How does a diya work?
A clay cup, a cotton wick and a spoon of oil: no moving parts, yet it pumps its own fuel uphill and burns it for hours. See oil climb the wick by capillary action, find out why liquid oil never burns, watch the flame turn into a blue dome without gravity, and count the watts and lumens of a row of lamps.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Four parts, no moving ones. A diya is a hand-thrown terracotta cup about 8 cm across with a pinched spout, a twisted cotton wick (batti), about 20 mL of oil or ghee, and a flame. The flame sits on the wick tip, never on the oil. The spout holds the wick at a slope with its tip just clear of the pool.
- The wick is a pump. Oil clings to cotton, so surface tension pulls it into the narrow gaps between fibres. Jurin's law says vegetable oil could stand about 16 cm high in gaps of 40 micrometres; a diya needs about 1 cm. The limit is speed, not height: thick oil trickles through narrow gaps at a few grams an hour, and less as the oil level falls.
- Only vapour burns. Liquid oil does not burn. The flame's heat boils oil off the wick and cracks it into gas, which burns in a thin shell where it meets air. Inside, tiny grains of soot glow yellow at about 1,200 °C and give nearly all the light. Hot gas rising gives the teardrop shape. On the Space Shuttle and Mir, with no gravity, candle flames became dim blue domes.
- Sixty watts, ten lumens. A small diya burns about 6 g of oil an hour. At 37 kJ per gram that is about 60 W, nearly all of it heat. It gives about 10 lumens, roughly one candle: 0.16 lumens per watt against about 100 for an LED. Soot at flame temperature glows almost entirely in the infrared.
- Oils, ghee, clay and brass. Til, mustard, coconut and castor oil and ghee all hold about the same energy but flow differently. Ghee and coconut oil are solid on a cool day, so the flame first melts a small pool. Low-fired clay is porous, which is why new diyas are soaked in water. Thin kerosene can climb a long wick, and a glass chimney adds a draught, as in Argand's lamp of 1780.
- A row of lamps. Deepavali means a row of lamps. Karthigai Deepam, Dev Deepawali at Varanasi and daily aarti use the same small flame. Ayodhya's Deepotsav lit 2,617,215 oil lamps on 19 October 2025, a Guinness World Record. Fire services ask that lamps stay clear of curtains and loose clothing and are never left unattended. LED diyas give the same glow from a tenth of a watt.
| Term | Meaning |
|---|---|
| Diya | A small oil lamp, most often a clay cup with a cotton wick. Also deepam, deep, pradip, divo and vilakku. |
| Capillary action | A liquid creeping into narrow gaps by itself, even upward, because it clings to the walls. It is what lifts oil up a wick. |
| Jurin's law | The height a liquid climbs in a narrow gap: 2γ cos θ / (ρ g r). Halve the gap and the height doubles. |
| Viscosity | How thick a liquid is. It sets how fast oil can flow up a wick, and it falls quickly as oil warms. |
| Diffusion flame | A flame where fuel vapour and air start apart and burn only in the thin layer where they mix. Diyas, candles and matches burn this way. |
| Soot | Tiny grains of carbon made inside a flame. Hot, they glow yellow and give the light. Unburnt, they are black smoke. |
| Buoyancy | The upward push on hot, light gas. It gives a flame its teardrop shape and pulls in fresh air at the base. |
| Luminous efficacy | Lumens of visible light per watt of power. A flame gives about 0.16, an LED about 100, and the limit is 683. |
| Terracotta | Clay fired at a fairly low heat, from about 600 °C. It is hard and fireproof but still porous. |
Seventeen thousand years from a hollow stone full of fat to 2.6 million clay lamps on one riverbank.
- 15000 BCE · A lamp in the painted cave (The painters of Lascaux, Lascaux, Dordogne)
- 200 · Karthigai lamps in the oldest Tamil poems (Sangam poets, Tamil country)
- 640 · A king's play names the lamp festival (Harsha, Northern India)
- 1012 · Ninety-six ewes for one lamp (Rajaraja I and the shepherds of Thanjavur, Thanjavur, Tamil Nadu)
- 1780 · Argand gives the flame air from inside (Aimé Argand, Geneva, Paris and London)
- 1853 · Surgery by kerosene light (Ignacy Łukasiewicz, Lviv (then Lemberg))
- 1860 · Faraday and the candle (Michael Faraday, Royal Institution, London)
- 1921 · The equation of a wick (Richard Lucas and Edward Washburn, Germany and the United States)
The full story, with 30 moments, charts, people and 50 sources: glassbox.how/e/diyaclear/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:
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|---|---|
diyaclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
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- Code: MIT. Use it, change it, ship it.
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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/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
js/diya.js |
Shared diya physics (Jurin's law, the wick's supply and demand, the Lucas–Washburn soaking time, heat and light sums, Planck's law for glowing soot), the oil table with its sources, the clay diya and flame models, chart boards and helpers |
glassbox.json |
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reel in each chapter |
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