This week we continue our four-part (I, II, III, IV) look at pre-modern iron and steel production. Last week we prospected our iron ore and extracted it from the ground and did some initial mechanical processing (washing, sorting, crushing). This week, we’re going to make our way from just rocks to an actual mass of metal rather than just some metal-bearing ore. As we’ll see, we are going to do this by applying heat and (more importantly) chemistry:
Warning: Many, many trees were harmed in the making of this iron.
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But 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…
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.
Forests and Fellers
The bad news is that while ancient sources are often very interested in trees (entire books about them, in fact), they are generally interested in trees used to make things like ships, buildings, furniture and weapons; essentially, elite products. They are not interested in trees used as fuel. Indeed, Latin marks this distinction, where wood for building was materia whereas wood for burning (but also, it seems, bulk wood being transported overseas) was lignum; our sources care greatly about the former and only minimally about the latter. And so as soon as we get very far into the question of the harvesting and preparation of fuel woods, our evidence just about drops away entirely, save for a few poor mentions of this or that tree being good for charcoaling (a crucial process we’ll get to in a moment).
Consequently, our ability to see the fellows felling the forests (say that five times fast) is limited. Medieval ‘foresters’ are often more visible, but much like we noted last time that when Georgius Agricola says ‘miner’ he means ‘mine owner,’ my understanding is that foresters in the Middle Ages were something closer to administrators of the forest (responsible for letting out contracts, catching poachers, etc.; essentially a sheriff but in the woods) rather than simple tree-fellers.
So who did the actual tree-cutting? I must confess, I have found relatively little evidence for the social standing of ancient tree-fellers. In quite a lot of cases, they must not have been meaningfully distinct from the local peasantry or other sources of unskilled rural labor. Clearly a lot of woodcutting was done by the rural population that bordered the forests to clear spaces for fields, gather fuel and firewood and so on, and consequently it seems like the basic skills of tree-felling may have been relatively common. The Latin word for a wood-cutter was a lignator (or sometimes a caesor, which meant ‘cutter’ but could mean of wood (lignorum caesores) or of stone), but that word most often appears in military contexts to mean soldiers tasked with cutting wood for fuel, not full-time lumberjacks. Evidence for the medieval period is somewhat better and also generally suggests that the local peasantry was employed in the wood-cutting itself (for this, note J. Birrell, “Peasant Craftsmen in the Medieval Forest” Agricultural History Review 17.2 (1969): 91-107). As we will see below, often wood cut for charcoaling was cut by the colliers themselves, who we will discuss below. It seems hard to imagine that there wasn’t some division of labor in larger operations (like on Elba or at Populonia), but how that might have been structured is not clear from the limited evidence.
Not all timber works were so easily acquired, of course. While ancient wood-cutters are hard to see in the evidence, ancient sawyers and carpenters are more visible; records from building programs in Athens and Delphi suggest that skilled sawyers (seemingly always assisted by at least one unskilled worker) were paid at least as well as citizen oarsmen in the Athenian navy and in some cases rather better. The presence of English surnames like Carpenter, Cooper, Fletcher, Bowyer, Turner, Sawyer and Wheeler speak to the fact that these were specialized crafts in medieval England; the absence of wood-cutting surnames further suggests that the bulk labor of felling was mostly done by the local rural workforce. Consequently, the social status of the average timber-cutter seems to have been about the same as that of a local peasant, serf or small-farmer, because by and large these seem to have been the same people; while the work done once the tree was down and barked might be done by specialists (but is far less important for trees that are going to be charcoaled). There were also clearly specialist timber merchants, even in the ancient world, and the degree of their visibility, especially in timber-rich regions suggests that they could do quite well for themselves (although, like most merchants, we effectively never see them penetrate into the ruling class), but again, these merchants were likely working with building timbers because, as we’ll see, charcoal wood doesn’t tend to travel very far.
The largest stock of forest-land was typically owned by the state, but private landholders owning their own forests also played a role, albeit generally a small one. In Macedon, the king owned the forests and controlled the supply of lumber, granting or revoking the authority for communities within his territory to take advantage of woodland resources; the practice seems to have been the same, Meiggs (op cit.) notes, in the Near East. In Roman Italy, a large amount of the forest-land was held by the state and contracted out for timber-cutting; Meiggs supposes that figures called saltuarii may have been responsible for making sure that these contracts were carried out properly (much like the later medieval forester, discussed above). We know also from Roman legal enactments later in the empire that it was common for large estates with woodlands and pastures to also have saltuarii, suggesting they might contract out their woodlands in much the same way. Likewise, large forests in the European Middle Ages tended to fall under royal ownership, but Birrell (op cit.) also notes significant timber exploitation from private wood owners in thirteenth century England.
In terms of the practice of timber-cutting, it doesn’t seem to have changed very much through the pre-modern period, although the availability of iron and later steel axes represented a significant improvement in efficiency. Wood-felling axes in the ancient world were the standard single-headed affair (with a heavier axe-head than axes for war; war axes tend to have very thin, light heads). Ovid (Met. 8.775) writes of using ropes on trees ‘loosened by the countless blows of the axe’ being used to pull the tree down, presumably to guide the direction of its fall; we see the same method being used on Pharaoh Seti I’s (r. 1290-1279) relief at Karnak to fell Lebanese cedars:
Pliny and Theophrastus, writing about timber, both place quite a lot of importance on the season of felling, but this was mostly for wood used in things like construction and ship-building; quite evidently (see below), wood would have been cut for the furnaces year round. The practice of pollarding and coppicing (which we have actually discussed before) has more relevance here; pollarded or coppiced trees are pruned in the upper branches to produce a dense set of relatively thin branches for easy harvesting. We know that the Romans did this (Propertius mentions it) and it seems to have been quite common in the Middle Ages for trees intended to supply fuel wood (probably mostly fuel for cooking and home-heating, but possibly also for charcoaling).
With just one exception, however, we cannot jump directly to using that wood to process our ore. Wood, even when dried, contains quite a bit of water and volatile compounds; the former slows the rate of combustion and absorbs the energy, while the latter combusts incompletely, throwing off soot and smoke which contains carbon which would burn, if it had still been in the fire. All of that limits the burning temperature of wood; common woods often burn at most around 800-900 °C, which isn’t enough for the tasks we are going to put it to.
Charcoaling solves this problem. By heating the wood in conditions where there isn’t enough air for it to actually ignite and burn, the water is all boiled off and the remaining solid material reduced to lumps of pure carbon, which will burn much hotter (in excess of 1,150 °C, which is the target for a bloomery). Moreover, as more or less pure carbon lumps, the charcoal doesn’t have bunches of impurities which might foul our iron (like the sulfur common in mineral coal).
That said, this is a tricky process. The wood needs to be heated around 300-350 °C, well above its ignition temperature, but mostly kept from actually burning by lack of oxygen (if you let oxygen in, the wood is going to burn away all of its carbon to CO2, which will, among other things, cause you to miss your emissions target and also remove all of the carbon you need to actually have charcoal), which in practice means the pile needs some oxygen to maintain enough combustion to keep the heat correct, but not so much that it bursts into flame, nor so little that it is totally extinguished. The method for doing this changed little from the ancient world to the medieval period; the systems described by Pliny (NH 16.8.23) and Theophrastus (HP 5.9.4) is the same method we see used in the early modern period.
First, the wood is cut and sawn into logs of fairly moderate size. Branches are removed; the logs need to be straight and smooth because they need to be packed very densely. They are then assembled into a conical pile, with a hollow center shaft; the pile is sometimes dug down into the ground, sometimes assembled at ground-level (as a fun quirk of the ancient evidence, the Latin-language sources generally think of above-ground charcoaling, whereas the Greek-language sources tend to assume a shallow pit is used). The wood pile is then covered in a clay structure referred to a charcoal kiln; this is not a permanent structure, but is instead reconstructed for each charcoal burning. Finally, the hollow center is filled with brushwood or wood-chips to provide the fuel for the actual combustion; this fuel is lit and the shaft almost entirely sealed by an air-tight layer of earth.
The fuel ignites and begins consuming the oxygen from the interior of the kiln, both heating the wood but also stealing the oxygen the wood needs to combust itself. The charcoal burner (often called collier, before that term meant ‘coal miner’ it meant ‘charcoal burner’) manages the charcoal pile through the process by watching the smoke it emits and using its color to gauge the level of combustion (dark, sooty smoke would indicate that the process wasn’t yet done, while white smoke meant that the combustion was now happening ‘clean’ indicating that the carbonization was finished). The burner can then influence the process by either puncturing or sealing holes in the kiln to increase or decrease airflow, working to achieve a balance where there is just enough oxygen to keep the fuel burning, but not enough that the wood catches fire in earnest. A decent sized kiln typically took about six to eight days to complete the carbonization process. Once it cooled, the kiln could be broken open and the pile of effectively pure carbon extracted.
Raw charcoal generally has to be made fairly close to the point of use, because the mass of carbon is so friable that it is difficult to transport it very far. Modern charcoal (like the cooking charcoal one may get for a grill) is pressed into briquettes using binders, originally using wet clay and later tar or pitch, to make compact, non-friable bricks. This kind of packing seems to have originated with coal-mining; I can find no evidence of its use in the ancient or medieval period with charcoal. As a result, smelting operations, which require truly prodigious amounts of charcoal, had to take place near supplies of wood; Sim and Ridge (op cit.) note that transport beyond 5-6km would degrade the charcoal so badly as to make it worthless; distances below 4km seem to have been more typical. Moving the pre-burned wood was also undesirable because so much material was lost in the charcoaling process, making moving green wood grossly inefficient. Consequently, for instance, we know that when Roman iron-working operations on Elba exhausted the wood supplies there, the iron ore was moved by ship to Populonia, on the coast of Italy to be smelted closer to the wood supply.
It is worth getting a sense of the overall efficiency of this process. Modern charcoaling is more efficient and can often get yields (that is, the mass of the charcoal when compared to the mass of the wood) as high as 40%, but ancient and medieval charcoaling was far less efficient. Sim and Ridge (op cit.) note ratios of initial-mass to the final charcoal ranging from 4:1 to 12:1 (or 25% to 8.3% efficiency), with 7:1 being a typical average (14%).
We can actually get a sense of the labor intensity of this job. Sim and Ridge (op cit.) note that a skilled wood-cutter can cut about a cord of wood in a day, in optimal conditions; a cord is a volume measure, but most woods mass around 4,000lbs (1,814kg) per cord. Constructing the kiln and moving the wood is also likely to take time and while more than one charcoal kiln can be running at once, the operator has to stay with them (and thus cannot be cutting any wood, though a larger operation with multiple assistants might). A single-man operation thus might need 8-10 days to charcoal a cord of wood, which would in turn produce something like 560lbs (253.96kg) of charcoal. A larger operation which has both dedicated wood-cutters and colliers running multiple kilns might be able to cut the man-days-per-cord down to something like 3 or 4, potentially doubling or tripling output (but requiring a number more workers). In short, by and large our sources suggest this was a fairly labor intensive job in order to produce sufficient amounts of charcoal for iron production of any scale.
As should be obvious from the complexity of the work, charcoal burning was a specialized profession; gaining a sense of how dense to set the pile (very dense, generally), how much fuel to add, how large to make it, and how to gauge the process of carbonization merely by the sight of the smoke are the sort of things one learned by experience, presumably first as an assistant or apprentice. In some cases, this job was done by peasants who did it part-time (the colliers certainly seem to come from the local peasantry); Birrell (op cit.) notes that in the fourteenth century records for the Cumberland Forest of Inglewood that the majority of licenses for charcoaling were for seasons rather than all-year, suggesting most of the colliers had other occupations. Nevertheless, the presence of ‘Colier’ (or collier), Askebrunner, Ashburner, or the Latin carbonarius or cinearius as surnames or professions appearing in thirteenth and fourteenth century English sources attests to the fact that this was a specialized skill-set, one which defined the practitioner, whether pursued part-time or full-time. In many cases, as Birrell notes, the fellows taking out licenses to burn charcoal (almost certainly for assistants or servants) were themselves smiths who also owned forges in the area.
And yet the colliers themselves seem not to have been well thought of by society despite these skills. Their occupation was a solitary one, since they had to attend their charcoal piles for the week or so it took for them to carbonize, which meant spending weeks at a time out in the forest. Mostly beneath the notice of our ancient sources (Pliny and Theophratus both describe the charcoaling process without mentioning the men doing that process); it seems likely that colliers fell under the same elite derision that attached to many overly smelly jobs (like tanners). German preserves an odd bit of this dislike with Köhlerglaube (“collier’s faith”) meaning a blind faith or loyalty in a rather negative sense, ostensibly because the colliers, alone in the forest (rather than in church) had no opportunity to learn the details of their own faith. Fourteenth century charcoal burners working directly for the English King (Birrell, op cit., 98n.2) were paid 3d (that is, 3 pence) a day, a decent but not extravagant wage at the time (for a broad comparison of contemporary wages, see this website, which amasses medieval price data), suggesting that the collier’s skill demanded some wage premium, but not necessarily a tremendous amount.
With our ore mined and our charcoal made, we are almost ready to get to our smelting, but first we have one final step to prepare our iron ore for the smelting process: roasting. Roasting solves two problems we have. The first problem is water: our ore, even if it appears dry, almost certainly traps small amounts of water inside the ore. We want to remove that before we subject this ore to extreme temperatures, both because water is just going to absorb the energy (heat) from our fuel, wasting it, and also because small pockets of water inside rock heated in excess of 1,200 °C can pose problems. Driving out the water at this stage also has the added benefit of cracking most ores into smaller bits that will reduce even easier once we get to smelting.
The second problem is chemical, because we are never going to melt this iron. Our furnaces can’t get that hot (and even if they could, melting this iron would cause it to absorb a lot of carbon from our fuel, which we do not want). So we’re going to be reducing our iron – that is, getting it to change chemically with the exposure of heat. That means the chemical composition of our iron matters a lot and we have to solve our chemical problems before we can smelt our ore.
The good news is that some ores of iron reduce fairly easily and directly, most notably hematite and the hydroxide-iron ores like limonite and goethite. They reduce fairly easily (but the latter two tend to come with lots of water that needs removing). But then we have magnetite, which while also an iron-oxide, doesn’t reduce nearly so easily as hematite, and siderite (and other carbonates) which has carbon in it, which we do not want. Moreover, our country rock might have some trace amounts of things like sulfur (or iron-sulfides) in them, which we very much do not want. Sulfur will absolutely ruin our final iron product, so we do not want it floating around when we get to the smelting process.
But let’s say we expose some magnetite (Fe3O4) to heat in an environment where there is some oxygen around – we can get a further oxidizing reaction (I dearly hope I have remembered my chemistry) whereby 4Fe3O4 get along with 1 O2 (your garden variety oxygen in the air) to make 6Fe2O3. And good news, that’s something we recognize – that’s hematite (Fe2O3) which we already know will reduce in our furnace in a bit. Likewise through a slightly more complicated reaction, we can get that pesky carbon out of our carbonate ores like siderite (FeCO3) which release its carbon and oxygen as carbon dioxide and end up forming Fe2O3, which again is our good friend hematite. Likewise, my understanding is that small amounts of sulfur will oxidize to sulfur-dioxide which is pleasantly a gas and so will be on its merry way out of ore as well.
(Edit: The more chemistry minded have pointed out that the hydroxide iron ores – which you have to roast to remove water anyway – will also reduce down to hematite when roasted through a reaction that looks like this: 2 Fe(OH)3 -> Fe2O3 + 3H2O releasing water which – just like the water trapped in the ore itself – boils away in the roasting process.)
The actual process of roasting the ore is in many ways less interesting than the chemistry that goes into why we roast the ores. This is the only step generally done with raw wood (typically dried to remove water), rather than charcoal. The temperatures we need to roast ore are fairly low, typically between 300 to 550 °C or so – enough to boil off the water and trigger our chemical reactions, but that’s it. We can accomplish that by setting a pile of dry wood within a brick kiln, putting the iron ore on top of them, and lighting the whole thing on fire. For very small quantities of iron ore, this can be done very simply (pictures at the link); doing larger batches of ore would have required more careful management of fuel and ore.
With that, we can finally move on to smelting in a…
All of that effort now brings us to a frustrating problem and the reason why nearly every ‘making the sword’ scene in a movie is wrong: iron melts at 1538 °C. That is very hot. To achieve those temperatures over the entire mass of ore (necessary to melt the iron out of it) we would need to get to around 2000 °C, which is unsurprisingly the temperature that modern blast furnaces work at. At those temperatures, a whole lot of chemical reactions happen and the resulting iron comes out molten, enabling it to be separated from the waste material (slag). The problems here are two-fold: first, that molten iron would pick up a ton of carbon, turning it into brittle and functionally useless pig iron and second, prior to the early modern period, most of the world’s furnaces couldn’t achieve those temperatures anyway (we’ll talk about exceptions in China and India in an addendum).
So we can’t melt the iron out of the ore. What if we melt the ore out of the iron? This is half of the fundamental principle of a bloomery (the other half being chemical reactions that take place in a carbon-rich environment with lots of heat and iron-oxides).
A bloomery was a type of furnace that produces a bloom, a sponge-shaped mass of (mostly) pure iron. The basic structure of a bloomery furnace is fairly simple: a conical shaft (or sometimes a lower bowl-shape) made out of some fairly heat-tolerant material (bricks, stone, clay) with one or several holes near the base (called tuyères) for air to enter at the bottom and either a bit beneath the shaft for slag materials to drain into or a larger hole at the base to allow the slag to be tapped. Some types of furnaces (particularly the bowl-types and furnaces which used a bit rather than having slag-tapping to deal with slag accumulation) were effectively single use, being broken up in order to extract the bloom. I am going to focus here on the more sophisticated low-shaft furnaces with slag-tapping because, as Tylecote (op cit.) notes, these were the sort that the Romans used and seem, so far as I can tell, to have provided the basis for later medieval European bloomeries.
That description sounds very complicated, so it is time for one of my trademark badly made diagrams:
With air entering through the tuyères (the diagram shows just one, but furnaces often had several, which may also have had air pushed through them by bellows), it feeds the combustion of the charcoal. To understand what that does for us, we need to get back into the chemistry.
The charcoal – a (nearly) pure mass of carbon – takes some heat and some O2 from the air and produces carbon dioxide (CO2) and a bunch more heat, which both heats up the iron ore we’ve stacked on top of it and the rest of the charcoal, which will react with the first oxygen it meets to repeat the process. But this process is rapidly going to be oxygen starved (this is important) and so some of these reactions are going to produce carbon monoxide (CO) which then, because it is very hot, goes racing up through our iron ore. And carbon monoxide is a lonely fellow – give him some heat and he starts looking for one more oxygen to form carbon dioxide…and would you just look at the hip, happening place he has stumbled into, because he is now surrounded by hematite (Fe2O3) with all that oxygen to spare. So each lonely-lad carbon monoxide grabs one more oxygen dance partner: Fe2O3 + 3CO -> 2Fe + 3CO2. The CO2, being a gas, exits the furnace at the top, flying off into the sky to utterly ruin our climate in revenge, leaving just the pure, metallic iron behind. This process actually begins to happen at only 800 °C or so.
But wait! That hematite we’re reducing didn’t come to us pure, it came to us embedded in other kinds of rock (called gangue). And this is why we need all of that heat. Most of these impurities from the ore are going to be silica (SiO2) and alumina (Al2O3). We need those gone too. In modern furnaces, a flux (typically limestone) is introduced into the mixture because it will readily bind to these compounds to get them out of the iron. In the Roman period, at least, this wasn’t understood for the smelting stage of the process; I’m not sure when exactly ironworkers realized that they could use limestone to deal with this. But iron can also do the job; exposed to enough heat, 2FeO (an iron-oxide we’ll have in abundance because of the processes above) + SiO2 forms Fe2SiO4 (alumina reacts much the same, but I cannot find the formula for the life of me). Of course, that means we lose a decent amount of our iron from having it serve in place of a flux, which further lowers our efficiency. Those new slag compounds melt around 1150 °C, well below the melting temperature of iron.
And so at long last we are getting somewhere. Our iron-oxides have mostly converted, by reaction with carbon monoxide (and a lot of heat) into pure metallic iron, while most everything that isn’t pure metallic iron has grabbed some of our iron (unfortunate) or some limestone (if we have it), lowering its melting point so that it becomes liquid, dripping down to the base of the bloomery furnace, where we can tap it out.
Once the charcoal burns all the way down and all of the ore has been reduced, what we’ll be left with is a mass of metallic iron – with some slag impurities still left, but tolerably few – in a sponge-shape. In contrast to a blast furnace, which will give us iron with a lot of carbon, our bloom’s carbon content will be extremely low, because the carbon that we had was burned into carbon-monoxide (and then oxidized into carbon dioxide) and so wasn’t sticking around to be absorbed into our growing bloom of iron. Which is fantastic, because we have no efficient way with medieval or ancient technology to decarburize iron (that is, pull the carbon out), so getting iron with super-high carbon counts would be really very bad (we’ll talk about iron and carbon more in the last post of this series when we get into steel).
Efficiency and Ecology
Ok, that was all a lot of fascinating chemistry, but you may well ask what does that mean from a practical standpoint? How much wood and how much ore do we need to produce a given amount of iron?
Obviously, those figures vary wildly based on the quality and type or ore, the quality of our furnace (how well it keeps heat, for instance) and any number of other considerations. Healy, using some of Tylecote’s figures, computes an estimate that Roman smelters, working continuously (two batches in 24 hours) might, in a day produce 16kg of iron from 100kg of ore and 80kg of charcoal. Healy also notes an experiment by E.J. Wynn with a more primitive bowl-furnace took 16lbs of charcoal to produce 1lbs of finished iron. Sim and Ridge, working from a different set of experiments, suggest that about 1kg of finished iron might require 12.3kg of ore and 14.6kg of charcoal (obviously a furnace batch would be larger than this). The variance here is considerable, but remember that the iron-content of the ore isn’t constant, so this is really a range of possibilities (there’s also a difference in experimental procedures here, particularly if bar- and billet-smithing are counted, or if we’re measuring the weight of the bloom).
And, for that charcoal, as you will recall, the charcoaling process averages a mass of finished carbon around 14% of the starting wood. If we take Sim and Ridge’s figures, the 14.6kg of charcoal required for each 1kg of iron would require around 105kg of wood (plus some fuel for the charcoaling process) to produce. And to be clear, we are not done: every stage of iron-working past this also involves losing mass, in some cases because we’re ejecting slag that’s found its way into our iron (so we’re not losing anything of value, but our mass is decreasing because we’ve counted things-not-iron in our 1kg iron bloom) and in some cases because our iron is oxidizing and we’re ejecting the resultant iron-oxide (read: rust) in the forging process, and in some cases because we’re going to need to polish, file and sharpen, which involves stripping off small amounts of iron.
To put that in some perspective, a Roman legion (roughly 5,000 men) in the Late Republic might have carried into battle around 44,000kg (c. 48.5 tons) of iron – not counting pots, fittings, picks, shovels and other tools we know they used. That iron equipment in turn might represent the mining of around 541,200kg (c. 600 tons) of ore, smelted with 642,400kg (c. 710 tons) of charcoal, made from 4,620,000kg (c. 5,100 tons) of wood. Cutting the wood and making the charcoal alone, from our figures above, might represent something like (I am assuming our charcoal-burners are working in teams) 80,000 man-days of labor. For one legion.
The ecological impact of pre-modern iron production was also significant. We know, for instance, that Elba was almost totally deforested during the Roman period to fuel the bloomeries smelting the ore and Pliny notes in his Natural History that smelting (not always of iron) had substantially reduced forest-stocks in parts of Gaul and Campagnia. Roman iron production in the eastern High Weald of England may have deforested something like 500km2 over the course of three centuries and there is reason to believe that Roman-period iron production in this area stopped because of scarcity of fuel, rather than ore. Iron-working was hardly the only factor in the steady deforestation of Europe, but it was a major factor.
Nevertheless, our trees have not all died in vain. We started this post with some crushed up iron-bearing rocks and some trees and we have managed to produce a bloom – our sponge-shaped mass of iron. Next week, we’ll apply even more heat (did I mention this process requires a lot of fuel?) and some mechanical force in the form of hammers when we finally get to forging.