While filming for Secret Earth: Yellowstone Supervolcano, a special for the History Channel, I got to spend time with one of the most creative minds I've ever met - Larry Progreba. Larry is a custom guitar maker who lives in the foothills of Montana in a home by a small creek that is completely off the grid. He made a cannon, several years ago, that shoots bowling balls over a mile and a half. When asked why he would build a cannon that shoots bowling balls he replied, "Well, I figured if I didn't, who would!"
When we finished shooting and filming the cannon he said he had one more thing to show us. He took out a small piece of large diameter drilling pipe, set it so the opening was facing the sky, dropped a piece of PVC pipe filled with gunpowder in the bottom, lit the fuze and placed a kitchen sauce pan over the top. The cameras were rolling but nobody was prepared for what would happen next!
Watch - you won't believe your eyes!
Video from Grizzly Creek Films
The Science
Gunpowder is a highly combustible solid. There is no simple equation to for gunpowder because the amound of charcoal and sulfur (not to mention the other components) can be different depending on the type/brand/maker of the gunpowder. A simplified equation for the combustion of gunpowder is:
10 KNO3 + 3 S + 8 C → 2 K2CO3 + 3 K2SO4 + 6 CO2 + 5 N2
The rapid expansion, caused by the solids of the gunpowder turning to gas as the temperature increases, changes the volume occupied by the combusing (burning) gunpowder. This expansion leads to an increase pressure in the "chamber". The metal pot over the opening is pushed skyward very rapidly, and the flat bottom is warped as the pressure wave pushes it upward rapidly. The resulting transformation outlines the sheer power of this ancient invention - gunpowder.
Oh, and it is a great way to make hard hats or medieval knight helmets...whichever you prefer!