[kronckew]

the powder contains the oxygen required to sustain the burning of the fuse charge, as it is not contained on the open end it burns rather than explodes. stone, being more insulating than bronze or iron/steel would enhance the burning as less heat would be lost to the surroundings. confinement is relative, the larger main charge is bigger than the fuse vent can ventilate, so you get a bang rather than a fizz. as vents wear, in metal cannon at least, they are replaced, drilled out & a new vent screwed in, to avoid loss of propellant gasses. i suspect in a stone cannon the thing would be discarded after the vent wears too big. being ceramic, this is likely still a long life. i'd worry more about it exploding after a limited no. of shots.

p.s. the 'spiking' of cannon so enamoured of authors only temporarily puts a cannon out of action, as the vent is fairly easy to drill out. artillerymen were well aware & usually had tools available. 19c cannon had pre-drilled threaded standardized plugs that could replace a vent in minutes. spiking only gave you enough time to get (hopefully) out of range of your own abandoned cannon before the victors turned them on your retreating selves. abandoning the guns is a shameful disgrace.

one reason the artillery's cannon are their 'colours', like a regiments regimental flag, given by the monarch is defended to the last man and losing your colours (or cannon) to the enemy is a shameful disgrace.

conversely capturing the enemies colours or cannon was the highest act, usually very well rewarded. capturing a french imperial eagle ranked at the top. britain only captured 5 eagles during the Napoleonic wars, 2 of those at waterloo.