During the Civil War, the sponging drill was changed. A damp or dry sponge became regulation. The purpose of dampening the sponge was to assure an air tight seal, not put out embers from the previous round. With #3 keeping a good seal on the vent, when the sponge is withdrawn it creates a partial vacuum. The resounding deep throated Blooommmm the sponge makes as it leaves the muzzle is very satisfying. Any residual live embers will be extinguished from the resulting lack of oxygen… of course, it isn’t perfect.
In the 1960’s Parks Canada did extensive testing to see how long a live ember would stay lit in the bore of a cannon. They lit all kinds of material & timed it. Nothing stayed lit for more than 7 minutes. The lack of circulation pit out the combination with oxygen starvation. The NPS rule is 10 minutes between shots, leaving an ample safety margin.
Parks Canada confirmed the discovery made by Civil War gun crews. The sopping wet sponge as described in this thread was the source of premature detonations. The liquid water chills the surface of the ember, forming a crust with a live ember inside. When the round is rammed, the crust is broken & the live ember can detonate the charge as in the example above. That is why NPS drill & instruction makes such a point of using a damp, not a sopping wet sponge.
I have a file folder full of premature detonations that occurred during rapid fire as is seen at re-enactments. The earliest one occurred when Army of the Cumberland soldiers were home on leave. Rapid firing a cannon resulted in a traumatic amputation for #1 & severe injuries among the spectators inflicted by the sponge rammer. Odds are, sometime this year, someone will reenact the sad experience of the A of the C veterans during rapid fire, but it won’t be in a National Park.
National Park Historic Weapons Manual is a public document that contains a wealth of information. It even covers Spanish American-Korean War weapons. Last time I looked, my wife is #2 in the center of the photo on the cover.
Google “Black Powder Cannon Accidents” for sobering up to date accounts of how dangerous not following safety procedures & drill can be.