… let’s start with the single largest input for our entire process, measured in either mass or volume – quite literally the largest input resource by an order of magnitude. That’s right, it’s … Trees
The reader may be pardoned for having gotten to this point expecting to begin with exciting furnaces, bellowing roaring flames and melting all and sundry. The thing is, all of that energy has to come from somewhere and that somewhere is, by and large, wood. Now it is absolutely true that there are other common fuels which were probably frequently experimented with and sometimes used, but don’t seem to have been used widely. Manure, used as cooking and heating fuel in many areas of the world where trees were scarce, doesn’t – to my understanding – reach sufficient temperatures for use in iron-working. Peat seems to have similar problems, although my understanding is it can be reduced to charcoal like wood; I haven’t seen any clear evidence this was often done, although one assumes it must have been tried.
Instead, the fuel I gather most people assume was used (to the point that it is what many video-game crafting systems set for) was coal. The problem with coal is that it has to go through a process of coking in order to create a pure mass of carbon (called “coke”) which is suitable for use. Without that conversion, the coal itself both does not burn hot enough, but also is apt to contain lots of sulfur, which will ruin the metal being made with it, as the iron will absorb the sulfur and produce an inferior alloy (sulfur makes the metal brittle, causing it to break rather than bend, and makes it harder to weld too). Indeed, the reason we know that the Romans in Britain experimented with using local coal this way is that analysis of iron produced at Wilderspool, Cheshire during the Roman period revealed the presence of sulfur in the metal which was likely from the coal on the site.
We have records of early experiments with methods of coking coal in Europe beginning in the late 1500s, but the first truly successful effort was that of Abraham Darby in 1709. Prior to that, it seems that the use of coal in iron-production in Europe was minimal (though coal might be used as a fuel for other things like cooking and home heating). In China, development was more rapid and there is evidence that iron-working was being done with coke as early as the eleventh century. But apart from that, by and large the fuel to create all of the heat we’re going to need is going to come from trees.
And, as we’ll see, really quite a lot of trees. Indeed, a staggering number of trees, if iron production is to be done on a major scale. The good news is we needn’t be too picky about what trees we use; ancient writers go on at length about the very specific best woods for ships, spears, shields, or pikes (fir, cornel, poplar or willow, and ash respectively, for the curious), but are far less picky about fuel-woods. Pinewood seems to have been a consistent preference, both Pliny (NH 33.30) and Theophrastus (HP 5.9.1-3) note it as the easiest to use and Buckwald (op cit.) notes its use in medieval Scandinavia as well. But we are also told that chestnut and fir also work well, and we see a fair bit of birch in the archaeological record. So we have our trees, more or less.
Bret Devereaux, “Iron, How Did They Make It? Part II, Trees for Blooms”, A Collection of Unmitigated Pedantry, 2020-09-25.
September 12, 2023
QotD: The largest input for producing iron in pre-industrial societies
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