Fundamentally, each stage of forging iron revolves around a basic cycle: by heating the metal, the smith makes it soft enough to work (that is, hammer into shape). Technically, it is possible to shape relatively thin masses of iron by hammering when cold (this is called cold-working) but in contrast to other metals (tin, copper and bronze all come to mind) nearly all serious iron-working was done “hot”. In smithing terminology, each of these cycles is referred to as a “heat” – the more heats a given project requires, the more fuel it is going to consume, the longer and more expensive it is going to be (but a skilled smith can often finish the work in fewer heats than an unskilled smith).

A modern blacksmith can gauge the temperature of a metal using sophisticated modern thermometers, but pre-modern smiths had no recourse to such things (and most traditional smiths I’ve met don’t use them anyway). Instead, the temperature of the metal is gauged by looking at its color: as things get hotter, they glow from brown to dark red through to a light red into yellow and then finally white. For iron heated in a forge, a blacksmith can control the temperature of the forge’s fire by controlling the air-input through the bellows (pushing in more air means more combustion, which means more heat, but also more fuel consumed). As we’ve seen, charcoal (and we will need to use charcoal, not wood, to hit the necessary heat required), while not cripplingly expensive, was not trivial to produce either. A skilled smith is thus going to try to do the work in as few heats as possible and not excessively hot either (there are, in fact, other reasons to avoid excessive heats, this is just one).

Once hot the metal can be shaped by hammering. The thickness of a bar of metal could be thickened by upsetting (heating the center of the bar and them hammering down on it like a nail to compress the center, causing it to thicken) or thinned by drawing (hammering out the metal to create a longer, thinner shape). If the required shape needed the metal to be bent it could be heated and bent either over the side of the anvil or against a tool; many anvils had (and still have) a notch in the back where such a tool could be fitted. A good example of this kind of thing would be hammering out a sheet of iron over a dome-shape to create the bowl of a helmet (a task known as “raising” or “sinking” depending on precisely how it is done). A mass of iron can also be divided by heating it at the intended cutting point and then using a hammer and chisel to cut through the hot, soft metal.

But for understanding the entire process, the most important of these operations is the fire weld. Much like bloomery furnaces, the forges available to pre-modern blacksmiths could not reach the temperatures necessary to melt or cast iron, but it was necessary to be able to join smaller bits of iron into larger ones which was done through a fire weld (sometimes called a forge weld). In this process, the iron is heated very hot, typically to a “yellow” or “white” heat (around 1100 °C). The temperature range for the operation is quite precise: too cold and the iron will not weld, too hot and it will “burn” making the weld brittle. Once at the right temperature, the two pieces of iron are put next to each other and hammered into each other with heavy blows. If done properly, the two pieces of metal join completely, leaving a weld that is as strong as every other part of the bar.

Bret Devereaux, “Collections: Iron, How Did They Make It, Part III: Hammer-time”, A Collection of Unmitigated Pedantry, 2020-10-02.

There are a few basic behaviors of iron that fundamentally control what blacksmiths are going to do with it in this stage. To begin with, we need to introduce some terminology to avoid this coming confusing: a given piece of metal can be hard (resistant to deformation) or soft; it can also be ductile (able to deform significantly before breaking) or brittle (likely to break without deformation). This is easiest to understand at the extremes: a soft, brittle material (like a thin wooden dowel) takes very little energy and breaks immediately without bending, while a hard, ductile material (the same dowel, made of spring-steel) bends more easily under stress but resists breaking. But it is also possible to have hard brittle materials (pottery being a classic example) which fiercely resist deforming but break catastrophically the moment they exceed their tolerances or a soft, ductile material (think wet-noodle) which bends very easily.

(I should note that all of these factors are, in fact, very complex – far more complex than we are going to discuss. In particular, as I understand it, some of what I am using “hardness” to describe also falls under the related category of yield strength. Hopefully you will all pardon the necessary simplification; if it makes you feel any better, ancient blacksmiths didn’t understand how any of this worked either, only that it worked.)

Of course these are not binaries but a spectrum. Materials have a degree of hardness or ductility; as we’ll see, these are not quite opposed, but changing one does change the other – increasing hardness often reduces ductility.

The sort of things that pre-modern people are going to want to be made in iron are going to have fairly tight tolerances for these sorts of things. Objects that had wide tolerances (that is, things which could be weak or a little bendy or didn’t have to take much force) got made out of other cheaper, easier materials like ceramics, stone or wood; metals were really only used for things that had to be both strong and relatively light for precisely the reasons we’ve seen: they were too expensive for anything else. That means that a blacksmith doesn’t merely need to bring the metal to the right shape but also to the right characteristics. Some tools would need to finish up being quite hard (like the tip of a pick, or the edge of a blade), while others needed to be able to bend to absorb strain (like the core of a blade or the back of a saw).

Bret Devereaux, “Collections: Iron, How Did They Make It, Part III: Hammer-time”, A Collection of Unmitigated Pedantry, 2020-10-02.