Incense clock

A replica of an ancient Chinese incense clock

The incense clock is a timekeeping devices invented by the Chinese during the Sung Dynasty (960-1279). Incense sticks or powdered incense that have been manufacted and calibrated to a known rate of combustion is used to measure relatively short periods of time from minutes, hours, to days.

While calibrated incense sticks can be burned to keep track of time in the same manner as candle clocks, sticks are often used used as a part of a more elaborate device. In many 18th- or 19th-century incense clocks, threads with weights or bells on either end were hung over an incense stick at desired intervals. As the incense burns, the threads burned one by one and the weights dropped to a sounding plate, pan, or gong below. Sticks of incense with different scents might be used at different times, so that the hours were marked by a change in fragrance.

Another form of incense clock are based on incense seals (香印, hsing yìn in Chinese; kodokei in Japanese). A fine layer of white wood ash is layed down on a small container and compacted. Seals that were in the form of patterned metals cutouts were simply laid down on the ash while the incense powder was poured it. After a light compaction of the incense powder by a tamper, lifting up the metal seal forms a long trail of incense powder that have been masked onto the ash. Other seals have a protruded pattern that creates a negative indentation in the wood ash. The incense poweder is carefully spooned into the indentation in the ash and then recompacted again with the seal. In either case, depending on the size of the seal, the trails of incense may burn for long periods of time from hours to days. Bits of frangrant woods and resins can be placed on the incense powder trails to signal time.