I don’t want to get too bogged down in the exact chemistry of how the introduction of carbon changes the metallic matrix of the iron; you are welcome to read about it. As the carbon content of the iron increases, the iron’s basic characteristics – its ductility and hardness (among others) – changes. Pure iron, when it takes a heavy impact, tends to deform (bend) to absorb that impact (it is ductile and soft). Increasing the carbon-content makes the iron harder, causing it to both resist bending more and also to hold an edge better (hardness is the key characteristic for holding an edge through use). In the right amount, the steel is springy, bending to absorb impacts but rapidly returning to its original shape. But too much carbon and the steel becomes too hard and not ductile enough, causing it to become brittle.

Compared to the other materials available for tools and weapons, high carbon “spring steel” was essentially the super-material of the pre-modern world. High carbon steel is dramatically harder than iron, such that a good steel blade will bite – often surprisingly deeply – into an iron blade without much damage to itself. Moreover, good steel can take fairly high energy impacts and simply bend to absorb the energy before springing back into its original shape (rather than, as with iron, having plastic deformation, where it bends, but doesn’t bend back – which is still better than breaking, but not much). And for armor, you may recall from our previous look at arrow penetration, a steel plate’s ability to resist puncture is much higher than the same plate made of iron (bronze, by the by, performs about as well as iron, assuming both are work hardened). of course, different applications still prefer different carbon contents; armor, for instance, tended to benefit from somewhat lower carbon content than a sword blade.

It is sometimes contended that the ancients did not know the difference between iron and steel. This is mostly a philological argument based on the infrequency of a technical distinction between the two in ancient languages. Latin authors will frequently use ferrum (iron) to mean both iron and steel; Greek will use σίδηρος (sideros, “iron”) much the same way. The problem here is that high literature in the ancient world – which is almost all of the literature we have – has a strong aversion to technical terms in general; it would do no good for an elite writer to display knowledge more becoming to a tradesman than a senator. That said in a handful of spots, Latin authors use chalybs (from the Greek χάλυψ) to mean steel, as distinct from iron.

More to the point, while our elite authors – who are, at most dilettantish observers of metallurgy, never active participants – may or may not know the difference, ancient artisans clearly did. As Tylecote (op. cit.) notes, we see surface carburization on tools as clearly as 1000 B.C. in the Levant and Egypt, although the extent of its use and intentionality is hard to gauge to due rust and damage. There is no such problem with Gallic metallurgy from at least the La Tène period (450 BCE – 50 B.C.) or Roman metallurgy from c. 200 B.C., because we see evidence of smiths quite deliberately varying carbon content over the different parts of sword-blades (more carbon in the edges, less in the core) through pattern welding, which itself can leave a tell-tale “streaky” appearance to the blade (these streaks can be faked, but there’s little point in faking them if they are not already understood to signify a better weapon). There can be little doubt that the smith who welds a steel edge to an iron core to make a sword blade understands that there is something different about that edge (especially since he cannot, as we can, precisely test the hardness of the two every time – he must know a method that generally produces harder metal and be working from that assumption; high carbon steel, properly produced, can be much harder than iron, as we’ll see).

That said, our ancient – or even medieval – smiths do not understand the chemistry of all of this, of course. Understanding the effects of carbuzation and how to harness that to make better tools must have been something learned through experience and experimentation, not from theoretical knowledge – a thing passed from master to apprentice, with only slight modification in each generation (though it is equally clear that techniques could move quite quickly over cultural boundaries, since smiths with an inferior technique need only imitate a superior one).

Bret Devereaux, “Collections: Iron, How Did They Make It, Part IVa: Steel Yourself”, A Collection of Unmitigated Pedantry, 2020-10-09.