Protein Fibres: Examples, Properties and Uses of Wool and Silk

Two of the most prized materials in clothing, wool and silk, look nothing alike. One is warm, springy and slightly fuzzy. The other is smooth, cool and shiny. Yet they belong to the same family. Both are protein fibres, which means they are made of long chains of amino acids, the same building blocks that make up hair, nails and muscle.

This guide explains what protein fibres are, where they come from, how their chemistry gives them their special properties, and how they compare with cotton and synthetic fibres. You will find a table of the main examples, a comparison of wool and silk, tips for dyeing and care and a worked example that counts how many fibres sit in a wool yarn. If you want to place protein fibres in the bigger picture, our guide on the classification of textile fibres shows where they fit.

What are protein fibres?

Protein fibres are natural fibres of animal origin, made of proteins. A protein is a long chain of amino acid units joined by peptide links. In textile terms, protein fibres are usually grouped as:

• Hair or fur fibres. Wool from sheep, and speciality hair such as cashmere, mohair, alpaca, camel hair and angora.
• Secretion fibres. Silk, which is a thread produced by the silkworm and other insects to make cocoons.

There is also a small group of regenerated protein fibres, made from proteins taken from milk (casein), soya beans or maize and spun into fibre. They are man-made and not widely used, but they appear in niche markets. See our article on milk fibre and casein fabric for one example.

Main protein fibres and where they come from

Fibre	Source	Main protein	Typical features
Wool	Sheep fleece	Keratin	Warm, elastic, crimped
Cashmere	Cashmere goat undercoat	Keratin	Very fine, soft, light
Mohair	Angora goat	Keratin	Lustrous, strong, springy
Alpaca	Alpaca	Keratin	Soft, warm, silky
Camel hair	Camel undercoat	Keratin	Soft, warm, golden colour
Angora	Angora rabbit	Keratin	Very soft, fluffy, light
Silk (mulberry)	Silkworm cocoon	Fibroin, with a sericin coating	Smooth, lustrous, strong
Wild silk (tussah)	Wild silkworm cocoon	Fibroin	Coarser, tan colour

Do not confuse angora rabbit hair with mohair from the angora goat. Both names come from the Turkish city of Ankara, but they are different animals and different fibres.

The chemistry in plain words

Keratin in wool and hair

Keratin is rich in an amino acid called cystine. Cystine has sulphur in it, and two cystine units can link to each other through a strong sulphur-sulphur bond. These cross-links tie neighbouring protein chains together. They give wool its strength and its ability to stretch and recover. Heat and moisture can break and reform some of these bonds, which is why wool can be set into a new shape in steam, and why it can also shrink and felt.

The surface of a wool fibre is covered with overlapping scales, a bit like roof tiles. These scales help the fibres grip each other in yarn, and they are also the reason wool felts when it is rubbed in warm, wet conditions. For more on how wool is processed, see our guide to wool processing from shearing to scouring.

Fibroin in silk

Silk fibroin is made mainly of three small amino acids: glycine, alanine and serine. Small side groups let the chains pack very closely in neat sheets. This makes silk strong and smooth. A natural gum called sericin coats the raw silk thread. It is removed by a process called degumming, which gives silk its soft hand and shine. Our article on silk fibre production explains how silk is formed and reeled, and our page on what silk is made of goes into the chemistry.

General properties of protein fibres

Protein fibres share a few features that set them apart from cellulose fibres like cotton and from synthetics.

Property	Protein fibres in general
Moisture absorption	High. Wool about 13.6% and silk about 11% standard regain
Comfort	Soft, warm in cold, breathable, good moisture handling
Elasticity	Wool is very elastic. Silk is moderately elastic
Strength	Wool is weak but flexible. Silk is strong
Heat	Chars rather than melts. Wool resists flame fairly well
Burning smell	Burning hair or feathers
Alkali	Damaged. Wool is attacked quickly by hot alkali
Acid	Fairly resistant to dilute acids
Chlorine bleach	Damages and yellows protein fibres
Sunlight	Silk weakens in prolonged sunlight
Moths and beetles	Wool and silk can be eaten by larvae
Dyeing	Take acid dyes, metal-complex dyes, reactive dyes and some natural dyes
Static	Low to moderate

These are general figures. Real values vary with the breed, the climate, the grade and processing. For wider comparisons, see our overview of the properties of textile fibres.

Wool and silk compared

Point	Wool	Silk
Form	Short staple, crimped	Very long continuous filament
Surface	Scaly	Smooth
Handle	Warm, bouncy, soft to prickly	Smooth, cool, fluid
Lustre	Low	High
Strength	Low to moderate	High for a natural fibre
Elasticity	Very good	Fair
Insulation	Excellent	Good, light
Typical fabrics	Knitwear, suiting, coats, blankets	Sarees, shirts, scarves, linings, eveningwear
Main weakness	Felting, moths, alkali	Sunlight, perspiration, water spots
Typical care	Gentle wash, dry flat	Dry clean or hand wash, low iron

Wool keeps you warm because of its crimp and trapped air. If you want that story in more detail, read why we wear woolen clothes in winter.

A worked example: how many fibres are in a wool yarn?

Wool spinners judge yarn quality partly by how many fibres sit in the yarn cross-section. Too few and the yarn is weak and uneven. This calculation uses fibre diameter and yarn count.

Given

• Wool fibre diameter: 20 micrometres (a common merino-type value)
• Wool density: 1.31 g/cm³ = 1,310 kg/m³
• Yarn count: Nm 48 (metric count, meaning 48 km of yarn weigh 1 kg)

Step 1: Cross-section area of one fibre.

Assume a round fibre. Area = π ÷ 4 × d²

d = 20 × 10⁻⁶ m

Area = 0.7854 × (20 × 10⁻⁶)² = 0.7854 × 4 × 10⁻¹⁰ = 3.14 × 10⁻¹⁰ m²

Step 2: Linear density of one fibre in tex.

Mass per metre = area × density = 3.14 × 10⁻¹⁰ × 1,310 = 4.115 × 10⁻⁷ kg/m

Tex means grams per 1,000 m, or kilograms per 1,000,000 m. So multiply kg per metre by 10⁶:

Fibre tex = 4.115 × 10⁻⁷ × 10⁶ = 0.41 tex (about 4.1 dtex)

Step 3: Linear density of the yarn.

Tex = 1,000 ÷ Nm = 1,000 ÷ 48 = 20.83 tex

Step 4: Number of fibres in the cross-section.

Fibres = yarn tex ÷ fibre tex = 20.83 ÷ 0.4115 = about 51 fibres

A common rule is that ring spun yarn needs at least 30 to 40 fibres in the cross-section to be spinnable and reasonably even. So an Nm 48 single yarn from 20 micron wool, at 51 fibres, is comfortably spinnable. If the spinner wanted a finer yarn of Nm 80, the tex would be 1,000 ÷ 80 = 12.5, and the fibres would be 12.5 ÷ 0.4115 = 30. That is near the lower limit, so a finer wool of 17 to 18 micron would be better for that count.

This is why fine yarns for light suiting and knitwear need finer wool. Fibre diameter limits how thin a yarn can be.

Moisture and trade weight

Wool trading uses a standard conditioned weight. Commercial regain for scoured wool top is often taken as 18.25 percent. So 1,000 kg of dry wool is traded at 1,000 × 1.1825 = 1,182.5 kg. A buyer paying by trade weight must therefore know the moisture basis in the contract. Compare this with the standard regain of 13.6 percent used in textile testing. If 100 kg of dry wool is conditioned at standard regain, the weight is 100 × 1.136 = 113.6 kg.

Uses of protein fibres

Wool and hair fibres

• Sweaters, cardigans, socks, hats and gloves
• Suits, jackets and coats
• Blankets, rugs and carpets
• Felt, for hats, crafts and industrial use
• Insulation and soundproofing
• Base layers and outdoor wear, using fine merino

Silk

• Sarees, scarves, ties and shirts
• Evening dresses and linings
• Bed linen and pyjamas
• Embroidery thread and surgical sutures
• Parachute cloth in the past, and some technical uses today

Speciality fibres

• Cashmere for luxury knitwear
• Mohair for shiny, textured jackets and yarns
• Alpaca for soft, warm scarves and sweaters
• Angora for very soft knits, often blended

Dyeing protein fibres

Protein fibres have both acidic and basic groups along the chain, so they can be dyed in acidic baths. The common dye classes include:

• Acid dyes. The standard choice for wool and silk. They bond with the basic groups on the fibre in an acid bath. Our guide on acid dyes: examples, properties and uses covers this class.
• Metal-complex dyes. Give deep shades with better fastness.
• Reactive dyes. Used on wool and silk for bright colours with good wash fastness.
• Natural dyes. Plant and insect dyes were used for centuries on wool and silk. See our page on natural dyes: sources and examples.

Dyeing temperature matters. Wool is usually dyed near the boil, but it must be heated gradually and not agitated too much, to avoid felting. Silk is dyed at lower temperatures, and care is taken to avoid fibre damage and uneven dyeing.

Care of protein fibres

1. Read the label. Some wool is machine washable, but much is not.
2. Use cool or lukewarm water and a mild, pH-neutral detergent. Alkaline detergents damage protein fibres.
3. Avoid rubbing and twisting. These cause felting and distortion.
4. Do not use chlorine bleach. It can dissolve or yellow wool and silk.
5. Dry flat and away from direct sun and heat.
6. Iron on low heat with a cloth, or steam.
7. Store clean items in breathable bags with moth protection. Moths lay eggs in soiled wool.
8. Dry clean delicate items as advised.

Our guide to care labels and wash symbols can help you decode the instructions.

How to recognise a protein fibre

A small burn test gives a quick hint (do this safely and only on a scrap):

• Protein fibres burn slowly, may self-extinguish when the flame is removed and smell like burning hair or feathers.
• The ash is a brittle black bead or crushable residue.
• By contrast, cotton burns quickly to soft grey ash, and polyester melts into a hard bead.

For a firm identification, laboratories use microscopes. Wool shows scales, and silk shows smooth, structureless filaments.

Another common lab method is the solubility test. Wool dissolves in hot sodium hydroxide solution, while cotton does not. Chlorine bleach also attacks wool and silk badly, but it hardly touches polyester.

Environmental points

Protein fibres are renewable and biodegradable, and they last a long time when well cared for. But they have costs. Sheep farming involves land use and methane emissions. Silk production involves the death of the pupae in many systems, which some buyers object to, and there are alternatives such as peace silk. Cashmere has raised concerns about overgrazing. Look for certifications for animal welfare, traceability and responsible farming when these matter to you. Compare these with natural and synthetic fibres to see the wider trade-offs.

Final thoughts

Protein fibres are made of amino acids, and that chemistry gives them a special mix of warmth, softness, elasticity and moisture handling. Wool and hair fibres are built on keratin, and silk is built on fibroin. They take acid dyes well, resist flame fairly well, and need gentle care because alkalis, chlorine, heat and moths can damage them.

Our worked example showed why fibre diameter matters: a 20 micron wool gives about 51 fibres in an Nm 48 yarn, which is easy to spin, but an Nm 80 yarn from the same wool would be near the limit. When you buy or specify protein fibres, think about the fibre diameter, the moisture basis of the trade weight and the care the final garment will need.

Frequently asked questions