Steel-making was an ancient art. Modern experiments have shown that good-quality steel can be made with technology that was around in the Iron Age and probably before. By the dawn of the Medieval period (c. 500 CE), steelmaking technology had changed little: it was a haphazard, almost magical art that produced fine weapons for a tiny noble elite. By the end of the medieval era, it was a mechanised process, the subject of manuals and treatises, mobilised to equip whole armies. Here, we’ll look at the basic building blocks of the process of steel-making.
What is Steel?
The most simple definition of steel is an alloy of iron and carbon. Alloys are metals that are comprised of more than one element – and their properties are often very different from the elements they are made from. Pure iron is a fairly soft, dull-grey metal that can easily be worked into shape, whilst carbon is a crumbly black substance, the most common form of which was charcoal. Steel, however, is a hard, strong metal that can be sharpened to a very effective edge, and, if properly heat-tempered, can demonstrate significant elasticity – ie., it can ‘spring’ back into shape when bent or hit. These properties made it extremely valuable in the medieval era for making weaponry and armor – like this 10th-century noble Viking steel sword (left, via Met Museum).
However, making steel is not a simple process, and for most of the medieval period, it signified that its bearer was wealthy and powerful, sitting at the pinnacle of a labor-intensive pyramid of manual labor and skilled craft to produce their steel sword or their steel chainmail. The precise steps involved in the production of iron and steel varied significantly across Medieval Europe, depending on time, place, available materials and known technologies. But what we shall do here is look at the basic steps required to go from minerals in the ground to an ingot of fine steel.
The Ingredients of Steel
The first step towards steelmaking is finding sources of iron and carbon. Iron occurs naturally in the form of iron ores. These are usually minerals that form inside rocks, and come in several different types like magnetite, haematite and the imaginatively named ironstone. These stones were well-known in the medieval period, having been identified by ancient peoples long ago. Mining in the medieval period was extremely dangerous and incredibly hard work, being undertaken with simple wooden props and hand tools. However, many places in Medieval Europe, particularly Scandinavia, had little or no surface iron ores – in these places, metalworkers had to make do with other sources like ‘bog iron’, low-grade iron ores that were deposited in wetland environments by iron-rich springwater. So, once you have your lumps of iron-rich ore, you need a source of carbon. Easily the commonest source of carbon is trees, which are around 50% carbon by mass. By burning wood very slowly in an oxygen-poor environment like a tightly-closed kiln, the other elements are driven off, leaving behind black, crumbly charcoal – almost pure carbon.
The Magic Bloomery
The iron in iron ore is bound up with oxygen, as well as silicon, phosphorus and all sorts of other undesirable impurities known as slag. We want to get the oxygen off the iron, and put it onto the carbon in order to make the gas carbon dioxide, which will be driven off as smoke (known as ‘reducing’ or a ‘reduction reaction’). At the same time, we want to introduce a little carbon into the pure iron, so that it produces an iron-carbon alloy, ie. steel. So our medieval metallurgist would have to do some magic, in the form of a chemical reaction:
The simplest tool that they would use to perform this reaction is called a bloomery furnace. It was a slightly tapered clay chimney with a pipe at ground level to feed it with air, either through the draw of the fire or with hand-bellows. They were usually about as tall as a person, although as the medieval period progressed they became much larger and more efficient! The bloomery would be filled up to the top with charcoal, and lit at the base. When it was hot, the iron ore (usually crushed or powered) would be added to the top of the bloomery. Experimental archaeology has shown us that, counter-intuitively, you didn’t want your bloomery to get too hot – if the iron gets hot enough to run liquid, then it’ll just drip away into the mass of slag and be lost. The ideal temperature for bloomery steel is fairly cool – around 750-900ºC (1400-1650ºF). Medieval metalworkers didn’t have thermometers, and so they would operate on skilled experience and guesswork.
Reducing the iron ore into bloomery steel would usually take a whole day. If everything went well, steel would slowly travel down the column of the bloomery and puddle in a ‘bloom’: a spongy mass made from a mixture of iron-carbon alloy and the slag waste from the ore, usually about the size of a football. After the metalworker was confident all of the iron had been reduced, they would break open the front of the bloomery and dig the bloom out from the mixture of ash and slag. The final stage of the process was to remove as much of the slag as they could. This was done in the most direct way – reheating it and beating it with hammers. The hot steel would ‘stick’ together, whilst the crumbly brittle slag would be driven off in the form of sparks and dust. Finally, after an exhaustive (and exhausting!) process, you would end up with a lump of steel, likely not much larger than a fist. No wonder this magical material was so valuable!
Beyond The Bloomery
Later in the medieval period, much more advanced steelworking techniques began to develop – like the blast furnace, and the cementation process. The permitted far greater quantities of steel that was far better consistency to be made. But the bloomery process continued to be used to make steel throughout the medieval period, and shows us the basic requirements of steelwork. Now, when you look at a warrior’s steel sword or a piece of early-medieval steel chainmail, you might be filled with a little of the same awe that it was intended to inspire!