Plumbata

[Renatus]

To add to Robert’s post, the July/August 2015 edition of Minerva magazine (distributed free to those who attended the Greek and Roman Armour Day on Monday) contains an article by David Sim on this very point. Noting that the shanks of some arrow-heads, javelins and plumbatae had been cold twisted and doubtful that this had been done for decorative purposes, he tested the effect of cold and hot twisting on the rigidity of iron. He explains that cold twisting involves twisting the iron at room temperature, whereas hot twisting involves heating the iron to red heat (usually 720ºC), twisting it and then either quenching it in cold water or allowing it to cool in air. He cut a number of samples from a single bar of wrought iron and cold or hot twisted them through 360º or 720º. He found that further twisting was not possible as cold twisted samples fractured at 724º. He then subjected the samples to a three-point bend test. This consisted of supporting the samples between two cylindrical bars and dropping a square bar with a rounded end on to them through a hollow square tube. The distance of the drop and the weight of the drop bar were the same in each case. The extent to which the samples bent was then measured. An untwisted bar was also tested as a control. The degree of bending suffered by each sample was as follows:

Untwisted bar: 6.9 mm
Cold twisted 360º: 6.0 mm
Cold twisted 720º: 5.0 mm
Hot twisted 360º: 9.5 mm
Hot twisted 720º: 11.9 mm

Thus, cold twisting increases the rigidity of the iron and the greater the degree of twisting the greater the rigidity, whereas hot twisting has precisely the reverse effect. The conclusion he draws from this is that, when twists were added when the metal was cold, the increased rigidity rendered the weapon more effective in penetrating its target.