GOLD is for the mistress – silver for the maid
Copper for the craftsman cunning at his trade!
” Good! ” said the Baron, sitting in his hall,
“But Iron – Cold Iron – is master of them all.”
– Cold Iron, by Kipling
But the fact is that any fantasy world is, sooner or later, this one. It may be in heavy disguise, but it can’t escape its origins. At a basic level even the language gives it away. However towering the distant mountains, however dwarf-haunted the local woods, any character wanting to eat a piece of zorkle meat between two slices of bread probably has no other word for it than sandwich. Every sentence the most exquisitely alien elf speaks will be filled with the echoes of Rome.
– Sir Terry Pratchett, Introduction to Unseen University Challenge.
Cold iron might be master of them all, but ironwood is what you want if you’re in Fantasyland and want to look like just a harmless traveller – many adventurers aim to look less than they are, a staff of ironwood is going to give you a slight advantage, especially if you know how to use it. I’d always assumed that ironwood was some sort of poetic license on the part of an author, but even on Earth there are some 30 or so different species of tree that are called ‘ironwood’ as a part of their common name, or as a colloquial term of reference. The key point, as you might expect is that the wood is exceptionally hard.
Returning to my slightly ignorant assumption, my imagination lead me to think of a wood that had an exceptional quantity of iron in the microstructure. This is not entirely daft: many plants do have a tendency to accumulate certain minerals. For example, nettles contain around 80 ppm titanium, and certain brassica are sometimes used to clean up contaminated ground because they collect heavy metals. The brassica can then be harvested and disposed of in order to prevent them from entering the food chain.
But what do we mean by hard? In materials, we generally mean resistant to being scratched, or indented. There are certain formulae which allow us to link the hardness of (some) materials to other properties, such as strength or toughness. In the case of wood, everyone knows that some woods are ‘hard’ and some are ‘soft’. Where it all gets very confusing is that one of the softest woods (in the sense that you can score it with a thumbnail) is balsa, which is classified as a hardwood. In the sense of timber then, the rule of thumb to work with is that hard woods are slow growing and soft woods are fast growing. From that perspective then it makes sense that most hard woods would be dense (and hard) and most softwoods would be less dense (and, erm, soft). There is a very slight wrinkle, which for the most part doesn’t matter at all, but which I include here for the sake of completeness. The wood that is laid down at different times of the year is slightly different and this is where we see the growth rings in a piece of timber that has been cut. Early wood tends to grow faster and is less dense and hence softer. In the case of softwoods, generally speaking, the early growth is much greater than the late growth and hence the rings are spread further apart. In hardwoods, the growth rings are much closer together.
Given that we’re using density as some kind of proxy, it might be worth making some sort of comparison. For reference then, water is 1 g/cm3 (at 4 oC, and can be rounded to 1 for convenience at most temperatures) and iron is 7.9 g/cm3. Balsa is the lightest wood, coming in at 0.16 g/cm3 (so about 1/6 the density of water), Scot’s Pine is 0.51 g/cm3. Oak varies between 0.6 and 0.9 g/cm3 and Black Ironwood (Krugiodendron ferreum), one of the densest woods, comes in at ~1.4 g/cm3, i.e. nearly an order of magnitude denser than balsa, but around 1/5 the density of iron itself.
So our hard wood is dense, but not as dense as iron itself (which makes sense really): a lump of ironwood will sink in water, which is of course rather atypical of most woods that we’re used to. Don’t try making a raft out of it! But you can see why it’s popular with the well prepared traveller in Fantasyland.