(I sometimes joke about the local volunteer fire departments having requested that I not be allowed to weld, or use a torch, or play with fire. . . or even convert motor vehicles to large wood-lathes. I think that as you read through this, you may understand why they would do this.)
Remember as a little kid, finding a fruit tree with ripe fruit, what did you do first? You picked the low-hanging fruit, right? Then you climbed the tree as high as you could, maybe fell out of the tree and broke your arm or something, but you picked the low-hanging fruit first because it was easy to get to. The low-hanging fruit of the iron ore world—widely distributed, easy to find, easy to refine, and renewable—is bog iron.
Bog iron is an impure iron oxide, typically found in swamps and bogs. The formation of bog iron begins when subterranean streams dissolve iron-bearing ore. When the streams break the surface and become springs, the iron in solution becomes iron oxides. When the water enters a swamp or bog, anaerobic bacteria that grow in the acidic water concentrate the iron, resulting in the formation of pea-sized lumps or nodules. Folks harvesting peat from bogs discovered the stuff. There’s never a lot in any one place, but if you are harvesting peat to burn for heating and cooking, you cut a lot of peat and find a fair amount of iron . . . and it is renewable in that you can go back to the same area in 20-30 years and find newly-formed nodules.
But finding and gathering bog iron is the easiest part of refining it. The next step is to heat it in a fire to dry it out and burn off organic contaminants. You can do this while burning the charcoal—and you’re going to use a lot of charcoal. . . . Coal would have been a more-efficient fuel, but charcoal was preferred for a couple of reasons. Iron refining seems to have developed from earlier refining of copper/tin/bronze, and charcoal was the preferred fuel for this because anywhere there are trees, charcoal can be made. They knew how to use charcoal, and it worked with the tools and technology they had then. Coal burns hotter, which can create problems of its own.
For about 2,500 years, Iron refining was done in bloomeries. A bloomery is rather like a chimney, in which charcoal and iron are mixed, and then the charcoal is burned in the presence of an air blast to increase temperatures. The burning charcoal generates carbon monoxide, which reduces the iron oxides in the ore into metallic iron. There are problems with this approach: the most obvious from a modern perspective being that the iron doesn’t really get hot enough to melt completely, so instead of an ingot of cast iron, your get a foul mass of iron and slag, commonly called a “bloom” or “sponge iron”.
I’m not so sure that is a problem, at least from the perspective of the early workers. A more-modern way is to process ore into cast iron (which is iron and various impurities including a very high level of carbon), and then further refine the cast iron by sorting out the impurities and burning off the carbon. Way-back-when, they didn’t really have a way of further refining cast iron, but they could convert the bloom into wrought iron. They used some of the impurities to their advantage, and worked out ways to convert the iron to steel. Granted, they were very labor- and energy-intensive ways to do so, but they were ways that worked on a small scale. Modern ways are more efficient, but only on a large scale.
Early bloomeries were small, producing a couple pounds of iron with each firing. By early medieval times in Europe, they were producing up to 25-30 pounds per firing. When the loads got larger, the extended time involved meant that some of the iron got hot enough to melt. This resulted in cast iron, which they saw as a waste product because they didn’t know what to do with it!
In the early ‘90s, I got my hands on a couple gunny-sacks of bog iron. Of course, when you’ve got something like this, you have to build a bloomery. Maybe that’s just me. Half of that bog iron was wasted when I made a series of stupid mistakes. I thought I—a product of the 20th century—knew more than the folks who invented the process.
I scaled up the bloomery to yield a roughly-25 pound bloom. I used coal, because it was “more efficient” (and available). And because I’d always used a blower on a coal forge, I brought that degree of forced air into the picture.
According to the probes I had in place on the bloomery, internal temperatures exceeded 1,300 degrees C (about 2,400 degrees F). The bonded-clay bloomery cracked. All I had to show for it were a twisted mass of useless cast iron and slag, and a request to move my experiments out of the city where I lived then. Oh well, I was going to have to make charcoal anyhow. . . . Before I was done, I’d converted a cord of mixed hardwoods—not a “face cord” but an actual 4’x4’x8’ cord—to charcoal.
My Mark II bloomery was smaller, with a projected yield of a 2 pound bloom. Air supply was via a single-chamber bellows, and the fuel was charcoal. The probes said the temps never exceeded 1,100 degrees C (roughly 2,000 degrees F). I still ran out of charcoal, but by the time I was done I’d converted over 200 pounds of bog iron to about 40 pounds of sponge iron.
From start to finish, my hypothetical umpty-great-grandpa who ran a bloomery 2,500 years ago would have had about 120 hours in producing that 40 pounds of sponge iron. And the work only gets harder from here. . . .