"Hoist with his own petard," an ancient phrase signifying that one's
carefully laid scheme has exploded, had truly graphic meaning in the
old days when everybody knew what a petard was. Since the petard fired
no projectile, it was hardly a gun. Roughly speaking, it was nothing
but an iron bucket full of gunpowder. The petardier would hang it on a
gate, something like hanging your hat on a nail, and blast the gate
open by firing the charge.
Small petards weighed about 50 pounds; the large ones, around 70
pounds. They had to be heavy enough to be effective, yet light enough
for a couple of men to lift up handily and hang on the target. The
bucket part was packed full of the powder mixture, then a
2-1/2-inch-thick board was bolted to the rim in order to keep the
powder in and the air out. An iron tube fuze was screwed into a small
hole in the back or side of the weapon. When all was ready, the
petardiers seized the two handles of the petard and carried it to the
troublesome door. Here they set a screw, hung the explosive instrument
upon it, lit the fuze, and "retired."
Petards were used frequently in King William's War of the 1680's to
force the gates of small German towns. But on a well-barred, double
gate the small petard was useless, and the great petard would break
only the fore part of such a gate. Furthermore, as one would guess,
hanging a petard was a hazardous occupation; it went out of style in
the early 1700's.