by Kevin L. O’Brien

Welcome to Part One of a series that examines technology and medieval machines that can be used in worldbuilding.

Many fantasy stories gloss over technological details that would be vital in a real-world setting. For example: how would a medieval-level society acquire the iron it needs for tools and implements? The natural assumption is that it would trade for it from a neighbor, but why wouldn’t it make its own? Again, the natural assumption is that ironworking is a specialized skill requiring a sophisticated technology that is difficult to develop in isolation and which would be kept a carefully guarded state secret. However, history shows that in fact an average village could smelt iron in its own backyard. All it would need is a bloomery.

At its simplest, a bloomery is a hollow furnace that resembles a chimney, made from heat-resistant material such as earth, clay, or stone. It has an opening at the top and in the base, and at least one pipe, called a tuyere, inserted near the bottom. The process begins by crushing iron ore, removing any large chunks of impurities, and then roasting it in a fire to drive off moisture. Meanwhile, the bloomery is preheated using charcoal or coke fuel derived from the slow anoxic pyrolysis of wood or coal, respectively, and when it reaches the correct temperature a 1:1 mix of fuel and ore is dumped inside through the top opening. The burning fuel releases carbon monoxide, which reduces the ore to metallic iron by removing oxygen from the oxide compound to create carbon dioxide, without melting the iron. The temperature and the ratio of fuel to ore must be carefully controlled to prevent the iron from absorbing too much carbon, otherwise it becomes too hard and brittle to be work-forged. Since the process is self-fluxing, no limestone needs to be added to produce slag to trap impurities.

The result is a spongy mass of iron and slag called a bloom. After it has cooled, the large external chunks of slag are broken off, and it is reheated and work-forged with hammers to drive out the interior slag and consolidate the iron into a bar. This is known as wrought iron. It still contains compressed layers of slag that make the iron rust resistant. Wrought iron is strong in tension—that is, bending—but weak in compression, so it is much like wood when used for construction. It can be used as is or converted into steel by adding more carbon, a process known as carburization. The slag from the bloom can be salvaged, crushed, and added to the ore of the next firing to recover iron still trapped therein.

A bloomery used by a village typically produces about 2 lbs of iron, but bloomeries can be made larger to produce more. Furnaces used by towns are large enough to produce about 30 lbs on average, whereas industrial-sized furnaces called Catalan forges can produce as much as 650 lbs of iron. Temperature is the key: the hotter the fire, the more iron can be produced, but care must be taken that the fire is not so hot that the iron melts, or it will absorb too much carbon and become pig iron, which cannot by work-forged. (Though it can be remelted and cast. Cast iron is weak in tension, but strong in compression, so it is much like stone when used for construction.)

Airflow in turn is key to temperature control: the stronger the flow, the more vigorously the fuel will burn and the hotter the furnace will become. The air enters the furnace through the tuyere. A small bloomery can use the natural draught created by the chimney-like design, but larger bloomeries require stronger flows, usually by one or more bellows powered by humans or animals, or by water or wind-driven wheels making use of cams, while the very large Catalan forges would require a trompe. This is a machine that uses falling water to compress air, creating high pressure. A waterfall is a convenient source of elevated water, but any source where a supply of water is suspended off the ground will work, such as a dam. The water is directed down a narrow pipe, into which air is introduced. The air is compressed by water pressure, which is produced by the hydraulic head, or the height through which the water falls. The greater the height, the stronger the head, the greater the pressure, and the more the air is compressed. When the water enters a separation chamber at the bottom of the pipe, the compressed air is released and exits through an air pipe to perform whatever work is needed; in this case to provide air for a Catalan forge.

Though a village may be able to get by with half a dozen bloomeries at most, and a town may not need more than a couple of dozen larger furnaces, the beauty of this technology is that it is scalable up to industrial levels. If a city has sufficient supplies of iron ore and charcoal or coke, and an elevated source of water, it could conceivably support a hundred or more Catalan forges and produce tons of iron (and from that, steel) that can be used for construction and manufacturing on a regular schedule; in short, it would have a medieval industrial revolution. Of course, such technology and the society based on it would have a number of cultural and environmental impacts, both positive and negative, that would alter the nature of a quasi-medieval fantasy world, and such a world could be the forerunner to a later steampunk world once steam engines develop.

This series continues in Part Two.

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Kevin L. O’Brien is a hybrid author who has sold stories to Weirdbook and Sword and Sorceress, and has published ebooks through Kindle and Smashwords. He primarily writes speculative fiction, particularly sword & sorcery and paranormal thrillers, but he has also branched out into other genres, including mainstream fiction. He is the creator of Team Girl. His general interest in science and history provides the basis for his posts. He lives in Denver with his three cats.