A Week of Iron Smelting at University College Dublin

A Week of Iron Smelting at University College Dublin

Last week, I finally took a little break from the keyboard-bashing and computer-staring, which forms the basis of my student life, by spending a week covered in dirt. As part of a postgraduate module on experimental archaeology at UCD’s School of Archaeology, we dedicated half the spring break to exploring the nature of iron production in early medieval Ireland.

Thus I spent 5 days assisting Brendan O’Neill – one of the school’s PhD students and the UCD Centre for Experimental Archaeology and Material Culture’s Resident Jack of All Trades. The aim of the whole week was to demonstrate the plethora of tasks, skills and entangled technologies which make it possible to transform rusty dirt into metal. With this in mind, Brendan and I set forth to provoke in our colleagues a critical interest in the mysteries of iron production and their connection to the archaeological record.

The issues which were trying to illustrate revolve around what is required to produce iron and how does this relate to archaeological evidence. On the one end this means the raw materials (ore, charcoal, clay, sand), features (furnaces, hearths, charcoal pits/mounds) and tools (bellows, hammers, tongs, etc.). On the other, it raises questions about what kind of trace this leaves in the archaeological record. What would archaeologists who happen to excavate a former iron production facility actually find, and in how many ways could they interpret it?

To tackle these issues we decided to dedicate ourselves to as many steps in he process as possible, from making our charcoal, to processing the bog ore, smelting it, trying to process the raw iron bloom and finally covering and abandoning the site, therefore transforming it into a continuing experiment which explores how the archaeological record is created.

I am also currently starting work on a postgraduate experimental project, which revolves around the smelting and processing of bloomery iron. Therefore I was able to use this little spring break project as a trial run, in order to see what challenges I might run into with Irish bog ore and the equipment which I have available.

Let me present to you a quick overview of the five days of experimental activities which we performed between the 20th and the 24th of March. During this time we posted regular updates on our activities in the ‘UCD Experimental Archaeology’ Facebook group. The constant barrage of progress photos raised quite a bit of interest and led to a considerable number of questions being raised in the comments. In attempt to address some of the issues, I will quickly summarise the processes of each and the rationale behind them.


Preliminary Steps – Leather & Wood

The whole process started a good week or two earlier since we first had to make a pair of bellows which would be suitable to the task of supplying a blast of air to furnace for hours on end. This needed to be relatively to use since there is nothing like struggling with semi-functional bellows for 5-7 hours, especially when most of the participants are new to the job.

The archaeological data on bellows is quite scarce, since they are made of organic materials (wood & leather), which degrade quickly and were discarded/recycled once they were past their use-by date.

The chosen design was an arrangement of two smaller bellows, which used in an alternating rythm, providing a constant blast of air. This set up can sometimes be seen on Viking age and (early) medieval art, such as the well known example on the Hyllestad stave church in Norway. The wooden portal of the church is covered in a complex assemblage of knot-work and pictures, which tell the story of Sigurd the Dragonslayer, a hero from the Völsunga Saga and Nibelungenlied. One of the scenes shows Sigurd and Regin in the smithy, forging the legendary sword Gram.

sigurd portal hyllestad
Sigurd and Regin forging the sword Gram, as depicted on the portal of the Hyllestad stave church. The setup of bellows, forge and anvil can be made out. (Source: KHM UiO)

This style of bellows functions like your standard fireplace bellows, with a lower and upper plate connected with leather. When the plates are pushed together, they force a blast of air out through the nozzle on the front. The version which I crafted with the most welcome help of my colleagues was 70 cm long including the nozzle part, 32 cm wide and opened to the height of roughly 35 cm.

bellows design
It all starts with a sketch. After that, it is a lot of measuring, cutting, chopping and hammering.

The basic materials were boards which we cut to shape, while the nozzle and leg were carved out of ash. The air intake was provided in the form of two holes on the top plate, with leather flap acting as a valve. This was nailed half way around the circle, allowing the other half to open and close in use. Everything was sealed of with a large piece of cowhide which was tacked on after a respectable amount of time spent hammering. The hinge between the upper plate and the front part was a piece of heavy leather, which was stitched on to the leather sides of the bellows to provide a seal at what is usually the more puzzling of the movable parts of this device.

The two finished bellows were finally connected with a hollow Y-fork made of wood, which worked reasonably well, and were pegged to the ground with the help of the leg to keep them from jumping around.

bellows construction
The wooden plates and nozzle cut out, ready to be glued before the leather is tacked on. Sadly I was forced to use plywood for the boards, due to the last-minute nature of the project. Proper wooden planks are in all aspects superior, so try to do better than me.
bellows construction
A bellows leg in the process of being carved out of an ash stave
bellows construction
The leather for the bellows roughly tailored and cut. In retrospect, I should have made this bit wider.
bellows construction
Two holes air intakes were drilled and a piece of thick leather tacked on as a valve. Adding a third air intake might improve the efficiency of the bellows.
bellows construction
The sides are stitched onto the leather hinge just before the last tacks are hammered home.
bellows construction
The bellows in various stages of assembly. At one point this almost looked like a production line.
The pair of bellows connected in a slightly makeshift manner. The legs protruding from the bellows allow them to be pegged to the ground, so that they do not jump all over the place.

They worked reasonably well, especially when used to power the forge. When used with the bloomery furnace they left us wanting a bit more air and pressure. I will improve this by stitching a bit of leather to the sides, increasing the height to which the bellows can open by another 15-20 cm. I will also add a 3rd air inlet on the top plate, because it seemed that the bellows were a bit slow to fill with air when opened. The reason why I want to add another air intake instead of increasing how much the existing valves can open is because smaller valves seem to close quicker at the top of the stroke, thus reducing the amount of air loss before the valves seal completely.

Moral of the story: I have yet build a pair of bellows which would perform quite as well as I would want it to on the first try.

Day 1 – Setting the Stage & Raw Materials

Monday was dedicated to clearing the chosen work area. We took the sod off a 5 X 5 m square of ground in the Experimental Centre. Then we focused our efforts towards preparing the ore and making the charcoal.

experimental archaeology charcoal
A landscape of industry  unveils itself as the preparations come in full swing. For 5 days, all our activities revolved around this 5 m square in the Experimental Centre.

We used bog ore from Co. Offaly. As the name implies, it is found in bogs, where it lies just beneath the surface in the form of rusty lumps. It was the ore most commonly used in early medieval Ireland. It is a hydrated iron oxide (FeO.H2O), which can vary greatly in its iron content, and reduces easily in a bloomery furnace. It can v Before smelting it needs to be dried well, so that it may be easily crushed to a uniform size. In order to do this lit a wood fire and gradually laid the lumps of ore on top, seeing them slowly start glowing in the blaze. We left this pile of roasted ore and glowing embers to cool overnight.

bog iron ore
Bog iron ore as dug from the peat fields of The Midlands. This may look like dirt, but it used to be the main source of iron across a large area of Europe.
The lumps of ore laid on the pyre.
bog iron ore
A pile of very hot and well-roasted ore. The ground around the fireplace has turned red from the heat of the pyre.

Iron smelting and the processing of raw iron requires a lot of fuel in the form of charcoal. Therefore we gave making our own charcoal a try in order to get an appreciation for the process.

There are two ways of making charcoal, the first is an above-ground mound and the other is a dug pit. Both make charcoal production possible by letting the fuel to catch on fire and then sealing of the supply of air with earth, allowing it to carbonise without burning down to ash. Charcoal pits first appear in Ireland during the early medieval period and before that we presume that all charcoal needed for metalworking was produced in mounds, which leave very little archaeological trace. We decided to fully engage with the materiality and try our hand at making both types.

For the mound we used dry hazel rods which we had left over from our afternoons spent building a wattle Viking house. Hazel was second most common wood used for charcoal in this time period in Ireland. It is surpassed only by oak and followed by a range of other hardwoods. The rods were cut to a uniform length stacked around a central pole with tinder in the form of birch bark, dry leaves and dry grass. The stack of wood was covered to the top in stiff mud. Afterwards, we left everything to settle overnight in the hopes reducing the chances of everything collapsing in a manner most catastrophic once it was lit.

A neat stack of hazel rods and kindling is slowly being covered in mud. The hazel rods lean against the central rod which is loosely secured in place by the 3 supports.
The charcoal mound is now fully covered in muck and being given a few finishing touches.
charcoal production
The finished mound, basking in the late afternoon sunshine.

At the same time another group was working on the charcoal pit. For this we dug rectangular pit, about 50 cm deep. When it comes to chosen fuel we decided to go a bit unconventional, by trying to make charcoal from peat. We have mentions of peat charcoal being made in North Dublin in the 17th century for use in foundries, where it was given much praise. We also find traces of what seems to be peat charcoal in the archaeological record. For example, at the monastic site Reask, Co. Kerry. Lastly, peat charcoal was used in Scotland both in prehistory and up to the early medieval period. Since Ireland contains an almost preposterous number of bogs, it made sense to give this a try.

A fire was lit in the pit and once that was going well, pieces of dry peat were added on top. Once these started burning, the rest of the pit of filled with soil to seal of the supply of air.

charcoal production
The dug pit for charcoal production, ready for a fire to be lit within it.
charcoal production
As the fire is lit and the lumps of peat lay waiting nearby, the novice charcoal makers find a moment gaze upon their work.
charcoal production
The peat has been laid on fire and is beginning to burn. Soon it will need to be covered, lest it be reduced to ash.
The pit is now filled back with a thick blanked of soil, while underneath, charcoal is made.

Day 2 – An Ode to Muck

On Tuesday it was time to build the furnaces in preparation for the smelt on Wednesday.

But first the now well-dried and roasted ore was removed from the fireplace and pounded to red sand in a bucket.

Also the charcoal mound was lit. First the central pole was pulled out, leaving an opening into the heart of the hazel stack. Several air inlets were also formed around the base. Then some embers were dropped into the opening at the top, allowing the stack to start burning. Once this was going, it was sealed off again in order to make the wood turn into charcoal instead of ash.

charcoal production
Brendan lights the charcoal mound the help of a few burning coals and a bit of panache. Note the air intake spread around the base of the mound.

Although traces of ironworking and iron production are very common in early medieval Ireland, especially in the form of waste materials known as ‘slag’, little remains of the smelting furnaces used at the time. In the vast majority of cases, all that we are left with is the basal bowl or pit which was dug into the ground. The chimney-like superstructure or shaft erodes away quite quickly. The best preserved case of a furnace shaft from this time comes from a site called Grange 2 in Co. Meath. There, 45 cm of a furnace shaft were preserved, although it is hard to say how much further the shaft on the original furnace might have extended.

Grange 2 shaft furnace
The parts of the furnace shaft are only rarely preserved. This example from Co. Meath is the best preserved early medieval shaft furnace in Ireland. (Source: Kelly et al. (2011) Final Report: Archaeological Excavations at Grange 2, M3 Clonee North of Kells motorway scheme.)

Following the age old adage which claims that ‘Fortune Favours the Bold’, we decided to build two slightly different furnaces, in the hopes that we might test them side-to-side. These sported the 2 types furnace bottom found at this time. The first is a shallow, rounded, bow-like depression and is by far the most common type. The other is a deeper, more rectangular cut, not unlike a bucket in its cross-section. Both furnaces were c. 35 cm wide at the base (internal diameter) and lined with clay. Both furnaces were identical above ground, with a c. 80 cm tall shaft which tapered down to roughly 16 cm at the top. The blowhole was in each positioned at roughly ground level.

shaft furnace design
A schematic representation of the two furnaces which we built. As it can be seen, the only difference is in the pit beneath the shaft.

The furnaces were constructed of a mixture of Wicklow clay with plenty of sand added, in order to improve heat resistance and reduce cracking. No organic temper, such as straw, was used since there is no evidence for this in the Irish material. As usual with this job, mixing the clay and sand was the toughest and most laborious task of the day. Luckily there were plenty of hands (and feet) around to help with the kneading, and once the clay was gritty to a satisfactory degree, the furnace was erected surprisingly quickly.

furnace construction mixing clay
Mixing crushed clay, a lot of sand and a bit of water. The act of building a furnace gets very hands-on quite quickly. (Photo: Aidan O’Sullivan)
bloomery furnace construction
The pit is lined with clay and the shaft is slowly built up out of lumps and coils of clay. To the left, the other pit awaits its coating of sandy clay.
bloomery furnace construction
The furnace with the bowl-shaped bottom is finished, while the slag-pit furnace still needs a shaft built. Shortly after the photo was taken, a blow-hole was cut into the wet furnace wall at ground level.

Day 3 – The Endurance Sport of Smelting

Wednesday was the big day of the smelt. It was also the day with some of the most miserable weather, with constant wind, rain, and the kind of damp cold that slowly seeps in your bones over the course of the day.

We assembled on that bleak morning, and despite certain notions about scrapping the daily agenda and relocating to a warm pub instead, we decided to finish the job which we set for ourselves. But to make it all a bit easier on us, and because we were still fiddling with the final set up of the bellows, we decided to run the smelt only in the furnace with the shallow bowl.

A tent was erected over most of the working area, covering the bellows and their operators, but leaving the furnaces exposed, lest we burn a hole in our shiny new cover. A fire was lit in the furnace so that it could start drying and preheating, while we started chopping the charcoal to evenly sized bits and sifting out the fines in order to ensure a steady burn. Since the charcoal from our mound and pit was soaked in the rains overnight, we made use of the bags of bought hardwood charcoal, which we had hidden away in the shed. (Always have a backup plan ready.)

While all of this was going on, we had a visit from a group of German archaeology students, on a visit to sites from early medieval Ireland, led by Joachim Henning. Thus they stood there shuffling around in the miserable weather and looking upon us in dismay, wondering what sort of madmen perform a smelt in such conditions.

Nevertheless, we told them the basic story about iron production in a bloomery furnace. Namely, that the reduction of ore, also known as the bloomery process, is a reaction in which the ore (iron oxide – essentially rust), reacts at high heat with the excess carbon in the furnace (carbon monoxide), leaving us with iron and carbon dioxide. Essentially, this is a process where rust is reduced to iron, by putting it into a hot furnace full of charcoal. In the process of reduction, the iron never becomes truly liquid, instead the unwanted (‘stone’) part of the ore is molten out of the reduced ore at temperatures exceeding 1100ºC. The molten waste forms a glassy, bubbly substance known as ‘slag’.

In the early medieval period in Ireland, the slag was not removed from the furnace during the smelt by tapping, instead the smelt was run until the bottom of the furnace filled up to the level of the blowhole. The iron is taken out of the furnace in the form of an unconsolidated mass of iron and slag, also known as a ‘bloom’. It is a black, spongy, lump of iron, full of holes and with a few pieces of charcoal sticking out. This then needs to be hammered into a bar, before it may be used for anything except as a dirty doorstop.

bloomery iron smelting
Brendan O’Neill explaining the mysteries of iron production while the furnace is drying.

Once the furnace was hot and filled to the top with charcoal, with violet flames gushing out of the top, the first charge of ore was added. We were charging charcoal and ore at 1:1 ratio by weight. Because we had a ladle laying around, which when completely filled held 250g of charcoal, this soon became the standard charge. Thus over the course of the next 6 or 7 hours we got through 10 kg of ore and plenty of small fluffy pieces of bloom from previous smelts. The slag which was accumulating at the blowhole needed to be pushed back several times with a stick, in order to keep everything from clogging up. A decision was made at the end of the day by my colleagues, to finish a bit quicker and not to open the hot furnace. The hot furnace was instead filled up more charcoal and the openings were sealed with clay. Thus the fruits of our labour had to wait for the next morning, before we would gaze upon them and pass judgement.

bloomery iron smelting
Once the furnace is sufficiently hot, the first charges of ore can be added. You can see that the furnace is almost completely dry at this point, with a few damp spots around the base. At this point, the furnace needs constant feeding for hours on end. In the background, my colleagues are chopping charcoal, lest the furnace go hungry.
bloomery iron smelting
You can catch a glimpse of the intense blaze in furnace through the blow-hole. It is important that a very hig, yellow to white, heat is maintained during the smelt.
bloomery iron smelting
The struggle continues. Along with the ore, we also added small bits of dirty, unconsolidated iron (gromps) from previous smelts. This is a good way of recycling any bits of iron which are not worth consolidating.
bloomery iron smelting
Despite the less than ideal weather, everyone persevered through the struggles. This was one tough and dedicated bunch.

Day 4 – Smiting the Results

We arrived on Thursday with warmth restored to our bodies by supper and a night’s sleep. As if to mock our past efforts, nature presented us with a sunny day again, leaving only puddles and muck as a reminder of the toils of yesterday.

As befits Thor’s Day, the planned activities were meant to revolve around swinging a hammer at forge in an attempt to tame and refine our bloom.

But this meant a blacksmith’s forge needed to be set up. Therefore I quickly made a simple ‘industrial hearth’ or forge, as it is commonly known outside of archaeological literature. I dug a shallow depression in the ground, lined it and built the wall/bellows protector of a clay and sand mixture similar to the one used to build the smelting furnace.

building an early medieval forge
A quick and simple blacksmith forge can be made from sandy clay and used immediately. (Photo: Juan Maddrell)

We have archaeological finds of shallow pits, similar to the bottom of the furnace, which are interpreted as ‘industrial hearths’ based on the types of slag present and the vicinity of an anvil-stone (more on those in a moment). We also have finds of clay discs with a hole in the centre, which show signs of intense heat on one side, these could have been used forge-shields or bellows protectors which focus the air-blast and allow the charcoal to be piled up against it, creating a deep hot-spot required for this type of work. We also have Viking finds of similar bellows protectors made of stone, which would have been more durable and also possibly used as part of a portable forge. Some kind of bellows protector can also be seen at Regin’s forge on the Hyllestad stave church portal (see above).

viking forge bellows protector
A decorated stone viking age bellows protector. Held on display at the Museum of Cultural History in Oslo.

But the hearth is only one part of the equation. We also needed a sizeable anvil to process the bloom. Therefore we set a wooden stump in the ground and set on it a squared-off granite block. Iron anvils were for a very long time quite small, since producing a block of iron large enough was very hard and very expensive to produce. There are some larger anvils known from Roman times due to the very industrialised nature of metalworking in the Roman Empire. But anvils in early medieval Europe were generally very small, often no larger than a fist. They only began to grow as the scale of iron production began to grow in the later middle ages.

So, what does one do when one needs a larger anvil, but cannot make it from iron and steel, like the anvils that we are used to today? The natural solution was to use a large stone boulder as an anvil. We find such anvil-stones on archaeological sites; they are mentioned in texts, such as the description of Skallagrim’s anvil in Egil’s Saga; and we have ethnographic examples of stone anvils still being used in certain parts of Africa.

early medieval stone anvil at Clogher, Co. Tyrone
A possible stone anvil, found in the early medieval metalworking area at Clogher, Co. Tyrone. (Source: Scott (1990) Early Irish Ironworking)
irish early medieval forge
An artists reconstruction of the forge area at Lowpark, Co. Mayo. (Source: Wallace & Anguilano (2010) Iron smelting and smithing: new evidence emeging on Irish road schemes)

Thus we had our little setup of the forge being blown with the same bellows which we used for the smelt, with a stone anvil next to it. Now we just needed to put it to good use. The stone anvil’s properties did raise quite a few questions from the commenters on social media. The short answer based on my experience is that if stone anvils were that great, we would not make the switch to steel as soon as it was possible. Unlike a good metal anvil with a hardened surface, a stone anvil has little in the way of rebound, making you work harder to move the hot metal. I also quickly found out that if you miss a blow, as you are bound to do when you haven’t been forging for months, there is a high chance of chipping away a part of the surface. Nevertheless, stone anvils do work well enough to get the job done and have proven to be a useful asset for millennia.

early medieval or viking forge
The forge is quickly puts to its first use, since we needed some kind of pointed rod to extract the bloom. (Photo: Juan Maddrell)
forging on stone anvil
Forging the poker on the stone. (Photo: Juan Maddrell)

 

After all of these preparations we were finally able to open the furnace, where we were surprised to find that bloom was actually mostly separated from the slag block beneath it. This made the removal of the cold bloom a bit easier than anticipated. Once we removed the slag cake as well, we realised that some of the dry grass which we used to initially light furnace survived the smelt. This is the sort of thing which an experimental archaeologist loves to find, since it gets you thinking about various excavated features and how they get interpreted based on what is found in them.

bloomery iron smelting
The furnace wall is broken through so that the raw bloom may be extracted.

 

unconsolidated iron bloom
The extracted unconsolidated iron bloom.
bloomery furnace remains
Preserved grass found on the bottom of the furnace. (Photo: Juan Maddrell)

The amorphous bloom was reheated in our little forge and my colleagues took turns at trying to hammer it into something approximating a standard of common decency. Despite the fun that everyone had trying to squish fluffy metal, it could quickly be seen that the smelt was not as successful as initially thought. The bloom falling apart when heated and crumbling under the hammer, indicating that it lacked a bit of heat at a crucial moment during the smelt. In the end, the best we could produce before we labelled the exercise futile, were a few slightly crumbly plates of coarse iron. The smelt failed, but there was still plenty work to be done on the site in the morning.

bloomery iron processing
Processing the crumbly bloom.
bloomery iron forged plates
It quickly became clear that this was not the most successful smelt. At best me could make a few plates, which when refined would maybe yield enough material for a nail or two. Better luck next time.
used forge
The forge after an afternoon of bloomsmithing. The wall of the bellows protector has partially vitrified due to the heat. In the initial processing a lot of slag drips out of the hot bloom and gathers beneath the blowhole, where it forms a dense cake. This cake eventually needs to be removed. You can see the slag cake fragments in front of the forge.

Day 5 – A Site is Made

Our experimental week was focused on more than just making a bit of iron. We were also interested in how the remains of ironworking may be recognised on archaeological sites. Therefore one of the aims was to reproduce a site which will be excavated by another group of archaeologists at a later data. This will show two things.

1. How do the various features which we made and used get preserved and what gets eroded?

2. How will somebody who does not know exactly what happened here interpret the finds?

In order to make this possible we documented how we constructed each feature. Once we were done with the experiments we took away the tools, produced charcoal and most of the iron, but we left the ashes and waste in the features. We also had a pit on the site where disposed of any waste which was in the way during the experiments. We took detailed notes of what each pit filled back with and the site was meticulously photographed so that a 3D model was produced.

experimental archaeometallurgy in progress
Documenting the stages of furnace construction. This was done with every feature on site.

Lastly we covered everything everything with the turf sods which we cut out on the first day. These will grow together and the site will lay dormant under green grass, awaiting the day of the excavation. But, that will be someone else’s job. So we vacated the area as well, moving ourselves to the sort of establishment which pours beer in pints and surrenders them to visitors. There we were able to look back on a week of working together.

experimental archaeology simulated site
Our working space is covered in grass again, ready to begin the gradual transformation into an archaeological site.

Final Reflections

Once I had the chance to get some rest and look back upon this week dirty work done in good company, what can I say about are achievements and misfortunes?

There is no avoiding the fact that we did not succeed in producing any usable amount of iron. But a person of sharp wit should be capable of learning from their failures. Therefore this is not the end of the world and our time was far from wasted.

From the point of view of experimental archaeology and learning, everybody present was able to learn a lot about the various processes and the amount of work which goes into producing a bit of iron.

From the point of view of my craft and my thesis in progress, I was able to use this as a little test run, which told me what to improve next time.

It would seem that despite the fact that we reached quite high temperatures in the furnace, the hot spot was not big enough, being confined to merely a small area just around the blowhole. This can be solved simply by increasing the pressure and amount of air being forced into the furnace. This would raise the temperature across the lower part of the furnace, increasing the amount of slag which separates from the ore and helping in fuse together into a more solid bloom.

As I already mentioned, this will be achieved by increasing the amount of air which the bellows can hold. Also the amount of elements in between the bellows and the forge, such as various pieces of leather, wooden tubing, etc., will be reduced. This should improve the air blast to the point where it makes the whole task of bloomery smelting just a bit easier.

Lastly, it is always recommended to get the bloom as hot as possible at the end of the smelt, open the furnace and immediately hammer the bloom so that it can be partially consolidated before it needs reheating. This often reduces the problem of the bloom crumbling in the forge.

As it is proper, I would like to express my gratitude to everybody who participated in this project and thus made it happen. Now I just have to make sure to run a smelt properly in the near future.

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