Incense clocks: How to measure time with fire and smoke
How do you know what time it is? Throughout history, we’ve traced the hours with shadows, sand, water, springs and wheels, and oscillating crystals. We’ve even planted clock-gardens full of blossoms that open and close at each hour of the day. Anything that moves with regularity, really, can become a timepiece. But I only know of one kind of timekeeper that was driven by fire: the incense clock.
The incense clock takes the form of a maze of incense, with a tiny ember slowly burning through it. Early in the Qing dynasty (1644–1911), incense clocks burned all night in Beijing’s tall drum tower, measuring out the time until the beating of the huge drum announced the end of the night watch.
According to historian Andrew B. Liu, incense had been used to measure time since at least the sixth century, when the poet Yu Jianwu wrote:
The incense clock takes the basic concept—timing by combustion—and elevates it to a new level of gorgeous complexity. Examining the example held by the Science Museum, I was struck by its diminutive size: no larger than a coffee mug. Yet its small compartments are carefully packed with everything it needs to operate. In the bottom tray, you’ll find a bite-sized shovel and damper; above that, a pan of wood ashes for laying out the incense trail; then, stacked on top, an array of stencils for laying out the labyrinths. As Silvio Bedini, historian of scientific instruments, explains in his extensive study of the use of fire and incense for time measurement in China and Japan, the variety allows for seasonal variation: longer paths to be burned through the endless winter nights, while shorter ones serve for summer.
To set the clock, start by smoothing the ashes with the damper until they are perfectly flat. Select your stencil, then use the sharp edge of the shovel to carve out a groove, following the pattern, and fill it with incense. Finally, cap it with the lacy lid to vent the smoke and control the flow of oxygen.
To track smaller intervals of time, place small markers at regular points along the path. Some versions had little chimneys dispersed across the lid, allowing the hour to be read based on which hole the smoke was venting through. And some users may have used different kinds of incense at different parts of the path, or inserted scented chips along the way, so that they could tell the time with just a sniff.
But just in case the scent of sandalwood wasn’t enough of an alert, people also contrived to create incense-based alarm clocks. A dragon-shaped fire clock offers a particularly beautiful example. The dragon’s elongated body formed an incense trough, across which stretched a series of threads. Small metal balls were attached to opposite ends of the threads. Dangling below the dragon’s belly, their weight held the threads taut. As the incense burned down, the heat broke the threads, freeing the balls to clink into a pan below and sound an alarm.
Bedini offers a description of incense clocks written by Father Gabriel de Magalhaen, a Jesuit missionary to China in the mid-1660s. De Magalhaen reported that he himself had made several clocks for the Chinese emperor, and he had observed the construction of many more, including a much more pedestrian version of the fire-clock concept, based around a spiral of hardened incense paste:
By the 1600s, mechanical clocks were available, but only for the very wealthy; timing by incense was cheap, accessible, and, as the passage notes, perfectly functional. Hence, no doubt, its surprising persistence: well into the twentieth century, writes Liu, coal miners continued to use the glow of incense to track the time they spent underground, while tea-roasters used them to approximate the time it took to toast batches of tea.
This article appeared on JSTOR Daily, where news meets its scholarly match.