiber.

Cotton: The Durable Cellulose

Cotton is the fiber attached to cotton plant seeds. It's almost pure cellulose - around 90% cellulose, with some water and trace minerals.

The fiber is short - individual cotton fibers are typically 0.5-2 inches long. These short fibers are twisted together in spinning to create continuous yarn. The fiber is also relatively flat and twisted - it looks like a collapsed, twisted ribbon under magnification.

Cotton is strong, especially when wet. Unlike most fibers, cotton actually gets stronger when wet. This is why cotton can withstand aggressive washing - it won't weaken or degrade from hot water and agitation the way wool would.

Cotton is absorbent. Cellulose attracts water. Cotton can absorb 24-27 times its weight in water. This makes cotton comfortable in hot weather (it wicks moisture away from skin) and useful for towels and similar applications.

For dyeing: Cotton resists natural dyes. The smooth cellulose structure doesn't provide good mordant attachment points. Cotton typically needs tannin pre-treatment before mordanting to achieve good color. Even then, cellulose fibers don't reach the color intensity that protein fibers do with the same dyes.

For weaving: Cotton is unforgiving. It has almost no elasticity - stretch cotton, and it doesn't spring back. Tension must be even across the warp. Cotton is harder to weave than wool for this reason. But the resulting fabric is durable and stable. Understanding yarn weights and their corresponding dent sizes becomes critical with cotton's lack of give.

For felting: Cotton doesn't felt. No scales, no felting mechanism. Cotton fabrics rely on yarn structure and weave structure for integrity, not fiber bonding.

Drawbacks: Cotton wrinkles. The cellulose structure doesn't have memory - fold cotton, and it stays folded. Cotton also shrinks when first washed (though pre-shrunk cotton addresses this). And cotton is heavy relative to warmth - it doesn't insulate the way wool does.

Linen: The Stiff Cellulose

Linen is processed from flax plant stems. The fiber is also cellulose, like cotton, but structurally different.

Linen fibers are long - up to 36 inches - and composed of multiple fiber bundles held together by pectin. The individual fibers are smooth and straight, with almost no crimp.

Linen is the strongest natural fiber. Higher tensile strength than cotton, and like cotton, it gets stronger when wet. This is why linen was used historically for everything from sails to armor backing - it's durable enough to withstand serious abuse.

Linen is stiff. New linen fabric is crisp and somewhat uncomfortable. It softens with use and washing - the pectin binding the fiber bundles breaks down gradually, and the fabric becomes more flexible. This is why vintage linen is often softer than new linen.

Linen also has natural luster. Not as pronounced as silk, but more than cotton. The smooth fiber surface reflects light.

For dyeing: Linen resists natural dyes just like cotton. Same cellulose challenges. Tannin pre-treatment and careful mordanting are required. The smooth fiber surface can make colors appear slightly more vibrant than on cotton, but it's still harder to dye than protein fibers.

For weaving: Linen is challenging. The fiber has no give - it's even less elastic than cotton. Tension must be precise. The fiber is also brittle when dry - dry linen yarns can snap under tension. Many weavers keep linen warps slightly damp during weaving to maintain flexibility. The warping method chosen matters even more with linen's brittleness.

For felting: Linen doesn't felt. Smooth cellulose fibers don't bond to each other without external binding.

Drawbacks: Linen wrinkles even more than cotton. The stiffness that makes it durable also makes it crease easily. Linen is also expensive relative to cotton - flax processing is labor-intensive. And linen sheds - the long fibers break at stress points and create lint.

Hemp: The Controversial Cellulose

Hemp fiber comes from Cannabis sativa stems. Same plant species as marijuana, different cultivar - fiber hemp has negligible THC content.

Hemp fiber is cellulose, similar to linen in structure. Long fibers, high tensile strength, naturally resistant to mold and UV degradation.

Hemp was historically important for rope, canvas, and sail cloth because it's strong and durable in wet conditions. The fiber structure resists rot better than cotton or linen.

For dyeing: Hemp behaves like cotton and linen - cellulose resistance to natural dyes. Tannin treatment and mordanting are required. Some dyers report hemp takes color slightly better than cotton, though not as well as protein fibers.

For weaving: Hemp handles similarly to linen. Stiff, inelastic, requires even tension. The fiber softens with use, like linen, becoming more comfortable over time.

For felting: Hemp doesn't felt. Cellulose fiber without scales.

Drawbacks: Hemp availability varies by region due to legal restrictions - even though fiber hemp isn't psychoactive, cultivation laws vary. Hemp fabric can be coarse, though processing improvements have created softer hemp textiles. And hemp shares linen's tendency to wrinkle.

Fiber Blends: Combining Properties

Blending different fibers creates textiles with combined properties.

Wool-silk blends combine wool's warmth and elasticity with silk's strength and luster. Common in luxury textiles where both properties are desired.

Cotton-linen blends create fabrics that are more stable than cotton alone (linen's strength) but softer than pure linen (cotton's flexibility).

Wool-cotton blends are less common because the fibers behave so differently. Wool felts and takes dye easily; cotton doesn't. But some textiles use this combination intentionally - the cotton provides structure and durability, wool provides warmth.

For dyeing blends: Each fiber component takes dye according to its own chemistry. A wool-cotton blend dyed with natural dye will show color primarily on the wool, with the cotton remaining paler. This creates heathered or two-tone effects. Some dyers use this intentionally. Others avoid blends because of the uneven color uptake.

Fiber Preparation: From Raw Material to Usable Fiber

Natural fibers don't come ready to use.

Wool requires shearing, skirting (removing dirty/damaged fleece), washing (removing lanolin and dirt), and carding or combing (aligning fibers for spinning). Some wool is sold "in the grease" (unwashed, lanolin intact). Most is sold washed and processed.

Silk requires killing the pupae (boiling cocoons), reeling (unwinding continuous filament), and sometimes degumming (removing sericin, the protein glue holding the cocoon together). Silk can be used with sericin intact (raw silk, which is stiffer) or degummed (smoother, more lustrous).

Cotton requires ginning (separating fiber from seeds), cleaning, and usually carding. Cotton is almost always sold already processed - raw cotton isn't common in consumer markets.

Linen requires retting (rotting the pectin that binds fibers to the plant stem), scutching (breaking the woody core), and hackling (combing to separate and align fibers). Linen processing is labor-intensive, which is partly why linen is expensive.

Hemp follows similar processing to linen - retting, breaking, combing. The machinery and techniques are comparable.

The processing affects fiber properties. Over-processed wool loses crimp and becomes weaker. Under-processed linen retains too much pectin and stays stiff. The quality of fiber preparation shows up in the final textile.

Fiber Diameter and Hand Feel

Fiber diameter determines how fabric feels against skin.

Fine fibers (15-20 microns): Merino wool, cashmere, fine silk. Soft, non-itchy, comfortable against skin. More expensive.

Medium fibers (20-30 microns): Most commercial wool, cotton, linen. Comfortable for most people, though some find medium wool scratchy.

Coarse fibers (30+ microns): Some wool breeds, some hemp. Typically scratchy, better for outerwear or items that don't touch skin directly.

Fiber diameter also affects dyeing. Finer fibers have more surface area relative to mass, so they can take up more dye. Fine wool often produces more intense colors than coarse wool given the same dye bath.

Durability and Wear Patterns

Different fibers fail in different ways.

Wool pills and eventually thins in high-abrasion areas (elbows, seat of pants). The scales gradually break down, and fiber fragments form pills on the surface. Wool is vulnerable to moth damage - the larvae eat the protein.

Silk degrades from UV exposure and acid. Sunlight weakens the protein structure over time. Perspiration (acidic) can damage silk in areas of high contact (underarms, collars).

Cotton wears out from repeated stress and washing. The short fibers eventually break and the fabric thins. Cotton is vulnerable to mildew if stored damp.

Linen develops characteristic softness with age as the fiber bundles break down. Very old linen becomes threadbare but often gets softer. Linen resists mildew better than cotton.

Understanding wear patterns helps in choosing fibers for specific uses. A baby blanket that will be washed constantly: cotton or linen. A garment that needs to last for decades: wool or linen. Something for hot weather: cotton or linen. Something luxurious: silk.

Elasticity and Recovery

Fibers respond differently to stretching.

Wool can stretch 30-50% and recover fully. This is why wool sweaters maintain their shape and why wool is used in fitted garments.

Silk has some stretch (10-20%) but less recovery than wool. Silk garments need careful handling to avoid stretching out of shape.

Cotton has minimal stretch (3-7%) and poor recovery. Stretch cotton fabric relies on elastic fibers or tight weave structure, not the cotton fiber itself.

Linen has almost no stretch (2-3%) and no recovery. Linen doesn't bounce back after stretching.

This affects textile construction. Wool can be woven with less precision because the fiber tolerates tension variations. Cotton and linen require more careful tension control. Fitted garments work better in wool than cotton for this reason.

Moisture Management

Natural fibers interact with water differently.

Wool absorbs moisture into the fiber structure (up to 30% of its weight) while the exterior remains relatively dry. This is why wool feels warm even when damp - the moisture is trapped inside the fiber, not sitting on the surface against skin.

Silk also absorbs moisture (up to 30%) and wicks it away from skin. Silk feels cool initially but regulates temperature well.

Cotton absorbs moisture readily (24-27% of its weight) and releases it slowly. Cotton feels wet when it's wet - the moisture sits on the fiber surface. Good for towels, less ideal for athletic wear where moisture needs to evaporate quickly.

Linen absorbs moisture even more readily than cotton (30%+) and releases it faster. Linen dries quickly. This is why linen is traditional for hot weather clothing - it absorbs perspiration and dries before becoming uncomfortable.

Environmental Considerations

Different fibers have different environmental impacts.

Wool production requires grazing land and water for sheep. Sheep produce methane (greenhouse gas). But wool is renewable annually, biodegradable, and requires minimal chemical processing.

Silk production requires killing pupae (ethical concern for some). Silk cultivation is energy-intensive (maintaining consistent temperature for silkworms). But silk is renewable and biodegradable.

Cotton production is water-intensive and typically involves pesticides and herbicides. Conventional cotton farming has significant environmental impact. Organic cotton addresses some concerns but still requires substantial water.

Linen/hemp production requires less water and fewer pesticides than cotton. Flax and hemp are relatively low-impact crops. Processing is energy-intensive but doesn't require harsh chemicals.

These considerations are increasingly relevant as textile production scales. The fiber choice has environmental consequences beyond its performance characteristics.

Historical Context: Why Certain Fibers Dominated Certain Regions

Fiber availability shaped textile traditions.

Wool dominated in regions with grazing land - northern Europe, Central Asia, the Andes. Sheep farming was economically viable, and wool provided warmth in cold climates.

Silk originated in China and spread along trade routes. Silk production requires specific climate conditions and expertise. Regions without access to silk cultivation imported it at high cost.

Cotton grows in warm climates with adequate water - India, Egypt, the American South. Cotton cultivation and processing developed where the climate supported it.

Linen grows in temperate climates - Europe, parts of Asia. Flax cultivation was widespread in regions too cold for cotton.

These regional patterns explain why certain textile traditions developed around specific fibers. Irish linen, Merino wool from Spain, Chinese silk, Indian cotton - these aren't arbitrary associations. They reflect where the fiber source was available and where expertise developed.

Choosing Fiber for Specific Applications

The fiber determines what works.

For weaving beginners: Wool. The elasticity forgives mistakes, and the crimp helps yarns grip each other in the cloth structure.

For durability: Linen or cotton. Cellulose fibers withstand repeated washing and wear better than protein fibers.

For natural dyeing: Wool or silk. Protein fibers take natural dyes readily with standard mordanting.

For drape: Silk. Nothing drapes like silk.

For warmth without weight: Wool. The crimp and scales trap air for insulation.

For hot weather: Linen or cotton. Cellulose fibers feel cool and manage moisture.

For felting: Wool. Only protein fibers with scales will felt.

The material imposes constraints. Understanding what each fiber actually does - not what you want it to do, but what its physical structure determines - changes what's possible. You work with the fiber's properties, not against them.

What Natural Fibers Share

Despite their differences, natural fibers have commonalities.

They're all biodegradable. Leave natural fiber textiles exposed to weather, and they'll decompose. This is an advantage (environmental) and a disadvantage (preservation).

They all respond to tension, moisture, and heat. The specific response varies, but all natural fibers change under these conditions.

They all require processing from raw material to usable fiber. None emerge ready-to-use.

And they all have been used by humans for thousands of years. The techniques for working with these fibers - spinning, weaving, dyeing, felting - developed alongside agriculture and animal domestication. The craft knowledge accumulated over generations is embedded in how we work with these materials now.

That continuity matters. Working with natural fibers connects to textile history in ways that synthetic fibers don't. The wool you're working with behaves the same way wool behaved in medieval Europe or ancient Persia. The silk has the same properties it had on the Silk Road. The techniques and knowledge transfer across time because the materials themselves haven't changed.

Natural fibers aren't better than synthetics in all applications. But they're different. And understanding what makes them different - what each fiber actually is and how it behaves - determines what you can create with them.