Gold is for the mistress — silver for the maid —
Copper for the craftsman cunning at his trade.
“Good!” said the Baron, sitting in his hall,
“But Iron — Cold Iron — is master of them all.”
– Rudyard Kipling: ‘Cold Iron’ (1910) –
In my Introduction to Experimental Archaeology, I have described experimental archaeology as the testing of archaeological hypotheses based on the excavated record by reproducing and using archaeological finds. In other words, testing our presumptions about nature of the past by trying to reproduce it. A large group of experiments revolves around ancient craft, or ‘How did people make ______ in the past?’ This time I will delve deeper into how we may seek understanding of ancient craft, more specifically iron production.
Archaeologist like to describe craft production as sequence or chain of actions, for which they like to use, and phonetically mangle, the French expression chaîne opératoire. When attempting to understand craft in the past, we are trying to experimentally reconstruct the missing links in this chain.
Therefore Experimental Archaeology tries to fill in the gaps between the various bits of archaeological data. This data most often comes in the form of a known raw material, a selection of possible tools and a known end result (artefact).
My task as an experimental archaeologist and craftsman is then to figure out how the tools were used to manipulate the raw materials in order to end up with an exact replica of the artefact. This seemingly simple task may include a multitude of questions:
How was the raw material recognized and collected in the first place?
Which tool was used for what task?
In what order were the tasks/steps performed?
Is there more than one way of doing this?
What kind of working space was required for the task and can it be recognized in the archaeological record?
What kind of waste is produced?
How many artisans/assistants/workers were required to perform the task?
Is this something that was done all year round, or is there a seasonality associated with it?
How does the craft fit in with other crafts and the rest of society?
How do I best record all of this?
So on, and so on… As you can see, this is a multilayered and ever-engaging discipline, ripe with pitfalls and slippery slopes which can make one descend into pure obsessive-compulsive madness.
The Case of Iron
The beginnings of experimental iron smelting can be traced to the 1950’s. Since then it has intensively researched and developed. Nowadays, one can often find iron-smelting symposia or festivals being carried out across Europe, where craftsmen and scientists meet to smelt iron and share their knowledge.
Having said all that, what is the archaeological data on (pre)historic iron production?
Raw Materials – Surface Ores, Fuel & Earth
For most of (pre)history only those ore deposits were available, which could be collected without the need for deep mining. In large parts of Europe, these were limonite and bog-ores, which form easily accessible sediments, but have fallen out of use in modern times, because their iron content is too low for industrial production. In certain areas richer ores, such as magnetite and haematite, were also available in the form of ore veins accessible from the surface or iron-rich sand. The smelting and working of iron required a lot fuel, which came in the form of charcoal, traces of which can be preserved. Lastly the furnaces need to be built from a heat locally available heat resistant material such as sandy clay or stones.
Excavated Features – Remains of Furnaces & Ironworking Waste
What normally remains of a smelting operation in the archaeological record is the base of the smelting furnace, which was dug into the ground. This can be a shallow bowl or a deeper pit, which is charred from the heat. Unless the furnace was dug into a hill bank, then the upper parts (shaft) of the furnace will not be preserved, except in the form of a few vitrified clay-wall fragments. Luckily smelting produces a lot of waste in the form of slag, which is a glassy mess of silicates, iron oxides, bits of charcoal and ash. This forms a cake at the bottom of the furnace, which can sometimes be found left in the furnace remains, or in a refuse pit nearby. The type of slag can sometimes tell quite a lot about the type of furnace being used. The subsequent forging of the iron also produces waste in the form of hammer-scale, which are flakes of oxidation which and separate from red-hot iron as it is worked.
Tools – Tongs, Hammers & Tuyeres
The set of tools required for smelting is much smaller, than what a specialised forging operation would use. We do find some tools in the archaeological record, such as larger tongs and (sledge)hammers, which would be used to extract and hammer a bloom. Occasionally, an ‘anvil-stone’ might be found the vicinity of the furnace. It should be kept in mind that the bellows which would have provided the air blast used in forging and smelting, are more or less absent amongst the archaeological finds, since bags of leather have a lesser chance of being preserved. Along with the bellows, tuyeres or clay blow-pipes, which supply and direct the blast of air into the furnace, should be mentioned. These are sometimes found in the form of fragments, or sometimes fused with a part of the furnace wall.
Product – Iron Artefacts & Semi-Products
Lastly we have the semi-products such as pieces of raw and various types of iron ingots, as well as the finished artefacts, which provide a testament to the ironworker craft. These can be observed using a technique called metallography, where they are cut to reveal a cross-section, polished, etched and then observed under a microscope. This can how the artefact was assembled; the differences between iron and steel; traces of hardening; quality of the material; production defects; etc.
All of this archaeological data points to iron production using the bloomery process, where a hot furnace was filled with layers of charcoal and ore. In a bloomery furnace the iron is never completely molten, instead the iron oxides in the ore are reduced to metallic iron, while only the waste material (silicates) are melted out of the ore in the form of slag. The iron is retrieved from the furnace in the form of a bloom – a spongy mass of iron and slag. This then needs to be consolidated into a bar of iron by forging. This was the only method of iron production until the later middle ages.
What experimental archaeology explores are the practical details which make this process possible.
The shape of the furnace presents an interesting question. Since in almost all cases, only the base is preserved, we have no definite proof for height or width of whatever may lie above. Information on the height of tuyere is also scarce. This allows us to reconstruct a variety of furnace designs from low wide furnaces, to tall tapering shaft furnaces and everything in between. We can test these by trying to run a smelt in the reconstruction. If a furnace design turns out to be usable, it presents a plausible reconstruction.
The other enigma are the details of the actual smelting run. How hot was the furnace run? There seems to be a minimum and a maximum temperature at which a bloomery furnace can operate. What was the ore to charcoal ratio? Was the wall of the furnace punctured (tapped) during the smelt to allow the excess slag to run off in a stream, or was the smelt stopped once the basal pit was filled? A lot can be determined by carefully studying the slags, debris and iron blooms found, yet the hypotheses still need to be verified by experimental testing.
During all of this we have to beware of thinking to scientifically about ancient craft. There is a great danger and temptation in experimental archaeometallurgy to rely our technological equipment and understanding in trying to approach iron production. The ancient artisans did not have adjustable electric blowers, thermocouples, precision scales and the modern understanding of chemistry and physics at their disposal. To rely completely on these means that we further distance from understanding the past.
We have to ask ourselves how one can monitor the process of smelting using our senses only, since that was available to those whose work we are trying to study. For example: How to determine the quality of ore by appearance and judge the temperature of the furnace by observing radiant colours of the blaze inside the furnace.
Experiments with Limonite from southern Slovenia
In order to demonstrate the ideas which you were reading about so far, let me offer you a retelling of one of my experiences with trying to unravel the mysteries of the ancients.
In 2015 I received a considerable load of limonite from southern Slovenia (Lower Carniola). This type of ore is extremely abundant in that region, staining the clay-rich soil in hues of red. The region itself contained important settlements in the Early Iron age, which began in the 9th century BC. Therefore there is a very good bet that this ore was used very early on, but no one has done any smelting experiments with it in recent times.
I also agreed to hold a workshop on iron smelting at a Viking/Early Medieval living history event in June 2016. This meant that I had to brush up on my iron smelting prowess. Of course I used this as an excuse to give my pile of limonite a test run. Like all innocently simple plans, the whole project turned out to be a lot tougher that originally imagined.
Over the Spring I spent countless hours building furnaces and smelting iron on the weekends. At some point it became standard practice for me to spend Friday night alone, building a furnace, lighting a fire for it to dry until dawn, and finally spending all of Saturday smelting ore.
The furnaces which I built were shaft furnaces, about 80 cm tall, built of a mixture of clay, sand and designed with slag tapping in mind, as was customary in most early medieval Europe. I used a single set of bellows to do the smelting, which I pumped for trance-inducing lengths of time. All in all I ended up performing 10 smelting runs, the results of which ranged from heart-breaking disappointments to a few successes at the end.
I ended up learning a lot about the craft of iron smelting and how to think about it in archaeology. Here are some examples:
If the results are not satisfactory, then raising the temperature of the smelt is the first thing to try. A high temperature is needed for the slag to separate from the iron and a solid bloom to form.
It is hard to scale down an iron smelting experiment. At least 10 kg of ore was needed for a successful result, and the yield improved greatly once at least 15 kg of ore was used. Trying to do smaller (quicker) runs is usually a waste of time and resources.
The furnace can be reused and recycled many times. I have reused some of the furnaces several times, and when it was necessary I rebuilt the furnace in the same spot, using the same base at least 5 times. Only the innermost part of the furnace actually turns to ceramic, therefore when rebuilding a furnace, I often crushed the walls of the previous incarnation and added them to the new clay and sand mixture. This means that even if only a few furnaces are found at a site, a lot of iron smelting might have been going on.
‘Poorer’ ores such as this limonite tend to produce a lot of slag at appropriate smelting temperatures. This determines furnace design and practice. The furnace e needs either a large basal pit to catch all of this, or you should be prepared to keep tapping the slag regularly.
Just because the iron content of an ore is relatively low, that does not make it bad. People often think that the higher the iron content of the ore, the better The main question is the quality of the iron produced. Many high-yielding ores, such as haematites can be challenging to work with. You are looking for an iron bloom which consolidates into a bar nicely and is easy to work, and not the highest possible yield. Provided that there is an abundance of this ore and forests to provide the charcoal, then producing enough iron from a ‘poor’ ore is just a question of running the smelt longer, so that more ore can be added.
Craft – particularly metallurgy – is problem solving and troubleshooting. Experience is crucial to a successful smelt, having taken part in a successful smelting episode, as well as having seen a few failures and possible solutions, creates great leaps in your knowledge.
You can go and paint the ‘transformative magic’ of the smelt in poetic terms, describing the hues of the flames shooting from the top, the sparks disappearing into the starry night, the hours spent dancing around a clay chimney and feeding the fire in hope of something emerging at the end, but in the end you also become aware of the pragmatic reality of the task. Thus you come to the realisation that at its heart, the whole procedure is comprised of a lot of hard work, diligence and patience. As much as it might trigger our imagination, there is nothing inherently arcane about this dirty job. But this also means that it does not require some mystical inborn ability on the smelters part. Instead, it can be learned by anyone who is willing to invest some time, sweat and tears.
For more on how to do this, stay tuned for future posts.