Why Silk Is So Expensive — The Ancient Biology Behind the World's Most Coveted Fabric

Silk costs far more than cotton, wool or polyester, and the reason is not mainly branding. Silk comes from a caterpillar. That caterpillar eats almost nothing but the leaves of one kind of tree, spins a single protein filament that can be well over a kilometre long, and then has to be killed inside its cocoon so the filament can be unwound in hot water without breaking. Every step depends on living animals and careful handwork, and the yield at the end is small.

This article explains the biology and economics behind the price. It covers the silkworm itself, the chemistry of the fibre, how cocoons are turned into thread, the long history of silk as a Chinese monopoly, and who makes the world's silk today, using production data from the International Sericultural Commission (ISC) and technical figures from the Food and Agriculture Organization (FAO).

Key takeaways

• Bombyx mori, the domesticated silkworm, spins a single continuous filament of 600 to 1,500 metres, of which about 80% can be reeled, according to the FAO's silk reeling manual.
• A single cocoon weighs only about 1.5 to 2.5 grams, and the silk shell is roughly 19 to 25% of that weight in modern hybrids.
• The cocoon shell is about 72 to 81% fibroin (the fibre) and 19 to 28% sericin (a gum that must be removed).
• World silk production was about 97,351 tonnes in 2024 (provisional ISC data). China (50,500 t) and India (41,121 t) together made about 94% of it.
• Silk-making in China is very old: biochemical traces of silk have been found in tombs at Jiahu dated to about 8,500 years ago.
• Lab-made silk proteins made by fermentation exist, but have not replaced silkworm silk at scale.

The silkworm: an animal that depends on us

Bombyx mori is the caterpillar of a moth, and one of the most thoroughly domesticated animals on Earth. Its wild relative, Bombyx mandarina, still lives in East Asia and is widely accepted as its ancestor. Over thousands of years of breeding, the domestic form has changed so much that the adult moths cannot fly well, and the caterpillars do not wander off in search of food. Silkworm populations exist only because people raise them. Genetic studies comparing over a hundred domestic strains with wild moths have traced how selection reshaped many parts of the genome. Domestication is commonly dated to roughly 5,000 years ago, although the exact date is uncertain.

For a broader picture of how this compares with dogs, cattle and other species, see our explainer on how animals were domesticated. The silkworm is an unusual case: an insect domesticated for a single product, with many of the hallmarks of domestication seen in mammals, such as tameness and dependence on people. The famous Soviet fox domestication experiment shows how quickly selective breeding can change behaviour; the silkworm shows how far it can go over millennia.

A diet of mulberry leaves

Silkworm caterpillars feed mainly on the leaves of the white mulberry tree, Morus alba. This ties silk farming to mulberry growing. Farmers must plant and maintain orchards, harvest fresh leaves, and time the rearing of each batch of silkworms to when good leaves are available. Leaves must be fresh and clean, and caterpillars need to be fed several times a day as they grow. In practice this means silk farming is labour-intensive agriculture, not factory production.

The life cycle

A silkworm hatches from an egg, eats and grows through five larval stages, called instars, shedding its skin between each. Over this period it grows thousands of times heavier. When fully grown, it stops eating and begins to spin. Over two to three days, it moves its head in a figure-of-eight pattern, laying down a single continuous thread around itself to form a cocoon. Inside, it changes into a pupa. Left alone, the adult moth would emerge after one to two weeks by releasing an enzyme that softens one end of the cocoon and pushing its way out, breaking the thread.

What a cocoon contains

The FAO's Silk reeling and testing manual gives the basic numbers silk producers work with:

Property	Typical value	Source
Total filament length	600–1,500 m	FAO silk reeling manual, ch. 2
Reelable share of filament	about 80%	FAO, ch. 2
Cocoon weight (pure breeds)	1.5–2.2 g	FAO, ch. 2
Cocoon weight (hybrids)	1.8–2.5 g	FAO, ch. 2
Shell as share of cocoon (new hybrids)	19–25%	FAO, ch. 2
Fibroin in shell	72–81%	FAO, ch. 2
Sericin in shell	19–28%	FAO, ch. 2

Source: FAO, Silk reeling and testing manual, Chapter 2: Characteristics of the Cocoon.

Put these numbers together and the problem is obvious. Most of a cocoon's weight is the pupa, not silk. Of the silk shell, a fifth or more is gum that will be washed away. And of the filament itself, about a fifth cannot be reeled into continuous thread. A cocoon of around two grams yields only a fraction of a gram of finished silk, so it takes thousands of cocoons to make a single garment.

The fibre: protein architecture

Silk is a protein fibre, like wool or human hair, but with a very different structure. The silkworm makes two main proteins in its silk glands:

• Fibroin is the fibre itself. A silkworm actually produces two strands of fibroin at once, one from each of its paired glands, which leave the spinneret together.
• Sericin is a sticky gum that coats and glues the two strands together and binds the loops of the cocoon.

Fibroin is built mainly from small amino acids, especially glycine and alanine, in a repeating pattern. These repeats allow protein chains to line up side by side and form flat beta-sheet structures held together by hydrogen bonds. These ordered crystal-like regions give silk its strength, while less ordered regions between them give it some stretch. Ancient traces of silk found in archaeology are identified partly by finding peptides from these crystalline regions.

For comparison, spider dragline silk uses a similar principle, mixing strong crystalline regions with flexible ones, and is tougher still. Our article on how spiders make silk explains the spider's version of the same chemistry.

Why silk shines

Silk's lustre comes partly from the shape of its fibres. Degummed silkworm silk filaments have a roughly triangular cross-section with rounded corners, so they reflect light at different angles, producing a soft shimmer. Most synthetic fibres are extruded through round holes and reflect light more uniformly, although manufacturers can also make "trilobal" synthetic fibres to imitate silk.

Comfort and moisture

Silk takes up moisture from the air without feeling wet; textile handbooks commonly put its moisture regain near 11%, higher than cotton's and lower than wool's. That helps explain why it feels comfortable against skin across a range of temperatures. It is also light for its strength, which is why silk was historically used in parachutes and surgical sutures before nylon and other synthetics took over many of those uses. Medical uses continue: silkworm gut once appeared in early medical devices, a history touched on in our article on the IUD.

From cocoon to thread: the reeling problem

Turning cocoons into raw silk thread is called reeling or "filature." The FAO describes it as combining several cocoon filaments, with a slight twist, into one strand. It sounds simple. In practice, each step affects quality and price.

Step 1: Stifling

To keep the filament whole, the pupa must be killed before the moth emerges. This is called stifling and is done with steam, hot air or other heat. Cocoons are then dried and can be stored. If moths are allowed to emerge, the filament is cut and can only be used for lower-value spun silk.

Step 2: Sorting

Cocoons are sorted to remove double cocoons (two silkworms spinning together), stained, thin or deformed ones. Cocoon quality is graded on measures such as filament length and reelability. In the grading example in the FAO manual, a sample lot had an average filament length of 1,344 metres and a reelability of about 61%, meaning many filament breaks occurred during reeling.

Step 3: Cooking

Cocoons are soaked in hot water to soften the sericin gum just enough that the filament can unwind. Too little cooking and the filament sticks and breaks; too much and the cocoon falls apart.

Step 4: Reeling

A reeler, or a machine, finds the ends of filaments from several cocoons at once and draws them together onto a reel. Because a single filament is far too fine to use alone, several filaments are combined into one thread. The FAO manual describes multi-end reeling using eight cocoons per thread, and automatic reeling working to a size of 21 denier per thread. As each cocoon runs out, or a filament breaks, a new one must be added immediately, or the thread becomes thinner at that point. Filament thickness also varies from the outside of a cocoon to the inside, so operators keep adjusting.

Automatic reeling machines now handle a large share of production, especially in China. But they need well-sorted, uniform cocoons and trained operators, and much of India's production still uses smaller-scale equipment.

Step 5: Degumming

Raw silk still contains sericin. Removing it, called degumming, usually with soap or mild alkali in hot water, reveals the soft, lustrous fibroin. Because sericin is around a fifth or more of the shell, silk loses a substantial share of its weight here. Over-harsh treatment can damage the fibroin itself.

Stage	What is lost or limited	Why it adds cost
Rearing	Fresh mulberry leaves, daily care	Labour-intensive farming
Cocoon	Most weight is pupa, not silk	Low shell share, about 19–25%
Reeling	About 20% of filament unreelable; breaks	Skilled labour or costly machines
Degumming	Sericin, about 19–28% of shell	Weight loss after purchase by weight
Waste	Broken and outer filaments	Sold as lower-value spun silk

Shares from FAO, Chapter 2.

What happens to the rest

Very little is thrown away. Broken filaments, the outer floss and cocoons with emerged moths are spun like wool into "spun silk" and lower-grade noil, which has a duller, more textured feel. The pupae are used in many places as animal feed or food and as a source of oil. These by-products matter to the economics of reeling factories, but they do not change the basic fact that only part of each cocoon becomes premium thread.

A very old secret

Origins in China

Chinese tradition credits sericulture to Leizu, wife of the mythical Yellow Emperor, around the third millennium BCE. Archaeology pushes the story back much further. In a 2016 study in PLOS ONE, Yuxuan Gong and colleagues reported biochemical evidence of silk fibroin in soil from tombs at Jiahu, in Henan province, about 8,500 years old, an age the authors infer from the site's wider chronology because the tombs themselves have no direct isotope dates. This does not prove the silkworm was domesticated then; people may have used wild silk. The FAO's manual says silk was already in use by the middle of the third millennium BCE.

The Silk Road

For many centuries, China was the only source of reeled silk for the rest of the world. Silk travelled west along the overland and sea routes that a 19th-century German geographer, Ferdinand von Richthofen, later called the Silk Road. Rome imported silk at high prices, and Roman writers criticised the luxury. Silk was valuable enough that it also served as a form of currency and tribute in parts of Asia.

China guarded the methods. Silkworm eggs and techniques did eventually spread east and west: to Korea and Japan, to Central Asia, and then to the Byzantine Empire. The Byzantine historian Procopius tells a story of monks smuggling silkworm eggs to Constantinople for the emperor Justinian in the mid-sixth century CE, though how literally to take it is debated. From Byzantium, silk production later spread across the Islamic world and into Italy and France, where cities such as Lucca, Venice and Lyon became famous silk centres.

The modern collapse in many countries

Japan became one of the world's largest silk producers in the late 19th and early 20th centuries and silk was a major export. Production declined sharply after the Second World War as nylon and other synthetics took over many uses and labour costs rose. ISC data show Japan producing only 8 tonnes in 2024, compared with tens of thousands of tonnes at its peak. Similar declines happened in Italy, France and South Korea.

Who makes the world's silk today

The ISC, an intergovernmental body for the silk industry, publishes national production figures. Its 2024 table, marked provisional, shows a total of 97,351 tonnes.

Country	2016 (t)	2020 (t)	2024* (t)
China	158,400	53,359	50,500
India	30,348	33,770	41,121
Uzbekistan	1,256	2,037	3,122
Vietnam	523	969	1,033
North Korea	365	370	370
Thailand	712	520	326
Brazil	650	377	300
Iran	125	270	260
Tajikistan	—	—	227
Japan	32	16	8
World total	192,512	91,765	97,351

Source: International Sericultural Commission, Statistics: Global Silk Production 2016–2024 (table published as an image; 2024 figures provisional).

Three points stand out:

1. China and India dominate. In 2024, China produced about 52% and India about 42% of the world total, roughly 94% together. The ISC also says Asia accounts for about 90% of mulberry silk and almost all non-mulberry silk.
2. China's share has fallen. The FAO's manual, from the 1990s, described China as producing over 70% of world output. In the ISC's 2024 figures it is just over half, and India's output has grown by about a third since 2016. Older claims that China makes "70–80%" of the world's silk are out of date.
3. The China series has a break. The ISC table shows China falling from 120,000 tonnes in 2018 to 68,600 in 2019. Such an abrupt fall is unusual for an agricultural product, and may partly reflect changes in how output is reported. Year-to-year comparisons across that break should be made with care.

To put the total in perspective, the FAO noted in the 1990s that silk made up less than 1% of the natural textile fibre market. Total silk production of around 97,000 tonnes a year is tiny compared with tens of millions of tonnes of cotton or synthetic fibres. Silk is a niche fibre, which limits economies of scale.

Jobs

Silk is also a major employer in the countries that make it. The ISC estimates around 1 million workers in China's silk sector and about 7.9 million people in India, much of it in rural areas, as well as around 20,000 weaving families in Thailand. Those numbers reflect how labour-intensive sericulture remains.

Not all silk comes from Bombyx mori

India is notable for producing several kinds of silk from different moth species, often grouped as "non-mulberry" or "wild" silks:

• Mulberry silk from Bombyx mori, the most common type.
• Tasar (tussar) silk from Antheraea species that feed on trees such as arjun and asan, with a coarser, golden-brown fibre.
• Eri silk from Samia ricini, which feeds on castor leaves. Eri cocoons have open ends, so the silk is usually spun rather than reeled, and the moth can emerge.
• Muga silk from Antheraea assamensis, produced in Assam, known for its natural golden colour.

These silks have different fibre structures and textures. Because the moths are often reared outdoors on host trees, output is smaller and less uniform than mulberry silk.

"Peace silk"

Some producers sell "ahimsa" or "peace" silk, in which the moths are allowed to emerge before the cocoons are processed. Because the filament is broken, it must be spun rather than reeled, giving a different texture. It is usually more expensive because yield is lower and more work is needed per kilogram.

Lab-grown and engineered silk

Scientists have long wanted to make silk proteins without silkworms or spiders. Spiders are especially hard to farm because many species are territorial and cannibalistic.

The main approach is to put silk protein genes into microbes, such as yeast or bacteria, grow them in fermentation tanks, and then spin the purified protein into fibres. Several companies, including Bolt Threads in the United States, Spiber in Japan and AMSilk in Germany, have worked on this, and some have made small runs of garments or materials with partners. The idea links to broader advances in protein design, covered in our article on AI-designed DNA and proteins.

The obstacles are significant. Silk proteins are large and repetitive, which makes them hard for microbes to produce in good yields. Spinning them into fibres that match the strength, softness and lustre of natural silk is also difficult, because the silkworm's gland controls how the protein chains line up as they are spun. So far, these products remain small in volume compared with the roughly 97,000 tonnes of silkworm silk produced each year.

Why silk costs so much: a summary

Bringing the threads together, silk's price reflects several layers of cost that cannot easily be removed:

Factor	Effect on price
Living animal on a single food	Requires mulberry farming and daily care
Small yield per cocoon	Thousands of cocoons per garment
Delicate processing	Stifling, cooking, reeling and degumming need skill
Weight loss	About a fifth or more of the shell is gum removed later
Niche market	Small volume limits economies of scale
Concentrated supply	Two countries make about 94% of world output

Other luxury materials share some of these features. Cashmere depends on combing the undercoat of goats, saffron on hand-picking crocus flowers. The common thread is a biological process that does not scale the way chemical manufacturing does. Silk also has a hidden ecological dependency on insects; for a sense of how much human life relies on them, see what if all insects disappeared.

Frequently asked questions

Q: Why is silk more expensive than cotton? A: Cotton grows on a plant and is harvested by machine. Silk is spun by caterpillars that must be fed fresh mulberry leaves, and each cocoon yields only a fraction of a gram of finished silk after the pupa, gum and unreelable filament are removed. Reeling and degumming also need skilled labour.

Q: How long is the silk thread in one cocoon? A: According to the FAO's silk reeling manual, a Bombyx mori cocoon contains a filament of 600 to 1,500 metres, of which about 80% can be reeled into continuous thread.

Q: Which country produces the most silk? A: China. International Sericultural Commission figures show China produced 50,500 tonnes in 2024 (provisional), about 52% of the world's 97,351 tonnes. India was second with 41,121 tonnes.

Q: Are silkworms killed to make silk? A: For conventional reeled silk, yes. The pupa is killed with heat before the moth emerges, so the filament stays unbroken. "Peace" or ahimsa silk lets the moth emerge first, but the broken filament must then be spun rather than reeled.

Q: How old is silk production? A: Very old. A 2016 PLOS ONE study found biochemical traces of silk in tombs at Jiahu, China, dated to about 8,500 years ago. Chinese tradition and the FAO place silk in use by the third millennium BCE.

Q: Can silk be made in a lab? A: Silk proteins can be made by engineered microbes and spun into fibres, and several companies have done so on a small scale. But matching natural silk's properties at a competitive cost remains difficult, and lab silk is still a tiny fraction of production.

The bottom line

Silk is expensive because its biology resists industrialisation. A domesticated caterpillar that depends on people and on mulberry trees spins a single, very fine filament. Most of the cocoon is not silk, part of the silk is gum, and part of the filament cannot be reeled. Turning what remains into thread needs heat, water and skilled hands. The world makes only about 97,000 tonnes a year, almost all of it in China and India.

The history adds to the mystique: silk-making in China goes back thousands of years, and the trade in it shaped routes and empires. But the price today comes down to arithmetic set by an insect.

Sources

• International Sericultural Commission, Statistics: Global Silk Production 2016–2024.
• FAO, Silk reeling and testing manual, Chapter 1: Introduction and Chapter 2: Characteristics of the Cocoon; Chapter 3: Cocoon Quality and Classification.
• Gong Y. et al., "Biomolecular Evidence of Silk from 8,500 Years Ago", PLOS ONE, 2016.
• "Transcriptome Sequencing and Positive Selected Genes Analysis of Bombyx mandarina", PLOS ONE, 2015.

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