Flaming swords are popular fantasy fare. They crop up in a number of mythologies around the world. One of my favourites is the sword Drynwyn from the Welsh Triads. The sword was owned by Rhydderch Hael, one of the “Three Generous Men of Britain”. Supposedly the sword would burn if drawn by one who was righteous. Any ne’er-do-well who tried to draw it would themselves be burned. Rhydderch appears to have had a rather direct sense of humour – he would allow anyone to borrow it if they could draw it…
Probably the most well-known flaming sword is the one that is placed to guard the Garden of Eden after Adam and Eve are expelled. Modern scholarship, at least that which views Genesis as being more allegorical than literal, suggests a couple of things about the story which are relevant to the materials science theme. One is that the interpretation of a sword might not be quite correct and the other is that the sword was probably made of copper. The argument goes that the story is probably based on an older Mesopotamian story, and might even be an account of the rise of agricultural society. I’m not going there, except to say that the highly polished copper weapons, particularly when they caught the Sun, might just give rise to the description of a weapon on fire. (Oh, and I quite like this explanation – by Isaac Asimov – which explains why Genesis is written the way it is: it explains a lot).
But coming back to the point, later interpretations of “flaming sword” are much more literal. The concept even carries through to today where we see circus performers and other entertainers setting fire to their swords for their act. Of course these swords are props, and are not even really swords, more like batons. What would happen to a real sword if you set it on fire?
If we are dealing with a metal sword then there are two properties that we need to balance – and of course these two properties are diametrically opposed. We achieve sharpness by having an edge that is very hard, but this leads to a material that is very brittle – it breaks easily if hit too hard, which can really put a crimp on your sword-fighting style. A softer metal is more ductile, and more importantly for something that is going to get banged around a lot, more ‘resilient’. Resilience is one of those funny terms that crop up in science from time to time: we know exactly what means, but it is a non-quantifiable term. Toughness for instance is defined as the amount of energy absorbed in creating two new surfaces (Joules per square metre, J/m2), whilst fracture toughness is the resistance to the growth of a crack (with the slightly unwieldy units of MPa√m, megapascals-square root metres). We might describe a tough material as being resilient, and of course in everyday usage the two are synonyms. Here, though, resilience has connotations of a resistance to deformation. A material may be tough, absorbing lots of energy as load is applied, but this is usually achieved by one or more mechanisms, most of which give rise to fundamental changes in the properties of the materials, so that a tough object can, potentially, survive a significant blow, but be changed to the point that it is no longer fit for purpose. A resilient one might be described as relatively elastic (i.e. any change is recovered and, once the load is removed, the structure of the material returns to its original state), whilst at the same time being quite stiff (resists extension for a given applied load) and strong (a significant load needs to be applied to cause change to occur).
There are a number of ways in which this dichotomy can be resolved. One is by taking a bar of metal and working it over to get the shape that you want, whilst at the same time hardening the bits that you want to be hard. In the case of a sword, then, the edge is hardened significantly, whilst the spine of the blade is kept relatively ‘soft’. The edge is then ‘tempered’ which is to say that it is heat-treated such that the hardness is retained but the brittleness is removed.* The lower the tempering temperature the harder the metal will remain – hard tool steels will typically be treated at 200 oC. Differential tempering was commonly used in the West to temper swords: significant heat (but shy of 400 oC) was applied to the spine, and the colour of the metal observed. When the colour reached the appropriate tint at the edges, the blade would be cooled.**
*This is the origin of “good tempered” and “bad tempered” as applied to people…
**There are other methods of achieving a similar result, including pattern-welding, and the use of damascene steel, which is where a soft and hard metal are folded over and over to produce a composite of the two.
Setting your sword on fire suddenly seems like a really bad idea. It might look pretty spiffy, something that might impress the yokels and scare the opposition, but it is going to ruin the temper of your sword – just think, the cool part of a candle flame is ~1000 oC and the hot part can get up to about 1400 oC, so with enough time and a big enough candle, you could heat up the blade enough to mess up the tempering. A smear of creosote or something might not get hot enough to cause problems, but it would be a bit close.
So how might we get the effect that we want, without ruining the edge, and potentially the entire body of our sword. We could put a thermally protective barrier coating on it, which is what they do in jet turbine engines (although of course these are operating at considerably higher temperatures). We could try and find something that burns with a relatively cool flame (although that would seem to defeat the entire object of the exercise). We could move away from iron-based metallurgy and think about using other metals.
But as Death has found with fiery steeds (which have a tendency to set the stable on fire and then stand around looking sheepish), a flaming sword might be more trouble than it’s worth…