The gunpowder revolution, c. 1300–1650
Few inventions have had an impact on human affairs as dramatic and decisive as that of gunpowder. The development of a means of harnessing the energy released by a chemical reaction in order to drive a projectile against a target marked a watershed in the harnessing of energy to human needs. Before gunpowder, weapons were designed around the limits of their users’ muscular strength; after gunpowder, they were designed more in response to tactical demand.
Technologically, gunpowder bridged the gap between the medieval and modern eras. By the end of the 19th century, when black powder was supplanted by nitrocellulose-based propellants, steam power had become a mature technology, the scientific revolution was in full swing, and the age of electronics and the internal combustion engine was at hand. The connection between gunpowder and steam power is instructive. Steam power as a practical reality depended on the ability to machine iron cylinders precisely and repetitively to predetermined internal dimensions; the methods for doing this were derived from cannon-boring techniques.
Gunpowder bridged the gap between the old and the new intellectually as well as technologically. Black powder was a product of the alchemist’s art, and although alchemy presaged science in believing that physical reality was determined by an unvarying set of natural laws, the alchemist’s experimental method was hardly scientific. Gunpowder was a simple mixture combined according to empirical recipes developed without benefit of theoretical knowledge of the underlying processes. The development of gunpowder weapons, however, was the first significant success in rationally and systematically exploiting an energy source whose power could not be perceived directly with the ordinary senses. As such, early gunpowder technology was an important precursor of modern science.
Early gunpowder
Chinese alchemists discovered the recipe for what became known as black powder in the 9th century ce: this was a mixture of finely ground potassium nitrate (also called saltpetre), charcoal, and sulfur in approximate proportions of 75:15:10 by weight. The resultant powder behaved differently from anything previously known. It burned rapidly on contact with open flame or a red-hot wire, producing a bright flash and dense white smoke. It also produced considerable quantities of superheated gas, which, if confined in a partially enclosed container, could drive a projectile out of the open end. The Chinese used the substance in rockets, in pyrotechnic projectors much like Roman candles, in crude cannon, and, according to some sources, in bombs thrown by mechanical artillery. This transpired long before gunpowder was known in the West, but development in China stagnated. The development of black powder as a tactically significant weapon was left to the Europeans, who probably acquired it from the Mongols in the 13th century (though diffusion through the Arab Muslim world is also a possibility).
Chemistry and internal ballistics
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 meters) 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.
Serpentine powder
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.
Corned powder
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.
Refinements in ballistics
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
where m is the mass of the projectile, M is the mass of the bob, and V is the velocity of the bob and embedded projectile after impact.
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.
The development of artillery
The earliest known gunpowder weapons vaguely resembled an old-fashioned soda bottle or a deep-throated mortar and pestle. The earliest such weapon, depicted in the English de Millimete manuscript, was some three feet long with a bore diameter of about two inches (five cm). The projectile resembled an arrow with a wrapping around the shaft, probably of leather, to provide a gas seal within the bore. Firing was apparently accomplished by applying a red-hot wire to a touchhole drilled through the top of the thickest part of the breech. The gun was laid horizontally on a trestle table without provision for adjusting elevation or absorbing recoil—a tribute to its modest power, which would have been only marginally greater than that of a large crossbow.
The breakthrough that led to the emergence of true cannon derived from three basic perceptions. The first was that gunpowder’s propellant force could be used most effectively by confining it within a tubular barrel. This stemmed from an awareness that gunpowder’s explosive energy did not act instantaneously upon the projectile but had to develop its force across time and space. The second perception was that methods of construction derived from cooperage could be used to construct tubular wrought-iron gun barrels. The third perception was that a spherical ball was the optimal projectile. The result was modern artillery.
Wrought-iron muzzle-loaders
The earliest guns were probably cast from brass or bronze. Bell-founding techniques would have sufficed to produce the desired shapes, but alloys of copper, tin, and zinc were expensive and, at first, not well adapted to the containment of high-temperature, high-velocity gases. Wrought iron solved both of these problems. Construction involved forming a number of longitudinal staves into a tube by beating them around a form called a mandrel and welding them together. (Alternatively, a single sheet of iron could be wrapped around the mandrel and then welded closed; this was particularly suitable for smaller pieces.) The tube was then reinforced with a number of rings or sleeves (in effect, hoops). These were forged with an inside diameter about the same as the outside of the tube, raised to red or white heat, and slid into place over the cooled tube, where they were held firmly in place by thermal contraction. The sleeves or rings were butted against one another and the gaps between them sealed by a second layer of hoops. Forging a strong, gastight breech presented a particular problem that was usually solved by welding a tapered breech plug between the staves.
Hoop-and-stave construction permitted the fabrication of guns far larger than had been made previously. By the last quarter of the 14th century, wrought-iron siege bombards were firing stone cannonballs of 450 pounds (200 kilograms) and more. These weapons were feasible only with projectiles of stone. Cast iron has more than two and a half times the density of marble or granite, and gunners quickly learned that a cast-iron cannonball with a charge of good corned powder behind it was unsafe in any gun large enough for serious siege work.
Wrought-iron breechloaders
Partly because of the difficulties of making a long, continuous barrel, and partly because of the relative ease of loading a powder charge into a short breechblock, gunsmiths soon learned to make cannon in which the barrel and powder chamber were separate. Since the charge and projectile were loaded into the rear of the barrel, these were called breechloaders. The breechblock was mated to the barrel by means of a recessed lip at the chamber mouth. Before firing, it was dropped into the stock and forced forward against the barrel by hammering a wedge into place behind it; after the weapon was fired, the wedge was knocked out and the block was removed for reloading. This scheme had significant advantages, particularly in the smaller classes of naval swivel guns and fortress wallpieces, where the use of multiple breechblocks permitted a high rate of fire. Small breechloaders continued to be used in these ways well into the 17th century.
The essential deficiency of early breechloaders was the imperfect gas seal between breechblock and barrel, a problem that was not solved until the advent of the brass cartridge late in the 19th century. Hand-forging techniques could not produce a truly gastight seal, and combustion gases escaping through the inevitable crevices eroded the metal, causing safety problems. Wrought-iron cannon must have required constant maintenance and care, particularly in a saltwater environment.
Wrought-iron breechloaders were the first cannon to be produced in significant numbers. Their tactical viability was closely linked to the economics of cannonballs of cut stone, which, modern preconceptions to the contrary, were superior to cast-iron projectiles in many respects. Muzzle velocities of black-powder weapons were low, and smoothbore cannon were inherently inaccurate, so that denser projectiles of iron had no advantage in effective range. Cannon designed to fire a stone projectile were considerably lighter than those designed to fire an iron ball of the same weight; as a result, stone-throwing cannon were for many years cheaper. Also, because stone cannonballs were larger than iron ones of the same weight, they left larger holes after penetrating the target. The principal deficiency of stone-throwing cannon was the enormous amount of skilled labour required to cut a sphere of stone accurately to a predetermined diameter. The acceleration of the wage–price spiral in the 15th and 16th centuries made stone-throwing cannon obsolete in Europe.
Cast bronze muzzle-loaders
The advantages of cast bronze for constructing large and irregularly shaped objects of a single piece were well understood from sculpture and bell founding, but a number of problems had to be overcome before the material’s plasticity could be applied to ordnance. Most important, alloys had to be developed that were strong enough to withstand the shock and internal pressures of firing without being too brittle. This was not simply a matter of finding the optimal proportions of copper and tin; bronze alloys used in cannon founding were prone to internal cavities and “sponginess,” and foundry practices had to be developed to overcome the inherent deficiencies of the metal. The essential technical problems were solved by the first decades of the 15th century, and, by the 1420s and ’30s, European cannon founders were casting bronze pieces that rivaled the largest of the wrought-iron bombards in size.
Developments in foundry practice were accompanied by improvements in weapon design. Most notable was the practice of casting cylindrical mounting lugs, called trunnions, integral with the barrel. Set just forward of the center of gravity, trunnions provided the principal point for attaching the barrel to the carriage and a pivot for adjusting the vertical angle of the gun. This permitted the barrel to be adjusted in elevation by sliding a wedge, or quoin, beneath the breech. At first, trunnions were supplemented by lifting lugs cast atop the barrel at the center of gravity; by the 16th century most European founders were casting these lugs in the shape of leaping dolphins, and a similarly shaped fixture was often cast on the breech of the gun.
Toward the end of the 15th century, French founders combined these features with efficient gun carriages for land use. French carriage design involved suspending the barrel from its trunnions between a pair of heavy wooden side pieces; an axle and two large wheels were then mounted forward of the trunnions, and the rear of the side pieces descended to the ground to serve as a trail. The trail was left on the ground during firing and absorbed the recoil of the gun, partly through sliding friction and partly by digging into the ground. Most important, the gun could be transported without dismounting the barrel by lifting the trail onto the limber, a two-wheeled mount that served as a pivoting front axle and point of attachment for the team of horses. This improved carriage, though heavy in its proportions, would have been familiar to a gunner of Napoleonic times. Sometime before the middle of the 16th century, English smiths developed a highly compact four-wheeled truck carriage for mounting trunnion-equipped shipboard ordnance, resulting in cannon that would have been familiar to a naval gunner of Horatio Nelson’s day.
By the early 1500s, cannon founders throughout Europe had learned to manufacture good ordnance of cast bronze. Cannon were cast in molds of clay, suspended vertically in a pit. Normally, they were cast breech down; this placed the molten metal at the breech under pressure, resulting in a denser and stronger alloy around the chamber, the most critical point. Subsequent changes in foundry practice were incremental and took effect gradually. As founders established mastery over bronze, cannon became shorter and lighter. In about 1750, advances in boring machines and cutting tools made it possible for advanced foundries to cast barrels as solid blanks and then bore them out. Until then cannon were cast hollow—that is, the bore was cast around a core suspended in the mold. Ensuring that the bore was precisely centered was a particularly critical part of the casting process, and small wrought-iron fixtures called chaplets were used to hold the core precisely in place. These were cast into the bronze and remained a part of the gun. Boring produced more accurate weapons and improved the quality of the bronze, since impurities in the molten metal, which gravitate toward the center of the mold during solidification, were removed by the boring. But, while these changes were important operationally, they represented only marginal improvements to the same basic technology. A first-class bronze cannon of 1500 differed hardly at all in essential technology and ballistic performance from a cannon of 1850 designed to shoot a ball of the same weight. The modern gun would have been shorter and lighter, and it would have been mounted on a more efficient carriage, but it would have fired its ball no farther and no more accurately.
Cast-iron cannon
In 1543 an English parson, working on a royal commission from Henry VIII, perfected a method for casting reasonably safe, operationally efficient cannon of iron. The nature of the breakthrough in production technology is unclear, but it probably involved larger furnaces and a more efficient organization of resources. Cast-iron cannon were significantly heavier and bulkier than bronze guns firing the same weight of ball. Unlike bronze cannon, they were prone to internal corrosion. Moreover, when they failed, they did not tear and rupture like bronze guns but burst into fragments like a bomb. They possessed, however, the overwhelming advantage of costing only about one-third as much. This gave the English, who alone mastered the process until well into the 17th century, a significant commercial advantage by enabling them to arm large numbers of ships. The Mediterranean nations were unable to cast significant quantities of iron artillery until well into the 19th century.