Against leather or cloth armour, a very sharp sword is an asset – leather or cloth armour is a relatively soft target, and, while it can absorb a great deal of force, both the cut and thrust would penetrate it.
As a result, for the early Middle Ages prior to the Norman Conquest, we tend to see a lot of swords with relatively little profile taper – types X and Xa in the Oakeshott typology – and a lenticular cross section, built primarily for cutting, but also capable of thrusting.
So, the first factor to consider in when it comes to the sharpness of a Medieval sword is what kind of blow the sword will be used for.
The slice requires a sharper sword than a cut, and a thrust does not require a very sharp sword at all.
Swords used in battle survive primarily as archaeological finds, with the edges so corroded that any indication of the original sharpness is long lost.
Determining the sharpness of a Medieval sword must usually be done through deduction, using the literary evidence, armour of the time, and blade type to figure out how sharp the sword needed to be.While the blade strikes the target, the target strikes the blade with equal force.This becomes very important when it comes to dealing with armour – hard metal armour has enough resistence to deliver force back into the sword, and the sharper a blade is, the thinner its edge is, and the more (BRITAL) it becomes.The sharper the blade, the greater the concentration of force upon impact, and the more powerful and damaging the blow.However, it is important to remember Newton’s Third Law: for every action there is an equal and opposite reaction.Represented by Oakeshott types XII to XIV, as well as XVI and XVIa, they became far more tapered, while the blade geometry remained optimized for cutting.At least one type of sword – the Type XVI and XVIa, appeared to be specifically designed to handle reinforced mail armour.This is one of the reasons that when talking about Medieval swords, it is best to use Oakeshott’s Typology – with no fewer than 22 different categories of sword blade in use from 1000 to 1500, it is far more useful in understanding how swords developed and how they were used.The Oakeshott typology is particularly useful as, particularly in the early Middle Ages, it is next to impossible to determine the original sharpness of the sword through surviving examples.This was in part because mail armour is far better at delivering force back into the blade – as mentioned above, too sharp a blade would leave too little material to receive this force, rendering the edge too brittle to handle the impact without taking unnecessary damage.This does not mean that swords of this period were dull – they needed to be sharp enough to bite into armour.