CommodeClear

How does a commode work?
No motor, no pump: a bent pipe that stays full of water, and a tank that empties in three seconds. Cut a commode in half in 3D, flush it two ways, and see why a fast bucket works and a slow one never does.

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1. A bowl, a bend and a tank. A commode is one piece of glazed ceramic with a bowl on top and a bent passage, the trap, cast in behind it. On the back sits the cistern, a tank holding one flush of water, with a fill valve, a float, a flush valve and an overflow tube inside.
2. The trap is a plug made of water. Water always stands in the bend of the trap, level with its lip, the weir. That 50 mm of water blocks the gas from the drain. It is a gentle seal: about 490 pascals of suction pulls it out, and a few weeks without a flush can dry it up.
3. A flush is a push or a pull. A wash-down pan, the usual kind in India and Europe, dumps the water in fast and pushes the contents over the weir. A siphonic pan, the usual kind in North America, fills its narrow trapway so the falling water sucks the bowl empty until air breaks the siphon. Either way it has to be fast, which is why a bucket works only when poured quickly.
4. Two valves and a float. Pressing the button lifts the flush valve and about six litres leave in three seconds, or three litres on the half flush. The float then falls, opens the fill valve, and rises with the water to shut it at the set line. The overflow tube catches any excess. A worn seal or a float set too high makes the cistern run on.
5. Indian pan, Western pan, same trap. A squat pan and a seated commode are the same machine underneath. A pour-flush Indian pan has a steep glazed floor and a shallow seal, so 1.5 to 2 litres from a mug does the job that takes a cistern 3 to 6 litres.
6. Then it needs somewhere safe to go. The outlet joins a soil stack whose top is a vent, open above the roof, so that falling water cannot suck the traps dry. At the bottom is a sewer, a septic tank, or the twin leach pits used across rural India, where one pit rests and dries while the other fills.

4,500 years from brick drains in the Indus cities to seats that wash, and the unfinished work of a safe toilet for everyone.

• 2500 BCE · Toilets and brick drains in the Indus cities (Indus Valley builders, Mohenjo-daro and Lothal (today Pakistan and India))
• 1596 · Harington's water closet (Sir John Harington, Kelston, Somerset, England)
• 1775 · The S-bend (Alexander Cumming, London, England)
• 1851 · Spending a penny at the Great Exhibition (George Jennings, The Crystal Palace, Hyde Park, London, England)
• 1858 · The Great Stink and London's sewers (Joseph Bazalgette, London, England)
• 1884 · One piece of pottery (Thomas William Twyford, Hanley, Stoke-on-Trent, England)
• 1970 · Sulabh and the twin-pit toilet (Bindeshwar Pathak, Bihar, India)
• 1980 · Two buttons, two flushes (Caroma, Australia)

The full story, with 30 moments, charts, people and 47 sources: glassbox.how/e/commodeclear/history. The data lives in history.json.

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