G'day Chris here, and welcome back to Clickspring.
As part of the process of investigating how the Antikythera Mechanism was made,
I've made a set of hand cut files,
using mild steel as a substitute for the wrought iron that was likely used in antiquity.
But of course when it comes to hardening those cutting surfaces,
there's a problem.
And its the same problem as that faced by the original makermore than 2000 years ago.
Mild steel, has insufficient carbon to simply heat and then quench harden.
So in this video I'm going to use the ancient process of case hardening
to harden the file surfaces.
It involves heating the work in close proximity to a carbon source,
to absorb carbon into the surface prior to quenching.
And although the recipes for the carbon source do vary a lot,
the active ingredients of the method that I'm using are easy to identify,
and were readily available in the ancient world.
Common table salt, flour to act as a binding agent,
and scrap leather as the carbon source.
For convenience, I used the same process I use to make char cloth,
to turn the leather into charcoal.
By sealing it inside a small tin, and then heating with a propane torch.
A few minutes under the heat, transforms the leather into a hard glassy charcoal.
But of course there were no such tins in antiquity,
so I thought it might be worthwhile confirming
that leather charcoal would have been just as easy to make using simpler methods.
So I made a simple ceramic pot to hold the leather,
and then placed it in an ordinary fireplace.
It took a bit longer to heat up and cool down, so it wasn't quite as convenient.
And it did produce excellent quality charcoal.
The charcoal was then ground into a fine powder,
in preparation for making the carbon pack.
I mixed the carbon powder with salt and flour in the proportions onscreen,
and then turned into a paste with a small amount of water.
The paste was then packed ar ound the file surfaces, and then left to thoroughly dry.
Now a reasonable degree of case hardening can be achieved
by simply heating this carbon pack, as-is.
But to optimise the chemical reaction,
the process is best conducted in an airtight environment.
And traditionally, this was achieved by enclosing the carbon pack in clay.
Again for convenience, I'm using a modern propane furnace to do the heating,
but a charcoal furnace from the period would have been quite capable
of raising the clay to a red heat.
And its at this red heat temperature that the transformation begins.
The metal is now in what's known as the Austenite phase,
and has a strong affinity for more carbon.
At the same time, the carbon pack is reacting at the steel surface
generating a surplus of carbon that diffuses into the body of the steel.
Effectively carburising the metal from the outside in.
The depth of this carburisation is dependent on time.
The longer that I leave it in this heat, the deeper the resulting layer of high carbon steel.
So while thats cooking, I'd like to show you the tests that I did earlier
to figure out just how long the steel needs to remain in the heat.
I started with the same mild steel stock as the files,
using a control piece and 4 carbon packed pieces.
All were enclosed in clay, heated to a red heat,
and then quenched in room temperature brine.
And you can immediately see and hear that the control piece is virtually unchanged.
A file easily cuts it.
And it bends pretty much the same as it did before.
So there's been no meaningful change to its hardness.
But a quick file test shows that the other pieces have certainly changed.
And an attempt to bend the material shows just how dramatic that change is.
The rods are no longer soft and ductile, but hard and very brittle.
The Austenite has been transformed into a hard crystalline microstructure known as Martensite.
That appears as a silver-white ring around the darker low carbon interior.
And you can clearly see it progressing towards the centre of the metal, as a function of time.
The longer the material spends in the heat, the deeper the layer of martensite.
Now a cutting tool like a file doesn't really need the hard layer to be particularly deep.
So I removed them after 20 minutes,
which i figured would give a suitable hardness to the teeth,
but also ensure that the interior remained reasonably ductile.
A file confirms that hardening has occurred,
and its interesting to now see that the metal has changed enough
to generate the unmistakable spark pattern of high carbon steel.
The files were then tempered to a very pale straw,
and the tangs softened to blue.
Today, modern steel composition means that there are several ways
to form tempered martensite,
in addition to the process that I've shown in this video.
And we use it in much of our day to day life,
mostly without even being aware of it.
But in the ancient world, before it even had a name,
this is one of the ways that it was formed.
Using a simple process, and the simplest of ingredients.
Thanks for watching, I'll see you later.
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Thanks again for watching, I'll catch you on the next video.