Welcome to another video.
This one is going to be fun.
In previous videos, I have shown the process for turning iron ore into wrought iron using
a bloomery furnace.
I also showed how to refine wrought iron and make blister steel.
In the following 8 minutes, I will show you how to make hearth steel in an Aristotle furnace.
This is something I have seen Lee Sauder write about but not a process I have ever tried.
My naive understanding was as follows: take charcoal, blow in air put iron or mild steel
on top of it and 30 minutes later you end up with high carbon steel.
We started the day by acquiring a bunch of high-temperature hard fire brick and got slightly
more than we needed but that gave us creative freedom.
The basic idea was to use the fire brick to form a furnace in which the charcoal could
burn.
Our reading indicated that the iron should sink in the fire below the air blast
where temperature was still high but the atmosphere was starved of oxygen.
Another goal we had which was perhaps not as well thought out was that bigger is better.
So, we wanted to make a big furnace that could produce a lot of steel.
Since we started from scracth, we first built a sieve to sieve out the charcoal fines.
We would later use the fines to line the bottom of the furnace where the fire would
really burn but where extra carbon might still be helpful.
Talking about carbon, turning iron into steel requires a process called carbon migration.
This is similar to making blister steel only that the hearth steel method
seems to be much more aggressive.
It actually does end up melting some of the iron or in our case the mild steel we are
using as a charge.
The mild steel for our first experiment is 1/2in round which we cut into long rods that
get fed into the fire.
As the iron heats up in the fire it slowly drops down with the rate that the charcoal
burns up.
The furnace has a tyuere that directs the air blast down.
One of the challenges here is to adjust the air so that
there is a zone below the tyuere where the iron comes to a rest that has a reducing atmosphere.
By reducing, I mean that there is more fuel than oxygen.
In our case, we want an excess of carbon as otherwise the oxygen would leach the carbon
out of the steel and we would produce wrought iron instead.
For our first furnace build, we will use a tyuere made out of fire brick that we angle
down at roughly 45 degrees.
Tony is splitting the brick on which the tyuere will sit.
This is somewhat rough on my hot chisel but that was the most appropriate tool we had
lying around.
We will dress the edge later and turn it back into fully working condition.
With that we have everything we need to start building the furnace.
This will be our first furnace where we were still fully subscribed to the bigger is better
mantra.
So, let me forewarn you, do not expect success.
In this build we are using basic clay to hold the fire brick and tyuere together.
Since we did not want to damage the concrete we are also creating a raised floor with all
the fire brick we acquired for this project.
As air supply we use the blower and tyuere that we used for making crucible steel before
just without providing propane and by significantly dialing down the amount of air.
In the first version of the furnace - we are showing you our magic build sequence here
- we created an air chamber before the actual furnace which essentially serves as an adaptor.
For the first running, we will be feeding 1.5kg of 1/2in round mild steel that we cut
into multiple bars.
This is a lot of material to charge but we are still operating under
the bigger is better fallacy.
As I had mentioned before we are filling the bottom of the furance with charcoal fines.
Trying to create bed on which the hearth steel can rest and carburize even more.
Lighting the fire is always fun but one observation to be made is that the tyuere is quite close
to the bottom of the furnace and in order to get all of the charcoal on
top to burn we need to provide a lot of air.
And the oxygen in the air really likes to combine with carbon.
Given that we want our iron to absorb a lot of the carbon, it seems that a lot of air
might not be such a good idea.
However, in the moment, everything seemed good to us as the fire was burning quite brightly
and with a lot of spirit.
Our somewhat unrefined process, also did not accont for the size of the charcaol.
We figured we just dump it in at whatever size it is at and it will burn just fine.
However, consistency is really important.
We want each load of charocal to burn through at roughly the same temperature and time.
Our aim was for 400g of charcoal every 4 minutes.
As you can see here the individual pieces of charcoal vary quite a bit in size and many
of them are quite large.
We later refine our approach to break up charcoal so that each
piece is not much larger than an inch.
We are also occasionally measuring the temperature.
We want our hot spot to be around 2400 degree Fahrenheit.
At this point, we have developed a good rythm and are charging charcoal roughly every 4
minutes and are watching the midle steel rods travel down in the furance.
As we had some doubts about the temperatures, we pulled out one steel rod and are definitely
noticing how the end of it has melted.
Once all the iron has been charged, it's time to take a look at what we produced.
We are quickly removing the bricks to get to the puck of hopefully high-carbon steel.
Once extracted, I am quickly moving to the power hammer to consolidate the material while
it's still hot.
One of observation already is that the material did not consolidate into a single mass and
instead still seems to have pieces of the original mild steel rod.
The true test of carbon content happens on the grinder.
We are expecting many exploding sparks indicative of a lot of carbon.
Unfortunately, what we are getting instead does not have any sparks.
In fact, it seems that we removed all carbon from the material.
It's back to the drawing board.
Bigger is not better and we are creating a smaller furnace instead in which the tyuere
is placed much higher in order to create a zone in the fire
that is oxygen starved and where the iron takes in a lot of carbon.
In principle, this should do the trick and we also get to recycle the material from the
previous run back into attempt number 2.
Let's see what we produced.
First, it's back to the power hamer but then we get to test for sparks again.
Unfortunately, we ran into exactly the same problem again.
There are no sparks and we produced iron without any carbon in it.
Fortunately, we have an ace up our sleeve and it's time to play it.
The ace is Emiliano Carrillo, a great swordsmith who has produced hearth steel for many years.
We get on a video call on WhatsApp with him and get precise instructions on Emiliano's
way of making hearth steel.
If you want to find out more about his work,
The first improvement was to sieve the charcoal so that the pieces are all roughly the same
size - around an inch in diameter.
The furnace itself is much smaller, literally just 7 firebricks standing on their ends with
a tyuere pointed down at a very slight angle.
Our charge of mild steel consists of much smaller pieces.
In this case, cut offs from a previous gate and fence construction project.
We also made the protocol more stringent.
We charge 150g of mild steel with 400g of charcoal every 4 minutes.
Once we figured this all out with the help of Emiliano, we actually got to relax a bit
and enjoy the fire.
We really should brought some food for a barbeque.
Another lesson learned and perhaps much more important was that we significantly reduced
the amount of air that we blew in through the tyuere.
We needed less air because the furnace was much smaller and that also reduced the
risk of having access oxygen leach carbon out of the high-carbon steel puck we were
aiming to create.
As you can see we are really aiming for perfection now.
Precisly 400g.
Just joking this much precision is not really neede here.
While hard to see on the video, every time we add charcoal we had also added mild steel
just before.
Given all the expert instruction and our meticulous execution of said instruction, we really hope
that it's going to work this time.
Or at the very least, that we get some more carbon than before.
It would be a pity if our result ended up with less carbon than already present in the
mild steel.
It's time to find out.
With the much smaller furance, we no longer need to remove bricks and instead just need
to remove some of the burning charcoal from the top before we can
extract the puck below.
The shape already looks pretty good as everything seems to be consolidated into a single mass.
However, before we share the results, we decided to run one more experiment.
In this experiment, the last charge was just charcoal without any mild steel added in.
We hoped this would give more time for carburization.
Before, you can really appreciat the results, let's check the sparks from O1 tool steel
which has roughly 1% carbon.
This will be a good reference.
We can definitely see multiple sparks here.
Let's look at the last puck in comparison.
What a difference.
There certainly seems to be a lot of carbon in there.
I am super happy with the result and will turn the hearth steel into a knife in a future
episode.
The 100g process loss is very small as well.
That's it for this episode.
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