Smelting iron: A first attempt

Last weekend I made iron from dirt. I used a bloomery smelter, a furnace design that dates back to the early centuries of the Iron Age, to transform hematite ore into metal that I can forge.

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The smelting furnace, just before we extracted the bloom.

Smelting is the process of turning naturally occurring iron oxides into useful metal. The earth is full of iron, but most of it isn’t naturally ready to be forged. Aside from the odd meteorite, natural iron on earth’s surface is an oxide–that is, some sort of rust. This rust isn’t useful until we do some chemistry to it. In basic terms, we have to strip away all the oxygen atoms in the ore, leaving only (useful) metallic iron behind. These chemical reactions are tricky science, but humans long ago mastered the process using some deceptively simple furnace technology. In the early middle ages, smiths made this chemistry happen inside a bloomery furnace: a short chimney packed with charcoal that transforms iron oxide ore to metal.

Modern craftspersons, living historians, reenactors, experimental archaeologists, and backyard hobbyists in North America, Europe, and Africa have spent the past several decades recreating these processes. Many have published their results, and researching smelting technology can take you down a long, exciting rabbit hole. If you’re interested in the academic work, Lee Sauder and Skip Williams have published a helpful starting bibliography. For practical advice on how to do it, a few modern smelters’ publications stand out: Lee Sauder and Skip Williams, Darrell Markewitz in Ontario, and Mark Green and many others who’ve posted years of hands-on experience on www.bladesmithsforum.com (an incredible community of craftspersons with deep archives–another good rabbit hole worth falling down into!).

Written how-tos are great, but smelting is a very hands-on process and the best teacher in my opinion is getting your hands dirty with an experienced team. Before trying it myself, I’d watched smelts twice. The first was a demo by Darrell Markewitz at the Kalamazoo International Congress on Medieval Studies in 2013. More recently, Mark Green let me join him for a smelt at his home (and Mark has given me invaluable behind-the-scenes advice and troubleshooting along the way–thank you, Mark!).

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Mark Green, extracting a bloom from his furnace when I visited in December

There are some great videos online as well–this one, of a team of West African smiths who decided to recreate the process their grandparents had followed (but which had, lately, almost died out) is one of my favorites.

This was my first try on my own. I learned a ton from the process, made one big mistake, but walked away with some beautiful new iron. Here’s what I did, what I learned, and the metal that I made.

The Ore

For my smelt, I used an ore formula developed by a team of Canadian smelters in the Dark Ages Recreation Company that Darrell Markewitz has named “DARC dirt bog ore analog.”

Many smelters dig ore in their back yards. Go to bladesmithsforum.com, and you’ll find detailed discussions of how to use old geological maps to identify ore seams, how to track down the land owners, and how to convince them to let you dig a barrow full of rocks to take home. Črtomir Harald Lorenčič posted a lovely blog entry last year about how to track down ore in peat bogs in Ireland. I’m jealous of them all. Down here in Florida I don’t have any easy source of natural ore nearby, because Florida’s land is just too young. Most of the earth’s iron was deposited almost 2 billion years ago, but Florida’s only about 500 million years old. I’ve heard there’s bog ore up around Tallahassee, and hopefully I’ll get to track some down one day, but for now I was out of luck with local options.

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Crushed magnetite ore

Rather than buy a box of mystery rocks on ebay (a risky gamble, as you don’t know whether the ore is good or bad until you smelt it, unless you take the time to drag it into a lab and do a chemical analysis), I opted to use a tested method, the “DARC Dirt.”

Darrell and the Dark Ages Recreation Company in Ontario developed this ore recipe while they were reconstructing the Viking smelting workshop at L’Anse aux Meadows. They, like me, lacked a reliable source of local ore, but their local pottery shops had bags of powdered hematite (an iron ore, Fe2O3) as a glaze ingredient (marketed as “Spanish Red”). Hematite ore smelts beautifully, but in a powdered form it’s hard to toss it into a furnace–the dust blows out the top with the smoke, carried away by the updraft! The DARC team solved this by mixing a bit of flour and water with the ore to form a paste that bound the powder together into what are basically great big rust biscuits.

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70 pounds of "DARC Dirt" ore (powdered hematite bound into solid chunks thanks to a bit of flour and water).

I followed their recipe, and it gave me 35kg of hard, crumbly hematite with an iron oxide content of about 65%. This is chemically comparable to some of the bog ores used in the early middle ages, so good enough to test my hand recreating the process. More importantly, it was a tested recipe: I knew that if something went wrong with the smelt, it wouldn’t be because I harvested a bad crop of ore.

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2lb of ore, crushed and ready to add to the furnace

The Charcoal

Charcoal is the magic ingredient that makes the chemistry inside a bloomery furnace work. Charcoal provides both the heat and the carbon needed to rip oxygen atoms off of the ore, transforming the rust into iron. Without charcoal, furnaces would just make hot ore. This is because, inside the furnace, the burning charcoal turns into carbon monoxide. Carbon monoxide (CO) molecules really want to be carbon dioxide (CO2) if they can, and they’ll grab the first free oxygen atom they can find (CO is poisonous to humans because, if we breath it, it’ll latch onto the oxygen in our blood and not let go). Inside a bloomery furnace, the CO gas latches onto the oxygen in the iron ore. As the ore settles slowly into the furnace, the carbon-rich “reducing” atmosphere slowly strips away all the ore’s oxygen in a series of reactions that, after about 40 minutes, will transform the ore into metal, CO2, and silicon slag. In a nutshell. The chemistry is a bit more complicated–another rabbit hole, for those interested in the science that happens inside the fire.

Any type of charcoal seems to work for this, so long as it’s lump charcoal (made from solid wood timber) and not briquettes (made of ground up wood and a bunch of other random chemical binders that can interfere with the chemistry of the ore). I used Cowboy charcoal from the hardware store.

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Lump charcoal--the most important ingredient

The charcoal need to be a consistent size, and relatively small. Too large, and the ore falls down through the gaps in the charcoal so quickly that the chemical reactions don’t have time to finish. Too small, and the furnace gets choked up on dust. To prepare it, I had to chop each bag of charcoal down to the right size. This took hours, and it was exhausting work.

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The charcoal comes out of the bag in all shapes and sizes. I need to chop this down to something more consistent, and much smaller.

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Several hours later, the charcoal is chopped to size

The Furnace Stack

Traditional furnaces were made from clay. They take a few days to dry and fire, which isn’t a problem if you’re smelting outside your medieval village or in your back garden. I was smelting at a friends house, however, and need to build a stack that would assemble and tear down in a weekend. So I used firebricks. If I’d had more time, I would have used a mix of clay, sand, and chopped-up straw (or horse manure).

First, I built a good base for the furnace to stand on. I didn’t want it to fall over (!), and I also wanted to insulate the furnace from the cold ground. Lee Sauder and Mark Green have shared some tricks for making this base, which I copied. I packed the center with charcoal fines (charcoal dust, so fine that it won’t catch fire), and this gave me an insulating floor between the furnace and the ground.

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Building a stable base for the furnace from firebricks

The furnace itself was 36″ tall, built from 4 concentric rings of firebricks. I lined the inside with clay to protect the bricks from the slag that forms as a byproduct of the melting ore (a good idea, it turns out! Some bricks were still damaged where the clay was too thin). And I filled in the outside seams with a mix of sand and clay, and finally tied it all together with several lengths of wire in case it shifted during firing (it did not, phwew!).

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The furnace, assembled and ready for the pre-heat

The Tuyere

The tuyere (air intake) is one of the most important parts of the furnace–and the bit of this process that I got wrong, it turns out. Fire needs oxygen to burn (and to provide the O that combines with the C in charcoal to make our reactive carbon monoxide gas). The tuyere is the pipe that delivers that oxygen to the center of the furnace.

We’re not sure what early medieval smelters used for their tuyeres. Most archaeologists assume they used clay. A few have suggested they might have used copper. The heat inside the furnace is hot enough to destroy both clay or copper if they’re not carefully designed (and iron melts even faster). I chose to use a ceramic kiln shelf support for mine, a clay tube designed to withstand very high temperatures. Several other experienced smelters have reported good results with this material, and I hoped it would hold up to the fire. As we will see below, it did not.

The Air Supply

To reach the temperatures we want, we have to force air into the tuyere. Bloomery furnaces need a constant stream of air for about 4 hours of operation. If the air cuts off, the temperature will drop and the molten slag and iron inside the furnace will freeze in place. Once frozen, a furnace is done.

Early medieval European smelters used bellows, and they must have gotten help to them. Pumping bellows is hard work. It can be social work as well–perhaps the farmers of the village all gathered round and pitched in. Or maybe they conscripted a few unfortunate slaves. I, working mostly by myself, used the modern cheat: a shop vac. This gave me a strong, steady supply of air. I used a router control dial to regulate how much electric current reached the motor, and this let me set air flow at precisely the volume I needed.

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This router controller made my life so much easier. It let me set precisely how much current went to the shop-vac, which allowed me to fine-tune the air volume going into the furnace.

I attached the shop vac to a pipe with a ball valve on one end. This valve let me gaze down into the furnace to check on the progress of the smelt, and it let me clean the tuyere if it started to get blocked by melting slag. It also gave me a way to reduce the air pressure even more if I needed to (I didn’t) by letting the extra escape.

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The air intake, connecting the shop-vac to the tuyere. The ball valve makes it easy to open the tube and peak inside, should that be necessary.

And with that, the furnace was ready to burn!

The Preheat

I built the stack on Friday night, and I spent all of Saturday morning chopping charcoal (ugh). Then I was ready to begin.

First, I built a small wood fire inside the stack to slowly warm up the insulating walls and dry the mortar that filled the cracks between the bricks.

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I slowly filled the furnace with wood, heating the bricks without wasting my carefully chopped charcoal

Once the fire got going, I turned on the air and it took off. I fed logs into the fire for about an hour, until all the bricks were warm to the touch and the clay was dry.

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Once I turned the air on, the furnace quickly heated up

Then I started adding charcoal, and the fun began for real.

Smelting the Ore

To turn ore to iron, you need a careful balance of heat, air, and time. Thankfully, other smelters have worked out a lot of these variables through hard trial and error, so I didn’t have to reinvent the wheel. Lee, Mark, Darrell, and others that I’ve linked above have found that furnaces of this size will burn about 4lb of charcoal, plus ore, every 8-12 minutes (or 3lb every 8 minutes, the ratio Mark suggested to me that I ended up following).

As the furnace burned, I timed how long it took to consume each bucket of charcoal. For the first few buckets, I added charcoal on its own, and adjusted the airflow until it was taking about 8 minutes to burn each charge. Once things seemed to be working right, I put in the first batch of ore.

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My wife Rachel joined me for a bit! We're adding charcoal and ore.

At first, we added 1lb of ore for every 3lb of charcoal. As the smelt progressed, I increased this amount to 2lb, and by the end was adding 3lb of ore for every 3lb of charcoal. Adding more (inflamable) ore to the fire lowers the temperature and slows the burn rate, so the larger batches of ore near the end helped keep the first burning at the right speed. Keeping the speed steady is important because, remember, it takes about 40 minutes for the ore to transform into charcoal. Too fast or too slow, and I risk producing iron-rich slag or useless super-high carbon cast iron (which can’t be forged, it’s too brittle).

I added charges of charcoal and ore every 8 minutes (or so) for about 4 hours.

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With the charcoal added, the forge began to produce the reduction atmosphere needed to turn the ore into iron. The blue, ghost-like flames at the furnace's mouth are what we want to see.

As the smelt continued, the furnace developed a few leaks at the seams. This was fine, and I patched them up with clay.

As time went on, I noticed that a lot of heat was breaking through the stack next to the tuyere, however. I knew that hot a spot by the tuyere was a problem–it meant that too much heat was concentrated near that wall, instead of being centered in the middle of the stack. In fact, it meant that my tuyere had started to melt, shifting the heat to the wrong part of the furnace. I didn’t realize this until the end, however, when I disassembled the furnace.

After the first hour, I could see a steady stream of molten slag and ore dripping down the inside of the furnace. I wasn’t able to photograph it, but you can still get an idea of what it was like to look into the furnace from this photo–a glowing portal into hell, bright enough to do some serious damage to your eyes if you should look in without protection (I used a pair of green-tinted shade 5 welding goggles to peak inside).

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A view inside the furnace, seen through my green-tinted shade 5 welding goggles

Slag Tapping

By the third hour, the furnace was starting to gurgle as slag rose to the level of the tuyere.

Slag (a glass-like mix of melted silicon–sand–from the ore with iron and other impurities) is a good thing in a bloomery furnace. Slag catches the iron particles that fall down the stack, cradles them, and helps them weld together into a solid lump of metal. But too much slag can block the airflow into the furnace, which is what was causing the gurgling sound. To fix this, you have to let the excess slag escape by knocking a small hole in the bottom of the stack to let some of the extra slag drain. This is called slag tapping.

I tried to tap the furnace at the base, but the slag was very thick inside, the consistency of cold molasses. It wouldn’t flow out. Ideally, it should have been runny, more like hot honey, and it should run or even squirt out of the furnace. This, like the hot spot by the tuyere, was also a problem. I wasn’t sure what it meant yet–looking back, I realize it was an indication that the hot zone had moved out of the center of the furnace and that the slag was too cold to behave properly.

I tapped again, this time higher up the stack right under the tuyere, and I got a great flow of runny slag. That part of the furnace, at least, was working as it should.

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A big, juicy slag tap. This once came near the end of the firing. By this point, the hot-spot by the tuyere had gotten very bad, and I gave up trying to patch it. This was a portent of what I would find when I opened the furnace.

Over the final hour of the smelt, runny slag was pooling right up under the tuyere and I had to tap it frequently to keep the airway from blocking. I made it through the last of my ore (barely!), though, and I wrapped up the burn by adding a few more buckets of charcoal to allow the last bit of iron to work its way down the stack and settle to the bottom.

We were ready to open the furnace and see if there was any iron inside!

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Before I opened the furnace I laid out my tools. The meter-long pair of tongs were fun to forge, and even more fun to use!

Where is the bloom?

I first opened the bottom of the furnace. If everything is perfect, you can pry the bloom out from below. Many smelters call this “birthing” the bloom. Done right, the bloom slides out from the furnace, leaving the stack intact for many more smelts. I had no such luck, however, so I removed more bricks to get at the hot lump of slag and (hopefully) metal in its center.

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The furnace, opened wide to extract the bloom

And then I found it! I hoped. There was a big mess of hot slag right where the bloom should be. If everything worked right, this slag should encase a hot meatloaf-shaped lump of spongy metal.

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Is this a bloom? (It's VERY hot!)

To get at the metal, we carried it over to a stump and began to hammer the material. The goal was to break off all that slag on the outside, and compress the iron inside into a nice solid cake of metal–the “bloom”.

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My hammer helpers and I striking the ?bloom to knock off slag and compress any iron it contains

But as we hammered, it became clear that we didn’t have any metal.

The whole lump was nothing but slag.

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Gently glowing lumps of slag litter the ground around the stump, but no iron to be found

I went back to the furnace again to check if I’d missed anything. The stack was still full of charcoal, and perhaps–

And I found it!

Not in the middle of the furnace where it was supposed to form, but off on the side, right under the tuyere in the rogue hot-spot that had been burning through the side of the furnace.

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THIS is iron!

I could feel the difference as soon as I grabbed it in my tongs, and my hammer helper felt the metal too as soon as she hit it.

Phwew!

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The bloom. Smaller than I'd intended, but good iron.

We made iron 🙂

Postmortem Analysis

In the end, instead of a big cake of iron, I made 4lb of fluffy iron fragments. I also made a bucket full of iron-rich slag that could have been bloom if the hot spot had stayed in the center of the furnace. But it hadn’t, because my tuyere had melted.

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Instead of a solid bloom, I got 4lb of fluffy iron (on the stump) and a bucket full of iron-heavy slag (scattered round on the grass)

I’d made my tuyere from a high temperature clay that I’d hoped would survive in the furnace. When I took the furnace apart, however, I could see that tuyere was gone. I lost more than 3″, the entire length that extended into the center of the furnace.

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A fresh tuyere (left) next to the fired tuyere (right). The furnace removed almost its length. This shifted the hot zone from the furnace's center to the sidewall, inhibiting the formation of a good bloom. Oops.

When the tuyere melted short, the hottest part of the fire moved. It was supposed to be in the center of the furnace. By the end, it had shifted off to the side, up against the furnace wall directly below the tuyere opening. One consequence of this was that the furnace’s wall near the tuyere melted and cracked. Remember that I had noticed this about an hour into the smelt, but I didn’t immediately realize what it meant.

A worse consequence was that the mix of slag and iron in the center of the furnace was too cool to allow the iron to weld together into a solid lump. This is why the center of the furnace was full of heavy, iron-rich slag instead of solid metal. The furnace had frozen, a few hundred degrees too cool to finish the process.

The only place solid metal formed was the side of the furnace, in the off-center hot zone beneath the shortened tuyere stump. The rest of the furnace was too cool, because the air couldn’t penetrate far enough into the stack.

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A diagram showing how the melted tuyere inhibited the formation of a solid bloom

So! Next time I need a better tuyere. Most North American smelters use copper, and this is what I’ll do next time. Copper has a low melting point, but it conducts heat quickly. A thick, long copper pipe that extends outside the furnace will cool itself enough to keep from melting. This ensures a steady airflow into the middle of the furnace, right where it belongs. It’s a cool trick, and may even be something early medieval smelters did to avoid the trickiness of melting clay pipes.

The good news is, I made metal. I can, first of all, re-smelt the metal-rich slag that I pulled out of the center of the furnace. This slag would have been solid iron if it had stayed hot enough to sinter together, and I can melt it into the furnace again to restart the process. A setback, but not a loss.

And the 4lb of iron I did make, while fluffier than an idea result, is forging into beautiful bars of solid metal.

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The first bar of iron that I forged from my fluffy bloom. It forged quickly into a solid bar--this hasn't even been folded to homogenize the metal, it's a clean bar straight from the furnace.

And that’s a wrap! I made iron, and learned a lot about the process. I also burned an embarrassing amount of charcoal. I’ll use a better tuyere next time, and hopefully I’ll manage to make myself a solid bloom on attempt #2.

[UPDATE: Read about the next step in processing this metal in my new post, “Forging a bloom into iron bars“.]

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