Iron has a much higher specific gravity than stone, giving a projectile able to reach its target with greater kinetic energy and thus cause greater damage. This will have been particularly important with smaller bore guns. While iron round shot may have been produced in a wide range of calibres, it is interesting to note that on the Mary Rose the iron rounds extend from 23 mm to 270 mm but they concentrate above 55 mm.
The earliest iron rounds were of wrought iron, but it appears from the Burgundian archives that these may have been relatively uncommon as the references are few in the first half of the century. Only from the 1470s onwards do iron round shot become more common. While in the earliest records they are simply ‘de fer’, the records begin in 1473 to refer to ‘de fer fondu’, suggesting that the increase in quantity is linked to the replacement of wrought iron with cast iron.99 Its use for projectiles may have become cost effective as soon as investment had been made in the technology to produce cast iron artillery pieces.
The earliest documentary reference to a blast furnace in England is in Sussex in 1496, and the first cast iron cannon manufactured in England may have been as late as 1509.100 However the English crown were already purchasing iron ‘gunstones’ from the Low Countries, along with normal gunstones, as early as 1472. 101 While we cannot be sure whether these were of cast iron, as it lies on the very cusp of the change, it seems likely. Certainly if such purchases continued then they will soon have been of cast iron. The earliest so far from a battlefield in the UK appear to be the two probable round shot recently recovered from the 1547 battlefield of Pinkie. It would be reasonable to expect them at Flodden, fought in 1513, but they may also have been used on English battlefields of the 1470s and 1480s.102
Casting will have produced a more consistent round than smithing and thus cast iron could begin to match the accuracy and consistency previously only possible with lead. This will have been a consideration as gun technology improved and tolerances became increasingly important. This is clearly seen from the La Neuveville assemblage, believed to have been captured from the Burgundians at the battle of Murten in 1476, which comprises both wrought and cast iron rounds. The cast ones are slightly smaller and of a much more consistent calibre than those of wrought iron.103
As we have seen, there are various cast iron spheres from the Bosworth area, while others are also reported from Barnet and Flodden.104 But the identification and dating of iron round shot is a problem, because cast iron spheres from later artillery and from industrial and other uses are quite common finds. Examples include things such as large ball bearings for canal swing bridges – and there is a canal which passes close to Ambion Hill (Figure 7.15 and 7.3). In Sweden the survey of at least one battlefield has been confused by the contamination of the site by large numbers of iron spheres from a modern ore crushing plant.105 At Barnet we have been able to demonstrate, through careful examination, that one of the cast iron rounds in the museum is actually a shot from the twentieth century intended for use in shot-put, as it has faint engraved numbers indicating its weight! Rarely will spurious items be so easily dismissed.
When in good condition distinguishing the two types of iron round shot should be straightforward. Those of wrought iron typically have numerous irregular facets across the surface and are of slightly irregular shape (Figure 7.17). In contrast, those of cast iron will be more spherical and may be expected to have manufacturing features similar to those seen on lead rounds.106 These include a mould line, where the two halves of the mould seated together, as well as evidence for the sprue, representing the channel through which the iron was poured into the mould, although in cast iron rounds the sprue will have been snapped off rather than snipped (Figure 7.16). Unfortunately, when recovered from battlefields, as with the two from Pinkie, after hundreds of years of corrosion such features may not survive, indeed even the calibre may have been altered. These corrosion effects also probably mean that the slightly different specific gravity of cast and wrought iron cannot be used as a guide to composition, so scientific analysis may prove necessary.