Time fuse

From PyroGuide

Time fuse is thick type of fuse. Comprised of multiple layers of twine, paper, and asphalt make it nearly impossible to ignite from the outside or even from an end, since the asphalt often melts and seals it off before the powder can ignite. Shell makers must cut into it and insert a short piece of cross match to ignite the time fuse's powder core. It provides a timed delay which can be controlled by its length. It ignites the burst charge or similar at a determined apogee in a shells flight. It normally burns at a rate of 3 seconds per inch.

Time fuses are commonly found as a delay element in shells and timed inserts.