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.

Bret Devereaux, “Iron, How Did They Make It? Part II, Trees for Blooms”, A Collection of Unmitigated Pedantry, 2020-09-25.