Incense Clock: How China Measured Time with Smoke
In China, a smoldering line of incense could mark the night watches, wake a sleeper, or even let you smell what time it was.
A clock made of slow fire
An incense clock works on a simple principle: incense burns slowly and fairly evenly. If you lay it along a path of known length, you can tell how much time has passed from how far the glowing ember has traveled.
The Science Museum in London describes Chinese incense clocks as serving three purposes, practical, recreational and religious, and as reading out the time through sight, sound, touch or smell. Its own example is a little brass and nickel-silver box only 85 millimeters high.
JSTOR Daily points out that in the 1600s mechanical clocks existed but were mostly limited to the very wealthy, while incense was cheap. That made fire one of the most accessible ways to measure time, especially at night.
How old are incense clocks?
Dating them depends on what you count. JSTOR Daily cites writing by the sixth-century Chinese poet Yu Jianwu that links burning incense with knowing the hour of the night, which suggests incense was already used to measure time by then.
The Science Museum points to a later and more specific record. In 1073, a severe drought left many municipal water clocks without water. Fire took their place in a public clock called the Hundred Gradations Incense Seal, which could burn for up to 100 nights.
These two dates are not really in conflict. One shows that people were telling time by incense early on; the other records an incense clock built as public infrastructure.
Much of what we know in English comes from two studies: Silvio Bedini's The Scent of Time, published by the American Philosophical Society in 1963, and Andrew B. Liu's article Incense and Industry, published in the journal Past and Present in 2016.
Seal clocks: a maze of powder
The most ingenious form is the seal clock, in which powdered incense is laid in a maze-like pattern. The Science Museum's example, cataloged as made in China sometime between 1600 and 1950, contains three trays, two maze stencils, a leveler and a latticework lid.
To use it, you smoothed a bed of ash, chose a stencil, pressed the pattern into the ash, filled the groove with powdered incense and replaced the lid. Lit at one end, the incense burned slowly along the whole path. Each stencil corresponded to a different span of time.
According to the historian of instruments Silvio Bedini, as summarized in JSTOR Daily, the paths could also change with the seasons: longer ones for long winter nights, shorter ones for summer. The lacy lid let smoke out and controlled the flow of air, and the Science Museum adds that it shielded the ember from drafts.
Some lids had small chimneys over different parts of the path, so the hour could be read from where the smoke was rising.
Not every incense clock was so elaborate. The Science Museum says powdered clocks were mainly personal, portable timekeepers for the home, while sticks and spirals, some with notches to track time, were used for religious timekeeping during festivals and rituals.
Smelling the hour
The most charming detail is that some incense clocks told time by scent. The Science Museum notes that incense sticks were made to standard lengths and that their scents could be used so that a person could tell the time by smell.
JSTOR Daily describes users laying different incenses along different parts of the path, or adding scented chips, so the fragrance in the room changed as the hours passed. In principle, you could notice the hour change without ever looking at the clock.
The Science Museum adds that the smoke drifting through the lid changed in strength over time, and that this slow change could stir scholars' emotions. An incense clock was a timer, but it was also an atmosphere.
Dragon clocks and incense alarms
Incense could also wake you up. Royal Museums Greenwich holds an early 19th-century Chinese fire clock shaped like a dragon, made of black and gilt lacquered wood. A trough along its center held a burning incense stick.
The Greenwich example is long and slim, about 695 millimeters from end to end but only 45 millimeters deep.
Threads were laid across the trough at intervals, each with small metal weights hanging from both ends. When the burning stick reached a thread, it burned through, and the weights dropped onto a metal tray below with a clang, marking the time. JSTOR Daily describes the same idea with metal balls falling into a pan.
In the 1660s, the Jesuit missionary Gabriel de Magalhaen described a version made from a spiral of hardened incense paste, marked to divide the night into five parts. When the fire reached the chosen mark, a small weight fell into a brass basin and woke the sleeper.
See also: the story of the alarm clock on OnlineClock.app.
From night watches to temple spirals
Incense clocks were not just household curiosities. JSTOR Daily reports that in the early Qing dynasty, incense clocks burned through the night in Beijing's drum tower, where the drum's beat marked the end of each night watch. Liu notes that coal miners and tea roasters were still using incense to track time into the 20th century.
Metal incense clocks, made of nickel, silver or pewter and lighter and smaller than older wood and stone versions, were popular gifts among scholars. They often carried symbols of growth and prosperity. The Science Museum's example has the double happiness character, linked with love and marriage, along with characters for good fortune and for spring, the latter tied to the spring festival.
Bedini's study, The Scent of Time, covered fire and incense timekeeping in both China and Japan. Today incense is still burned in East Asian temples, mainly as an offering rather than a timer; the Science Museum points to the incense spirals of Man Mo Temple in Hong Kong.
See also: a free online alarm that needs no incense on OnlineClock.app.
Questions people ask
How does an incense clock work?
Incense is laid along a path of known length. Because it burns at a steady rate, the distance the ember has traveled shows how much time has passed.
Could incense clocks work as alarm clocks?
Yes. In dragon-shaped fire clocks, the burning incense cut through threads holding metal weights, which dropped onto a tray with a clang.
How old are incense clocks?
Writing by the poet Yu Jianwu suggests incense was used to tell the time in China by the 6th century CE, and a large public incense seal clock is recorded in 1073.
Could you really tell the time by smell?
Yes. Some users placed different incenses along different parts of the path, so the scent in the room changed as time passed.