Question about WWII balloons from Operation Outward

From Wikipedia:

At launch, a slow-burning fuse was lit; its length was calibrated to the estimated time to arrive over German-controlled territory. At first, the balloon rose rapidly and expanded in size until an internal cord tightened, preventing further increase in altitude beyond 25,000 feet (7,600 m) by releasing some gas; the balloon would then begin a slow descent due to the hydrogen gradually leaking away.[6] After a while, the slow burning fuse would release a bung in a can of mineral oil; as the oil slowly dripped out, the balloon's payload would lighten, arresting its descent. The same slow-burning fuse was also used to release the balloon's weapon.
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Heck of a fuse that can burn from London to Germany, light enough to travel by balloon. Can't seem to find any details as to the composition of such a fuse; anyone know the details? I'm curious.