How does silk form?
A caterpillar pulls a liquid through a nozzle thinner than a hair, and it comes out a solid thread stronger than steel for its weight. Open the worm, turn coils into crystals, reel cocoons into a thread, work out what a sari takes, and meet India's four silks, including the plain fact about the pupa.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- The worm and the fibre. A silkworm is the larva of the moth Bombyx mori. Two long glands store silk as a liquid. One fibre is two strands of the protein fibroin, glued with a gummy protein, sericin, which FAO puts at 19 to 28% of the cocoon shell.
- Liquid to thread, by pulling. Silk does not set by drying or heat. As the liquid moves down a narrowing duct the pH falls, and the pull lines its chains up. They lock side by side as β-sheets held by hydrogen bonds. Cocoon silk breaks near 0.6 GPa: below high-tensile steel at 1.5 GPa, but more than twice as strong per kilogram. Nylon beats it by weight.
- The cocoon. The worm grows about 10,000 times heavier over some 25 days and five instars, then winds one continuous thread of 600 to 1,500 m in a figure of eight. A 0.4 g shell over 1,000 m works out at under 3 denier: grams per 9,000 m.
- The reel to the sari. Hot water softens the sericin, and filaments from several cocoons are drawn together: seven of 3 denier make about 21. Degumming removes about a quarter of the weight and brings out the shine. Then twist, dye and weave. By our sum, a plain 500 g sari takes roughly 2,000 good cocoons; smaller cocoons and heavier silk take several times that.
- Four silks and the pupa. India is second in world silk and first in use, with 41,121 tonnes in 2024-25: mulberry, tasar, eri and Assam's GI-protected muga. To reel a whole thread, the living pupa is killed by heat. Eri and peace silk let the moth leave first, and are spun instead.
- Care and history. Sun, hot irons and alkali all wear silk down. Woven silk goes back to about 3630 BCE in China, wild silk threads to about 2450 BCE at Harappa, then come the Silk Road, the eggs taken to Byzantium, Tipu Sultan's Mysore, Pasteur and pébrine, nylon and the Central Silk Board.
| Term | Meaning |
|---|---|
| Fibroin | The strong protein at the core of a silk fibre. |
| Sericin | The gummy protein that glues the two strands together. |
| β-sheet | Protein chains lying side by side, tied by hydrogen bonds. |
| Hydrogen bond | A weak pull between a slightly positive hydrogen and an oxygen or nitrogen nearby. |
| Instar | A growth stage of a caterpillar between two moults. |
| Denier | The mass in grams of 9,000 m of thread. |
| Reeling | Unwinding filaments from several cocoons at once into one thread. |
| Degumming | Washing the sericin out of silk. |
| Geographical Indication | A registered name that ties a product to its place of origin. |
From a chemical trace in a Chinese tomb to a record Indian harvest, a moth's thread has moved trade, empires and science.
- 6500 BCE · Silk proteins in Neolithic tombs (Researchers at USTC and Henan archaeologists, Jiahu, Henan)
- 3630 BCE · The oldest woven silk found so far (Yangshao culture, Qingtaicun, Rongyang, Henan)
- 2225 BCE · Wild silk threads in the Indus world (Studied by Irene Good, J. Mark Kenoyer and Richard Meadow (2009), Harappa and Chanhu-daro)
- 130 BCE · Silk Road trade opens (Han China and Central Asian traders, China to the Mediterranean)
- 552 · Eggs in hollow canes (Two monks and Emperor Justinian I, Constantinople)
- 1786 · Tipu Sultan seeks silkworms for Mysore (Tipu Sultan, Mysore)
- 1863 · France's cocoon harvest collapses (French sericulture, Southern France)
- 1865 · Pasteur takes on the silkworm disease (Louis Pasteur, Alès, France)
The full story, with 26 moments, charts, people and 21 sources: glassbox.how/e/silkclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
| File | What | Size |
|---|---|---|
glassbox/reel.mp4 |
Reel / Short, with captions and soundtrack | 1080×1920 |
glassbox/video.mp4 |
YouTube video, with captions and soundtrack | 1920×1080 |
glassbox/slide-1…10.jpg |
Instagram carousel | 1080×1350 |
glassbox/thumb.jpg |
YouTube thumbnail | 1280×720 |
glassbox/cover.jpg |
Share card and repo social preview | 1200×630 |
glassbox/history-reel.mp4 |
“History in 10 moments” Reel / Short | 1080×1920 |
glassbox/history-slide-*.jpg |
History carousel | 1080×1350 |
glassbox/post.json |
Post copy and schedule used by the publish kit |
This box has no accounts and no ads, and it ships its own fonts and libraries. When you run it yourself it sends nothing anywhere. On glassbox.how, the site's /bar.js also loads Glassbox's analytics: Google Analytics to count visits (it asks first in the EU, UK and Switzerland, and stays off when your browser sends Global Privacy Control or Do Not Track) and ClickTrust to detect bots.
It remembers a few things in your own browser only, and never sends them anywhere:
| Browser storage key | What it holds |
|---|---|
silkclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
Exactly what each one sees is at glassbox.how/privacy.
- Code: MIT. Use it, change it, ship it.
- Explanations, text, images and videos (
glassbox.json,glassbox/): CC BY 4.0. Credit “Glassbox, glassbox.how/e/silkclear”. - Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
- The Glassbox name and logo aren't covered by either licence. See the terms.
Found a mistake? Open an issue. Corrections happen in public.
It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.
python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
The page and its styles |
js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/silk.js |
Shared models and numbers: the worm, cocoons, protein chains, duct model, denier and fibre figures |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
glassbox.json |
Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how |
reel in each chapter |
The storyboard the Glassbox studio records into short videos |
glassbox/ |
The published video, slides, thumbnail and post copy |
fonts/, vendor/three/ |
Self-hosted Geist and Instrument Serif (SIL OFL 1.1) and three.js (MIT) |