I’ve spent a lot of hours over the last couple of days trying to express every variable of bloomery iron smelting and their complex and dependent relations. At the moment, I can’t seem to do it in any way that doesn’t look like a spider covered in multi-coloured ink had a seizure on my page.
The best I can do is below – and although I think it looks pretty it only covers the general shape of smelting, it isn’t as awesome as I was planning. Anyway, I thought I’d share the first draft with you for two reasons. I hope that if anyone has any better ideas, comments, corrections or other reading they will comment or get in touch, and I may be able to improve the diagram. I also hope that anyone trawling the web for information on this topic will find the diagram to be of some use.
Iron smelting is a much more complicated process than copper smelting, despite that fact that both could be conducted in essentially the same structure and using much the same techniques and materials.
During copper smelting liquid copper can be produced, because copper has a melting point of 1084 C and furnaces can conceivably get up to or above that temperature. However iron melts at around 1536C, a temperature which is incredibly difficult to reach without modern machinery, and at which the ceramics of pre-modern furnaces are likely to melt and collapse.
As a result, iron is very rarely liquid during the smelting process. When it is produced, it is generally thought to remains solid. Luckily when iron oxide and silica are present they can melt together, and this forms the slag that is the most common archaeological evidence for smelting. I like to think of the slag as having a similar beneficent effect to ‘primordial soup’ – it allows the transport of the tiny fragments of iron metal so that they can conglomerate as a solid mass, and it covers it from the strong oxidising effect of the air being pumped into the furnace.
For good slag formation we need enough carbon monoxide to be circulating inside the furnace system, and high enough temperatures, so all three of these components effect the outcome of the smelt and are inextricably linked. But of course, you can’t just throw in more CO if its not working. What you can do is fiddle with the proportions of fuel to ore you put in, the size of these pieces, the amount of air you force into the furnace, and you can patch the furnace so that the CO doesn’t escape through the furnace walls.
The diagram below is a summary of this. Oh, except for the fact that you can also add ‘flux’ to ease the formation of slag, and I haven’t put that in. But you can imagine it in the bottom pile of variables if you like. Enjoy!
The full reference is:
Tylecote, R.F., Austin, J.N. & Wraith, A.E., 1971. The mechanism of the bloomery process in shaft furnaces. Journal of the Iron and Steel Institute, 209, 342-363.
It’s a rather full-on and large report of a campaign of experimental work undertaken by Tylecote and his colleagues, but it’s essentially invaluable. Most of what we know of how the various parts of the process contribute to the final product (the bloom) and how they interact comes from experimental work, of which Tylecote is the dominant contributor.