An Introduction to Iron Smelting – Part V: Building the Furnace

An Introduction to Iron Smelting – Part V: Building the Furnace

Your tools are gathered, your clay mixed, your ore processed, and you have chosen a furnace design to follow. Now you are ready to give building the furnace a try.

How well you do this will have an influence on how smooth the whole smelting operation runs. After all, you do not want to be standing in front of a leaning shaft, with ever larger cracks spreading across the furnace, wondering if the thing will make it to the end. Especially not on your first smelt.

Therefore I recommend that you take your time and do not rush this on your first few tries. Later you can figure out just how much sloppiness you can get away with. Yet for now, you should expect to dedicate at least several hours to this step in the smelting process.

In this post I will present you with a few basic steps and little tricks, which will guide you through the process of building a furnace with success. This will cover everything from preparing the area, to forming the furnace, doors and tuyeres, and finally drying them in preparation for the smelt.


This post is part of an instructional series on bloomery iron smelting. The aim of the series is to to present how iron used to be made in ancient times, while also teaching you how smelt your own iron and steel.

Therefore, should you have just joined in on the course, or if you would like a quick refresher, then I urge you to read through the previous installments:

Part I: Theory

Part II: Tools of the Trade

Part III: Raw Materials

Part IV: Furnace Design


I. Preparing the Area

building archaeological forges and furnaces

The building of the furnace will begin with preparing the spot on which the furnace is meant to stand.

The area should be cleared of any obstructions, possible brush or branches which might get in the way or catch on fire at some point.

Therefore…

If there is any tall grass where the furnace is meant to stand, you should probably give it a quick trim. (Yes, you can use the grass as organic temper.) You might even need to give the terrain a quick working over with a mattock to even things out, or raise a specific spot.

Give the area one last check, making sure that there is nothing flammable hanging over the future furnace.

Pick a spot where you will not be stumbling over large rocks or holes in the ground. Keep also in mind that you need space for the bellows, the person operating the bellows, all the equipment charcoal and ore. You also need to be able to access the furnace to feed the fire and solve any problems, should they arrive.

Keep in mind how you want to fix and operate the bellows. Beyond having enough space for the bellows, I like to check if the terrain allows me to hammer in a peg or two, so that I may keep the bellows from creeping forward during use.

Looking for more advice on how to select an appropriate general area for smelting? Take a look at the 2nd part of this series: Tools of the Trade.

Picking the ideal spot for the furnace will also depend on the type of furnace that you want to use:

Slag-pit furnaces can be put up on any type of terrain where you can dig a shallow hole.

Slag-tapping shaft furnaces, on the other hand, are built above ground. Therefore the need for digging is dropped. At the same time, a slag-tapping furnace is best built on slightly elevated ground. How come?

It is easier to access the furnace with the poker and tap the slag when you approach it from slightly lower ground. Slag also starts solidifying once it drips bellows the hot-spot. Therefore in order to tap out the liquid slag, you will often want to hammer the iron rod through some of the semi-solid slag and into the liquid part above it. This is most easily done from a position bellow the furnace.

When building an embanked furnace, you will need to take a pick or mattock and cut out a vertical shaft into your selected terrace. If you plan on tapping slag out of this sort of furnace, it is again best if the base of the furnace lies slightly higher than where you plan on placing the bellows.

Regardless of what furnace type you have chosen, this will be the moment when you define the inner diameter of the furnace base. You shall do this by smearing the bottom of the furnace with some of your metallurgical clay mixture. This will also define the border between the bottom of the furnace and the soil beneath it.

Once you have the spot selected and the base established, it is time to build the shaft.

bloomery iron smelting building the furnace
Clearing the area and cutting out some grass will get the job done in most cases.

 

bloomery iron smelting guide furnace construction
A clay-lined slag pit and a finished furnace. By covering the bottom in clay, the base diameter of the furnace is established.

II. Building the Shaft

bloomery iron smelting furnace construction

The main challenge of building a bloomery furnace begins once you start building upwards. After all, this is where your work has to start defying gravity.

How you proceed with building the above-ground portion of the furnace will depend on your chosen type of furnace.

Free-Standing Shaft

This is the most commonly seen, and the most basic, type of shaft furnace. Other shaft types will also involve building a clay tube in the last part of their construction. Therefore it is best that you acquaint yourself with necessary methods and their challenges right away.

First of all…

Do not be misled by the seeming simplicity of building a clay shaft. Doing this really well might actually the more challenging of the construction methods. So if you can pull off this one right, then the rest should not give you trouble.

There are two ways of building a standing clay shaft: with or without an internal support.

The biggest challenge lies in convincing the clay to maintaining its shape as you build upwards, thus keeping if from slumping, collapsing, or toppling over.

At the same time, you want to keep the inside walls even, without overly large bumps or sudden changes in diameter which could cause the charcoal to get stuck during the smelt.

In order to keep things upright and define the internal shape of the furnace, many smelters choose to build the furnace around an internal support, which later gets removed or burned away. This usually takes the shape of a bundle sticks, reeds or straw.

An example of using internal supports while constructing the furnace is given by Lee Sauder.

Some then also decide to battle the clay’s tendency to start sagging and slumping down under its own weight by tying a piece of rope around the shaft at regular intervals.

Nevertheles…

While these things may help, they are not necessary. I have always built my furnaces without any support, and I have yet to see a shaft just topple over during or after constructions.

The trick lies in getting the clay mixture right and building as evenly as possible. If the building material is stiff enough, and the weight of the shaft is evenly distributed during construction, then there is no reason why it should not stay in place.

In other words: There is a bit of skill and experience involved in the endeavour of building a furnace. (This is a craft after all.)

With that in mind…

Start of by forming lumps of well kneaded clay mixture. You may use these as balls, potato-shaped oddities, or sausages of varying lengths. The latter method is especially useful if you are using longer strands of organic temper (hay or straw).

If you unsure about what kind of clay mixture to use, then stop right now. Go and (re-)read the 3rd part of this series, which covers everything about the raw materials and their preparation.

bloomery iron smelting furnace construction
Big or Small, Short or Long – Be prepared to make A LOT of these. Hopefully you were able to find an assistant or two. (Photo: K. Lokajickova)

Start by defining the walls at the base, making sure that the material fuses well with the bottom of the furnace and the ground. This will be the both the widest part of the furnace and the part where the walls are thickest.

Since you want to clay mixture to be as stiff as possible, you might find that the lumps do not want to fuse together easily. This is not a problem. Dip them in water just before you add them to the structure. This makes the surface softer (stickier), while keeping the body of the lump dry.

Once you have the first row done, just add the next one on top.

Push each lump into the row bellow it so that they fuse. This will compress and deform things slightly, making the wall somewhat lower and thicker. Correct this by moulding the wall with your hands, pushing the clay together and upwards. You will get a feeling for it quickly.

bloomery iron smelting furnace constrution
Furnaces are built in horizontal rows of clay lumps. In this case the decision was made to cut the openings after the shaft was finished.

You should also decide whether and how you want to make space for a furnace door, since building over a large hole in the wall is a recipe for disaster.

There are again two ways of making the door arch:

Option A: Build the whole shaft and cut out the opening at the end. By the time you reach the rim, the clay mixture will have stiffened up enough so that the doorway should not collapse.

bloomery iron smelting guide furnace construction
The furnace on the right is finished, except for cutting out an opening for the door, or just a small hole for the air intake.

While this works, it requires a bit more clay while you build.

Therefore, there is also…

Option B: Build the arch over a support. Inserting a log, or a stack of firewood, where you later want the door. Once the shaft is finished, simply pull out the support and smooth out the opening.

bloomery iron smelting furnace construction
Stuffing some firewood into the door opening ,until you are finished with the shaft, gets the job done well enough. You will find a use for the firewood, when the time comes to dry the furnace.

Beyond this, it is just a matter of adding row upon row clay lumps until you reach the desired height of the shaft.

You should take care to either keep the walls parallel, or have the diameter gradually decrease as the shaft rises.

The walls can also get thinner as you move upwards, since they need to carry less and less weight.

Do try keep the shaft as circular and symmetrical as possible, since this helps distribute the forces evenly, thus forming a stronger and straighter furnace.

TIP: Make sure not to stay in one spot while building the furnace. There is a high chance that this way you will end up with lop-sided shaft. The trick to building symmetrically is in moving around and observing the furnace from various angles. This way you can correct any irregularities as the emerge, rather of trying to do a lot of fixing right at the end.

But what do you do when things really go wrong?

Sometimes, despite your best attempts at the keeping the muck in its rightful place, the stuff just refuses the stand upright – sagging and leaning all over the place. You most probably mixed the clay a bit too wet, or built something a bit off-centre.

Despair not! Cursing in front your dirt idol will not save the project.

Take a break from the build and wait for the shaft to dry enough in to firm up. Half an hour on a breezy day might already stiffen things up enough so that you push onward. Use the time to think about what went wrong and possibly mix more clay. In the worst case, you can always continue on the next day.

Once you have finally finished the shaft, it is time to remove the door arch supports. If you were building without an internal support, then at the end you can reach into the furnace and give everything one more working over – making sure that things are nice and even.

Should you want to smoothen the outside of the furnace, then dip your hands in water and rub out irregularities.

That is it. The furnace is now ready for drying.

bloomery iron smelting furnace construction
Once you have pulled out the door supports, you can give the arch a few finishing touches. Give the furnace one last check, making sure that you are pleased with it, before you start drying it.

Shaft With A Stone Base

This type of furnace is nothing but a stone box with a clay shaft on top. Therefore you already know how to build the upper part.

Now let me walk you through the process of erecting the base.

You will be using fire-resistant stones and your clay mixture to build the base. Repeating from last time: granite is great, soapstone amazing, sandstone shall do as well. Anything with layered structure, such as slate, might be prone coming apart at high heat. Also try to stay away from limestone.

To mix the mortar, you should not add organic temper to you clay mixture. You can make it wetter than usual as well. This shall make filling the gaps between the stones easier.

You will start by laying the stones so that they start forming the 3 walls of the lower part of the furnace.

Ideally, these should fit together as neatly and tightly as possible, thus using the minimal amount of mortar. For the best results, use the clay mortar as something that helps seal the gaps between the stones, and not as a glue which holds them together. After all, you are using this type of construction to minimise the amount of clay used.

The size of the stones will determine how many you end up using. Therefore the base can be built from a 3 large boulders or a score of fist-sized stones.

Once you have reached the desired size of the door, you can top it off with a long stone – forming a stone lintel. Should you not find a stone long enough to bridge the gap between the sides of the door, then you can make a a clay arch by inserting some wood as a support, just as you did when building the clay shaft.

Beyond that, it is just a matter of building a clay shaft on top of the stone structure. All the guidelines given in the previous section still apply.

You can now proceed to drying the furnace.

bloomery iron smelting furnace construction
Once you have stone base mortared together, then you can start building a clay shaft on top of it. (Photo: Mikk Freiberg)

Embanked Furnace


This is probably the most fool-proof method of building a furnace. Provided that you dug out the shaft straight and vertical, there are few things that could go wrong now.

At this point, you should have dug the negative of the furnace shaft in the side of your selected terrace. You have also covered the bottom with sandy clay.

Now you may shall cover the walls of the cut with more metallurgical muck. This will even things out and define most of the diameter of the furnace.

Congratulations. You have just formed the bottom/slag bowl, back and sides of the furnace.

Now you just need to build the front.

You will again begin by defining the door arch with a log or a stack firewood.

With the supports it place, start stacking, fusing and moulding lumps of tempered clay until they form the front of the shaft to the height of the embanked sides. This is in no way different to building a clay shaft, except that there is a lesser chance of the shaft collapsing.

Once the front, back and sides all reach the same height, you may want to finish it all off with a short section of clay shaft. This way you can add some extra height to the furnace.

bloomery iron smelting embanked furnace construction
My colleague Dominik building the front wall of one his famous, hastily made embanked furnaces. (Photo: B. Beranek)
bloomery iron smelting embanked furnace construction
Finishing the door arch. (Photo: K. Lokajickova)
bloomery iron smelting embanked furnace construction
A finished embanked furnace, slowly drying along with a row of tuyeres. (Photo: K. Lokajickova)

This type of furnace can be built very neatly – not unlike a free-standing shaft – or it can be quickly slapped together.

A Czech colleague of mine almost exclusively uses these embanked furnaces, building them quickly and in a very blasé manner. During the smelt, he keeps a barrel of good, but relatively wet metallurgical muck on the side. When cracks start forming, he nonchalantly throws a handful of the muck at the offending area. There is nothing elegant about his furnaces – they are a patchwork of clay lumps. But he keeps them going season after season and produces a lot of quality blooms.

So try your best, but do not make too much fuss about the whole thing. Finish the furnace and dry it before smelting. Then build some more furnaces.

But first let us take a short excursion and take a look at making doors and tuyeres.


III. Doors & Tuyeres

bloomery iron smelting making a tuyere

You will have noticed that all of my furnace designs feature a door. Having a sacrificial door, which you can break out and replace, will greatly improve the longevity of your furnace.

So far you have built in an opening to take a door into your furnace shaft. Now it is time to consider the door itself.

There are two options on this front:

Option A: Make a door ahead of the smelt and let it dry. Then before you start the smelt, all you need to do is insert the door and seal with some wet metallurgical muck.

You will make the door from more of your sand-tempered clay. Chop the organic fibre very short or omit it completely. Shape this door roughly 3 cm thick, and of a size which will fit the door opening.

While you are at it, make a couple more. It is always good to have a spare laying around.

With this method you have cut out the blowhole before you start drying the door. So think about how high in the shaft you want your air intake. A tuyere height of 10-15 cm above the furnace bottom fits most occasions.

Once finished, all that you need to do is dry the door. This is best done while you are drying the furnace.

Option B: Right before the smelt, fill in the door with fresh metallurgical muck.

All you need is to have some leftover tempered clay from building the furnace. The furnace also needs to be dried and fired for this to work properly.

You build the door from fresh clay, and since the furnace has already been fired, the two elements will seal but will not fuse properly.

The advantage of this method is that you can decide where exactly to put the air intake at the last moment. You merely form a blowhole into the door or inserting the ready-made tuyere at the desired angle and elevation.

Then you start preheating the furnace for the smelt. By the time you start charging ore, the door will have dried.

bloomery iron smelting furnace construction
A door can be easily constructed by filling in the opening right before the smelt. (Photo: K. Lokajickova)
bloomery iron smelting furnace construction
Do not forget to install the tuyere at the desired height and angle. (Photo: K. Lokajickova)

The Tuyere Dilemma – Some Ado on Clay Tubes

In the previous post on furnace design, I have already mentioned that tuyeres, or blowpipes, are not necessary for smelting. But I nevertheless tend to use one to create a conical opening in the furnace.

Therefore let me walk you through the process of making one of my preferred tuyere types.

I like to use relatively short, flared tuyeres. These will have a wall thickness of up to 2 cm and a front opening. 2-3 cm in diameter. The opening on the back end widens out to up to 10 cm, which makes it easier to point a pair of bellows at the opening, and poke a stick down the whole, should this be necessary. The total length of the tuyere will usually fall in the ballpark of 15 cm. 

I will talk more about the furnace-poking arts in the future post on running the smelt.

bloomery iron smelting ancient ironworking tuyere
My preferred smelting tuyere design – narrow at the front and wide at the back.

The tuyere is initially formed around a wooden core. Therefore you will start by whiling a slight taper on wooden stick. The width of the stick should match the desired inner diameter of the finished tuyere.

As raw material, you will again be using more of your metallurgical muck. Do not use organic temper and make sure that you really add as much sand to the clay as possible.

Start by wrapping the clay around the wooden stake. You can do this by wrapping a sheet of clay, or by impaling a large enough ball of muck.

Mould and sculpt the rough shape of the tuyere, making sure to fuse shut any cracks. Otherwise these will come back to haunt you later. Do not worry about putting a lot flare on the rear end at this point. You will deform this part when you slip the tuyere of the stake. Focus on getting a solid tube with an even wall thicknes, and try to get the front part close to its final shape.

Push the tuyere blank off the stake. You can do this by wrapping your hand around the stake bellow the tuyere and slipping it off carefully. This will upset the rear end of the tube.

Pull out the flared part out of the thickened end. Keep a bowl of water on hand, wetting your hands when necessary, as you smooth out the shape of the tube.

Do the final correction on the front part as well, smoothing out any irregularities.

When you are satisfied with the shape of the tuyere, set it down with the flared end on the ground. This way it will not deform too much under its own weight. Preferably somewhere near the drying furnace.

bloomery iron smelting how to make tuyere
Making a clay tuyere: step by step.

Pro Tip: Make some more tuyeres. It is always great to already have them ready for the next smelt.

Once the tuyeres are dry, you should ideally have them pre-warmed and put them in some embers for a light firing. Having the tuyere at least partially turned to ceramic will reduce the chances of you cracking and damaging the flared end during the smelt. Pre-firing the tuyeres will also eliminate any faulty examples, since these should crack at this point.

If this is not an option, do not despair, most of the tuyere will quickly turn to ceramic as you begin smelting.

This whole process is again easily done while drying the furnace, so let me guide you through that part of the process.

bloomery iron smelting forging how to make tuyeres
Firing the tuyeres in an hearth. such as a campfire, ore even a forge, does the trick as well.

IV. Drying the Furnace

bloomery iron smelting furnace construction

Drying the furnace is a simple matter of lighting a fire in it. Then gradually feeding it with firewood.

If you really wanted to make a furnace as solid as possible, then you would first let it slowly dry and the fire it, starting off small and then increasing the heat until the furnace is ready for smelting. The whole process would then resemble the way we make pottery, crucibles or moulds.

Luckily, we are not making any of those things right now. We are building a furnace, which needs to make it through a smelt and preferably still be ready for a couple of runs after that.

It just needs to be good enough.

There is no significant disadvantage to force-drying a furnace. Sure, a crack or two will form, but this will not diminish the structural integrity of the shaft. And should any of the cracks spread to wide, you can still patch it up before or during the smelt.

Should you really be pressed for time, you could even start smelting in a freshly built, wet, furnace. It would take a bit longer to heat up in the beginning, but provided you mixed your clay right, it should make it through the smelt.

Therefore, with a clear conscience, you will start by lighting a fire in the bottom of the furnace. The large, vacant door opening should make this quite easy.

As the fire catches properly, you can start feeding it firewood through the top opening. The blaze will gradually increase, and the furnace should quickly start steaming and drying out visibly. At this point you can decide to reduce the air intake by putting a large stone in front of the door arch. This will insure a slightly slower burn.

This is also the time when you can start setting any furnace doors and tuyeres around and in front of the furnace to dry them as well. I often start making the tuyeres right after starting the fire.

Warning: This initial stage of drying produces of smoke, due to the combination of a fresh fire and the walls practically sweating water, which keeps escaping and re-condensing in the form of white clouds of vapour.

bloomery iron smelting furnace construction drying
A steaming furnace in the later stages of drying. The door is partially closed off with a large stone, and the tuyeres are also slowly drying in front of the furnace.

Keep feeding the fire for at least another hour or two.

At the end of this period, you should have a decent pile of glowing coals, embers and firewood at the base of the furnace. If any of the tuyeres are dry enough at this point, you may lay them on the coals. You will find that this fires them in a most satisfactory manner.

You may now prolong the last part of the burn, by closing off the front and possibly even the top of the furnace with fire-resistant stones.

bloomery iron smelting furnace construction drying
Once the fire is going well ,and the furnace shaft starts firming, the clouds of smoke will go away as well. You can see that the top of the shaft is already dry, while the thicker parts near the base are still damp. At this point you could let the fire burn down and fire the tuyeres in the embers. (Photo: Mikk Freiberg)

Provided that you started building the furnace in the morning, then the afternoon sun should now be shining upon your works. Evening might already be closing in. Let the fire slowly go out on its own.

The furnace will keep cooling and drying overnight. By the morning you find it warm to the touch and dry, with the possible exception of a damp spot or two by the base.

Congratulations! Now you are finally ready to begin smelting! More on that in the next post.

bloomery iron smelting magic how to make iron
Admittedly, the whole ritual of drying the furnace can also be performed at night, and used as an excuse to share a couple of cups of ale around a fire. Telling tall smelter’s tales and thinking of the next day’s smelt. (Photo: B. Beranek)

Conclusion

The whole process can be summarised as such:

Build a furnace from the bottom up, making sure to move around and check for symmetry as you go along. Once you are satisfied with the shaft, light a fire in the furnace and start making the tuyeres. Keep firing the furnace and dry the tuyeres in front of it. At the end, toss the tuyeres in the coals and let the fire die out. Call it a day and get some rest before the smelt.

Building a furnace shaft is a matter of how neat you want it to be, and how long you want it too last. Most furnaces will make it through a few smelts. They would really have to be built clumsily and from atrocious materials in order not to make it through the first smelting run.

Most cracks can be mended by pushing in a bit of wet muck, and the inside of the shaft should largely turn to ceramic and vitrify, thus forming a solid internal skeleton.

As a beginner you are more likely to severely damage the furnace while you try to extract the bloom. Failing that, a lot of damage will be done by the shaft being exposed to the weather after the smelt, and few furnaces survive a winter frost.

Therefore, you should definitely put some effort into building your furnaces. There is no need to be sloppy on purpose. At the same time you should not overthink this in trying to get everything perfect the first try. You primary concern is in getting it done and working. You will surely build more furnaces – and get better at it in the future.

Do you have any questions to ask or advice to share? Did you ever have a furnace fail catastrophically? How? Post in the comments bellow!

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