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.
August 3, 2023
QotD: Blacksmith forge techniques
Filed under: History, Quotations, Technology, Tools — Tags: BretDevereaux, Materials, Metalworking — Nicholas @ 01:00
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