After being invented in Asia, the fire clock, also known as a combustion clock, arrived in Europe around the 8th century CE. Replacing the hourglass, which had to be turned over at regular intervals, it allowed time to be measured by using a combustible material whose burning was both slow and uniform, and thus predictable – such as a candle or oil in a lamp. Let’s discover the history of this clock.

The Fire Clock, a Lauded System

The fire clock originated in Asia, particularly in China – Japan later adopted it. Traces of its use date back to the 6th century BCE.

The principle is simple: it uses sticks coated with a combustible paste, what we now call incense. Because it burns at a fixed rate, incense is an excellent indicator of passing time. The stick is placed on a horizontal support, most often a hollow lacquer object. As it burns, the incense stick passes markings that allow the elapsed time to be calculated.

Another way to mark the hours is to place weighted silk threads at regular intervals along the stick, on each side, with small suspended weights (typically metal beads). When the combustion reaches a thread, it burns and releases the weights, which fall into a metal receptacle, producing a recognizable sound. Thus, not only did the Chinese invent the stopwatch (calculating a relatively precise duration, represented by the length of the incense stick), but they also conceived the alarm clock!

With the aesthetic care characteristic of ancient China, the lacquer supports for incense clocks, on which the sticks are placed, are far from ordinary receptacles. The archetypal model is what is called a dragon boat: a base in the shape of a vessel, whose bow and stern represent the head and tail of a dragon. Around the support, two receptacles are designed to receive the beads held by the silk threads.

Measuring Time with a Candle

The fire clock arrived in Europe in the 8th century, possibly adopted (or reinvented?) by a monk who needed a time-measuring system to count the nocturnal hours between each prayer. The combustible stick of the Chinese incense clock was replaced by a candle, one meter high, regularly graduated to mark the hours. Thus, the burning of the candle indicated the number of hours elapsed, with a precision that depended on several criteria: the regularity of the candle, the quality of the wax, the thickness and distance between the markings, and, of course, any potential drafts.

Indeed, the first uses of the combustible clock were reserved for religious congregations, so that they could best adhere to prayer schedules during the night. Three one-meter candles were generally needed to last until dawn. This device nevertheless quickly spread throughout medieval Europe.

The candle has therefore always been a symbol of the passage of time, and its burning a representation of time’s fleeting nature – even to this day, when we continue to blow out candles to celebrate birthdays. Before the invention of the oil lamp, candles were used in theaters to light the stage and actors – the duration of an act was thus conditioned by the burning time of the candle. In the Middle Ages, it also became a symbol of wealth: the wax candle, more luxurious than its tallow equivalent, was the prerogative of wealthy families. And it could not be used as a fire clock in less affluent households, who had to preserve it for as long as possible.

The Evolution of the Combustible Clock

Between Asia and medieval Europe, another model of fire clock developed, inspired by both the candle and incense: the labyrinth clock. On a support, using a grid, a “path” is created with incense paste, or any combustible powder, along the convolutions of a mandala-shaped labyrinth (in a closed system), whose length depends on the desired time. The end of the combustion indicates the elapsed desired duration.

The principle of the combustible clock was modernized from the 15th century with the invention of oil lamps. These were then equipped with graduated reservoirs that allowed the passage of hours to be appreciated as the wick burned and the oil level dropped along the markings.

Throughout its evolutions and its journeys from one continent to another, the fire clock retained its unalterable basic operating principle. It took the invention of mechanical watches and clocks to end the dominance of this ingenious time-measuring system.