Powder strength was originally tested by setting a small amount afire in the open air, and observing the results. Eprouvettes ("provers"), which ignited the powder in a confined space simulating a cannon barrel, provided more useful data (von Malitz 163ff). An early eprouvette was described by William Bourne (1578). This was essentially a box with a hinged and ratcheted lid and a small fuse hole. A set quantity of powder was placed inside, and set off. The force of the combustion gases would drive the lid upward, and the lid would be kept from dropping back into place by the ratchet. The angle reached by the lid was a measure of the strength of the powder.
In Bourne's eprouvette, the propulsive force was resisted by the weight of the lid, but some later devices used a spring mechanism, and Du Me's eprouvette (1702) employed water resistance. Also, some were engineered so the propelled object moved linearly rather than angularly. And, instead of measuring that movement, one could measure the eprouvette's recoil.
Most of the eprouvettes just worked on an indicator object like Bourne's lid, but the mortar eprouvette actually fired a projectile at a fixed angle, usually 45°, so the power was inferred from the range achieved.
In storage, gunpowder could become damp, and once its moisture content rises above 1 %, it begins losing explosive power. (Kelly 59). Keeping it away from seawater isn't enough because it can actually absorb water vapor. (Douglas 199). It follows that what's needed is airtight storage or, if that isn't possible, storage together with some desiccant.
Powder was examined for dampness and if damp, it was dried. This was a ticklish operation as the drying could melt the sulfur or even explode the grains. (200). If the powder were past redemption, one could at least attempt to recover the saltpeter, which was a rare and valuable commodity in Europe (201).
Alternative Propellants
Steam. Jacob Perkins received a British patent in 1824 for a steam gun. This was no "paper patent"; in an 1825 demonstration an 800 (or 900) psi boiler projected one ounce musket balls out a barrel, achieving penetration of quarter-inch iron plate and eleven inches of pine at a range of 35 yards. Moreover, he developed a rapid-fire gravity feed enhancement. (Smith). The rapid fire version was later a major attraction at the National Gallery of Popular Science (1832). Some outrageous claims were made for how fast it was, but I am inclined to believe the ten balls/minute that Perkins' son asserted in 1861. (Bruce 138).
In 1828, Perkins designed for the French a 1500 psi steam gun, with a barrel six feet long and three inches caliber, firing four pound balls. It worked, but its range was only half that of a conventional cannon of the same caliber. Not only was the barrel pressure much lower than in a "powder" gun, it may have suffered much more acutely from bore-windage because of that difference. Another problem was weight; the 1825 model had a five-ton boiler. (BPHS).
EB11/Explosives alludes to the Winans (Dickinson) steam gun, built for the Confederacy. It was never put to use; Mythbusters Episode 93 suggests that it would have gotten off five rounds a second and had a maximum range of 700 yards, but expressed doubt that the impact velocity beyond point blank range was high enough to be lethal.
The concept of using steam to throw a projectile wasn't new; Leonardo da Vinci had speculated that Archimedes had used a steam cannon at the siege of Syracuse, and drew one. In 2006, an MIT team figured out how to implement Leonardo's concept. They were deliberately coy about the particulars of steam generation, but they built a steam cannon designed for 3,500-4,000 psi, and fired a one pound projectile (i.e., equivalent to that of a robinet) with a muzzle velocity over 300 m/s. The bore was 2 feet long and 1.5 inches diameter. They were able to fire one round every two minutes. (MIT).
While I am sure the projectiles made a satisfying whizz, the fact remains that "steamer" muzzle velocity is low compared to that achieved with powder guns. A bit of a back-of-the-envelope calculation puts this into perspective. Let's assume that we have a large enough reservoir of steam so that we can maintain constant pressure. Let's also assume that the barrel is horizontal (so we can ignore gravity), frictionless, and without windage. If so, the projectile accelerates at a constant rate and the muzzle velocity will therefore be
sqrt (2*L*P*A/m),
where L is bore length, P pressure, A cross-sectional area of the projectile and m mass. For a standard projectile (one pound, one inch diameter) this reduces to
24.56 sqrt(LP) (L inches, P psi).
Perkins' 1828 gun thus has a theoretical ideal muzzle velocity of 1345 fps, and the MIT gun, 951. But note that the actual muzzle velocity for the MIT gun was a bit less than a third of the theoretical value.
Compressed Air. The blowgun is the earliest compressed air weapon, limited in propulsive force by the ratio of the volume of air one can huff (about 60 cubic inches) to the bore volume of the blowgun (14 cubic inches for a six footer with a half inch caliber). (Gurstelle 142). A "pneumatic rifle was built at the beginning of the seventeenth century," and some Austrian jaegers carried the model 1780 rifle (300 m/s muzzle velocity), which was a great weapon for covert operations against French occupation forces. (Rossi 232).
The USS Vesuvius (1888) carried three 15-inch pneumatic "dynamite" guns. Compressed air from a 1000 psi reservoir was fed into the barrels, which were only 55 feet long (!), partially below deck, and mounted at a fixed elevation of 16 degrees. Range was changed by adjusting the pressure. The guns couldn't be traversed; you aimed the ship to aim the gun. The guns fired finned projectiles filled with up to 600 pounds of dynamite; this high explosive was too sensitive to be used in an ordinary gun and indeed even the muzzle velocity of the pneumatic gun had to be limited. The maximum range was 5,000 yards, with a subcaliber (6") shell. (NAVWEAPS; NAVSOURCE; Hamilton; Clark).
Because of the low pressure, a 20-inch gun could have a steel or aluminum bronze barrel that was one half an inch thick. In trials, the gun had good accuracy, and could fire about one round a minute. (Zalinski). The Zalinski gun is discussed approvingly in EB11/Pneumatic Gun.
The secret to understanding the dynamite gun is to think of it, not as a gun, but as a torpedo launch system. Ship armor had reached the point at which ordinary shells weren't reliably penetrating it. The projectiles fired by the dynamite gun were conceptualized as "aerial torpedoes," traveling faster and farther than any underwater torpedo and exploding underwater against the unarmored bilge of the enemy craft (Parkerson 83).
At a U.S. Naval Institute proceeding, the commentators conceded that the gun would be useful for countermining, that is, using explosives to set off enemy mines-stationary targets. They were less sanguine that it could be used effectively against a rapidly closing foe, as the elevation limited the zone of danger for the target and ranges are difficult to estimate. If the pressure were reduced because the enemy was close, the projectile would have a lower velocity and be more vulnerable to deflection by the wind. (Zalinski). The naysayers doubted that the countermining advantage was sufficient to justify building a ship with the dynamite gun as the main armament
In practice, USS Vesuvius proved reasonably useful for shore bombardment-the quietness of the pneumatic action meant that the enemy didn't hear the guns fire-but the system was quite obviously impractical for use against another warship. USS Vesuvius would have been outranged by conventional guns, and the inability to traverse the gun other than by turning the ship meant that a fast attacker could evade its fire. (McSherry).