Haven't you heard the story about the farmer plowing up the old rusty CW musket barrel? He was heating it up or beating on it or something happens and the thing goes off and blows his head off--and a dug up CW artillery shell, setting on the bricks next to the fire place, gets too hot from the fire, blam it blows up the fire place? I suppose that powder like that can remain active as long as it does not absorb moisture over time. However, stuff that is in or on guns can contaminate the powder load, so there are chances of a fizzle too. Presumably heat can melt something like lube in wads under the ball, or groove grease in a mini ball could migrate into the powder, or any oil that was in the chamber before loading could affect the powder, etc. Well that is a good excuse if your gun fizzles when you are at a shooting match? I would think a dab of melted wax around the caps would seal them from moisture in the air? I suppose that if the gun is loaded perfectly, with a good seal on the cap and around the ball or bullet, then it is maybe equivalent to a sealed brass cartridge case loaded with BP in which case it might go boom after a long time? Then again if the powder attracts or holds moisture, is it possible to cause rust in the chamber if the thing is not just perfect and mess up the gun? Best to not leave it loaded if you have no purpose for it. Or buy a cartridge conversion cylinder.
Your gun is like your nose, it is just wrong for someone else to pick it for you!