What effect did gunpowder have on land-based empires?

Black powder differed from modern propellants and explosives in a number of important particulars. First, only some 44 percent by weight of a properly burned charge of black powder was converted into propellant gases, the balance being solid residues. The high molecular weights of these residues limited the muzzle velocities of black-powder ordnance to about 2,000 feet (600 metres) per second. Second, unlike modern nitrocellulose-based propellants, the burning rate of black powder did not vary significantly with pressure or temperature. This occurred because the reaction in an exploding charge of black powder was transmitted from grain to grain at a rate some 150 times greater than the rate at which the individual grains were consumed and because black powder burned in a complex series of parallel and mutually dependent exothermal (heat-producing) and endothermal (heat-absorbing) reactions that balanced each other out. The result was an essentially constant burning rate that differed only with the grain size of the powder; the larger the grains, the less surface area exposed to combustion and the slower the rate at which propellant gases were produced.

Nineteenth-century experiments revealed sharp differences in the amount of gas produced by charcoal burned from different kinds of wood. For example, dogwood charcoal decomposed with potassium nitrate was found to yield nearly 25 percent more gas per unit weight than fir, chestnut, or hazel charcoal and some 17 percent more than willow charcoal. These scientific observations confirmed the insistence of early—and thoroughly unscientific—texts that charcoal from different kinds of wood was suited to different applications. Willow charcoal, for example, was preferred for cannon powder and dogwood charcoal for small arms—a preference substantiated by 19th-century tests.

The earliest gunpowder was made by grinding the ingredients separately and mixing them together dry. This was known as serpentine. The behaviour of serpentine was highly variable, depending on a number of factors that were difficult to predict and control. If packed too tightly and not confined, a charge of serpentine might fizzle; conversely, it might develop internal cracks and detonate. When subjected to vibration, as when being transported by wagon, the components of serpentine separated into layers according to relative density, the sulfur settling to the bottom and the charcoal rising to the top. Remixing at the battery was necessary to maintain the proper proportions—an inconvenient and hazardous procedure producing clouds of noxious and potentially explosive dust.

Shortly after 1400, smiths learned to combine the ingredients of gunpowder in water and grind them together as a slurry. This was a significant improvement in several respects. Wet incorporation was more complete and uniform than dry mixing, the process “froze” the components permanently into a stable grain matrix so that separation was no longer a problem, and wet slurry could be ground in large quantities by water-driven mills with little danger of explosion. The use of waterpower also sharply reduced cost.

After grinding, the slurry was dried in a sheet or cake. It was then processed in stamping mills, which typically used hydraulically tripped wooden hammers to break the sheet into grains. After being tumbled to wear the sharp edges off the grains and impart a glaze to their surface, they were sieved. The grain size varied from coarse—about the size of grains of wheat or corn (hence the name corned powder)—to extremely fine. Powder too fine to be used was reincorporated into the slurry for reprocessing. Corned powder burned more uniformly and rapidly than serpentine; the result was a stronger powder that rendered many older guns dangerous.

Late medieval and early modern gunners preferred large-grained powder for cannon, medium-grained powder for shoulder arms, and fine-grained powder for pistols and priming—and they were correct in their preferences. In cannon the slower burning rate of large-grained powder allowed a relatively massive, slowly accelerating projectile to begin moving as the pressure built gradually, reducing peak pressure and putting less stress on the gun. The fast burning rate of fine-grained powders, on the other hand, permitted internal pressure to peak before the light, rapidly accelerating projectile of a small arm had exited the muzzle.

Then, beginning in the late 18th century, the application of science to ballistics began to produce practical results. The ballistic pendulum, invented by the English mathematician Benjamin Robins, provided a means of measuring muzzle velocity and, hence, of accurately gauging the effective power of a given quantity of powder. A projectile was fired horizontally into the pendulum’s bob (block of wood), which absorbed the projectile’s momentum and converted it into upward movement. Momentum is the product of mass and velocity, and the law of conservation of momentum dictates that the total momentum of a system is conserved, or remains constant. Thus the projectile’s velocity, v, may be determined from the equation mv = (m + M)V, which gives

The initial impact of science on internal ballistics was to show that traditional powder charges for cannon were much larger than necessary. Refinements in the manufacture of gunpowder followed. About 1800 the British introduced cylinder-burned charcoal—that is, charcoal burned in enclosed vessels rather than in pits. With this method, wood was converted to charcoal at a uniform and precisely controlled temperature. The result was greater uniformity and, since fewer of the volatile trace elements were burned off, more powerful powder. Later, powder for very large ordnance was made from charcoal that was deliberately “overburned” to reduce the initial burning rate and, hence, the stress on the gun.

Beginning in the mid-19th century, the use of extremely large guns for naval warfare and coastal defense pressed existing materials and methods of cannon construction to the limit. This led to the development of methods for measuring pressures within the gun, which involved cylindrical punches mounted in holes drilled at right angles through the barrel. The pressure of the propellant gases forced the punches outward against soft copper plates, and the maximum pressure was then determined by calculating the amount of pressure needed to create an indentation of equal depth in the copper. The ability to measure pressures within a gun led to the design of cannon made thickest where internal pressures were greatest—that is, near the breech. The resultant “soda bottle” cannon of the mid- to late 19th century, which had fat breeches curving down to short, slim muzzles, bore a strange resemblance to the very earliest European gun of which a depiction survives, that of the Walter de Millimete manuscript of 1327.