Long after the Bronze Age was over for swords, knives and pole-weapon heads, the prehistoric alloy was still used for cannon. Why?
Because while iron and early, uneven-quality steel were fine for contact or melee weapons, they weren’t a sure thing for containing the violent deflagration of gunpowder that launches cannon projectiles towards one’s enemy. Bronze could be cast and machined with high consistency.
It turns out that this question has already been studied at length — and we’ll quote from a 2002 thesis by Chuck Meide at the College of William and Mary. (The whole thesis will be attached at the end of this post as a .pdf. It’s full of gems like this).
Writing at the end of the muzzle-loader era, British artillery officer Manley Dixon in 1858 summed up nicely the required material qualities necessary to create ordnance:
The material should be hard, so as not to yield too easily to the action of the ball when passing out of the bore; tenacious, so as to resist the explosive power of the Gunpowder and not to burst; and lastly, elastic, so that the particles of the material of which the Gun is composed should, after the vibration caused by the discharge, return to their original position (McConnell 1988: 15).
Bronze and iron were the only two metals with these requisite qualities available to historic gunfounders, and bronze was long considered the superior metal for ordnance manufacture. Up until the third quarter of the sixteenth century, however, iron guns outnumbered bronze pieces, though the former were almost all wrought iron, of decidedly inferior quality. The most powerful guns had to be cast, not hand-wrought, and as cast iron guns were overly heavy or dangerously unreliable, bronze was the material of choice throughout the 16th century. Though Henry VIII’s Mary Rose (wrecked in 1545) displayed a marked diversity of bronze and iron ordnance (Guilmartin 1994: 148) by 1569 the decision was made to equip Queen Elizabeth’s navy entirely with cast bronze guns (Lavery 1987: 84).
The main disadvantage of bronze guns was their price, which was generally three to four times higher than iron guns (Cipolla 1965: 42). In 1570 England, iron ordnance cost £10 to £20 per ton while bronze cost £40 to £60. With improvements in iron casting techniques, the price of iron began to fall by the turn of the century, and the difference in cost began to steadily increase, so that by 1670 iron cost only £18 per ton, while bronze cost £150 for the same amount (Lavery 1987: 84). As the principle maritime powers continued to increase the size of their navies in the 17th century, this cost became prohibitively expensive. An example, to put this greater cost in perspective: the four small bronze cannons carried as cargo on the French ship La Belle (wrecked in Matagorda Bay, Texas in 1686) cost more to manufacture than did the entire vessel! (personal communication, John de Bry, 1996)
Not surprisingly, rulers in the first half of the 17th century began to mandate and subsidize experimentation in iron gunfounding, in order to improve the quality of iron ordnance. Other than expense, however, bronze guns were still superior to iron ones in almost every way. Bronze was stronger, withstood the shock of discharge better, and lasted longer at sea. Bronze also was easier to cast, could be re-cast, and could be easily embellished with decoration. Because of this last quality, along with their hefty price tag, bronze guns also served as status symbols, an aspect whose importance should not be overlooked in the 17th century, when capital ships represented not only the might but the prestige of the king.
Despite the fact that bronze is 20% heavier than iron, bronze guns were lighter than their counterparts because the stronger metal could be used to make thinner guns of the same caliber (Tucker 1989: 10). The dramatic weight differences between bronze and iron guns of the same caliber are illustrated in Table 1 (keeping in mind that a gun of the same size and metal could vary by as much as 2-3 hundredweight or 224-336 lbs) (Tucker 1989: 10). The reduced weight of bronze ordnance was particularly important for field artillery.
|Gun Size||Bronze Guns||Iron Guns|
Table 1. Comparison of the weight of bronze and iron British naval guns, ca. 1742. Adapted from Gardiner1979: Table 8. Original source, undated table in British Admiralty records (ADM 106/3067). “cwt”=hundredweight, or 112 pounds.
One especially salient advantage was that bronze guns were less likely to break while firing, and when they did the barrel usually bulged or split open longitudinally at the breech rather than explode. When iron cannon burst they tended to shatter and fly to pieces, which caused much more catastrophic damage to nearby personnel (Tucker 1989: 10; Kennard 1986: 161; Guilmartin 1983: 563). Figure 7 illustrates the striking difference between two failed guns, one of bronze and the other of iron. Improved iron casting techniques and gun design, however, would help solve this problem, though the reinforced guns had thicker metal at the breech and reinforces, increasing their weight. While iron guns were never considered as safe as bronze pieces, by the 1630s both England and Sweden were exporting iron guns of reputable quality (Cipolla 1965: 43; Kennard 1986: 161).
Figure 7. Two guns, one bronze and one iron, that have catastrophically failed (burst). The upper gun is a bronze 6-pdr, cast by Richard Gilpin in 1756 (526 lbs/238.8 kg, 4’ 6”/1.372 m long). This English gun burst at St. Lucia in 1783, tearing open longitudinally at the first and second reinforces. Currently housed at the Museum of Artillery Rotunda, Woolwich, from McConnell 1988: Figure 15. The lower gun is all that is left of an iron cannon that exploded, with massive loss of life, while firing on English troops invading St. Augustine on 10 November 1702. Photograph by the author.
The sole disadvantage of bronze as a gunfounding material was its propensity to heat up quickly, which meant that when firing a great number of shots in continuous action it was prone to becoming soft and susceptible to sagging or other bore damage (McConnell 1988: 15). Due to the nature of 16th and early 17th century naval tactics, however, this defect was not readily apparent; by the time of the great broadside to broadside slugfests of the 18th century, ships had already exchanged their bronze guns for iron ones. It would not be until the sieges of the Peninsular campaigns of the early 19th century that this defect became widely known (Kennard 1986: 162; Fisher 1976: 279-280).
What caused bronze to finally lose its throne? First came some centuries of technological improvements, which had English and Swedish gun makers producing solid, safe iron naval guns by the time of the American Revolution.
Terrestrial artillery was another matter. Bronze first was replaced in large siege guns in the early 1800s, but the weight advantage kept bronze 6-pounders and mortars in the field for the British Army in the 1850s and 60s respectively, and the US didn’t give up the smoothbore 12-pounder bronze Napoleon until long after other armies had done so — until the 1880s, in fact.
The thesis covers all this and more (including what all the confusing traditional names for muzzle-loading bronze artillery, such as culverin and falconet, signify).