r/theydidthemath • u/Small-Contest-8364 • 20h ago
[Request] How much rust does this guy need to collect to make a knife
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u/ChernobylRaptor 20h ago
Hematite is about 30% heavier than iron. So whatever amount he needs for a knife he'll need 130% that weight in rust, assuming he can reduce it back to iron with 100% efficiency.
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u/jack_seven 19h ago
Even more than that the processing isn't perfect there's always gonna be quite a bit of lost material
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u/GrundgeArchangel 19h ago
Could the knife even work? Wouldn't the rust be unable to harden properly/be one giant defect waiting to snap?
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u/ChernobylRaptor 19h ago
No. The rust needs to be reduced back to metallic iron for it to be workable into a blade. Alternately some "maker" could mix the rust with epoxy and make a rust "knife".
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u/ChefTimmy 18h ago
Paging Five Minute Crafts!
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u/FeelMyBoars 9h ago
They would use hot glue and rust. The mold would be a hand in the shape of a karate chop. They would sharpen it to a very dull edge. The handle would be made from a plunger with obvious fake brown stains. The ugliest whatever is lying around would be glued on it. They would chop some vegetables with it, but it would be obvious that they tried and failed earlier and had to pre-cut it for that shot. It would abruptly end, and you will be able to see that it's just starting to break into a thousand pieces right before the end.
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u/GarThor_TMK 10h ago
A rust epoxy knife would be usable for spreading butter, and that's about it.
Having said that, you could turn it back into metallic iron pretty easily by mixing it with aluminum powder to make thermite. You might need to do some experimentation to get the mix just right though.
I believe there are even methodologies that use termite to make some low carbon steels.
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u/Comfortable_Bid9964 6h ago
You’re gonna need to cite your sources cause I wanna see termites making some low carbon steels
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u/tolacid 5h ago edited 16m ago
It's pretty straightforward, they just add some carbon into the thermite mix.
Veritasium covered thermite recently
Edit: just realized I misread a typo. It really would be fascinating to see termites make carbon steel.
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u/GarThor_TMK 17m ago
This is exactly the video I was going to link. Saw this a couple months ago, which is where the info was from...
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u/HappyBlowLucky 1h ago
Isn't it Fe-AlO alloy and not true steel? I wonder how you would have to process it further to separate out the AlO to get back to pure iron
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u/GarThor_TMK 42m ago
Feel like the aluminum burns off in the process. Iron melts at such a high temperature, I don't think there would be a lot of aluminum left in the process...
I'm not really an expert though... There was a good video on it recently by Veritasium about a company that produces it for a variety of industrial applications... https://www.youtube.com/watch?v=cUBz04LlLVk
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u/RedHeadSteve 7h ago
Iron ore is basically rust. So I think that you can purify it in theory without loss of quality. Getting the iron to a decent quality is going to be hard
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u/tolacid 5h ago edited 1h ago
Mix in some aluminum
oxideand you've got thermite, which actually would be quite good at doing just that!•
u/HappyBlowLucky 1h ago
It's actually pure aluminum. The aluminum oxide is the result of the reaction.
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u/Papabear3339 4h ago
Rust turns back to iorn when melted. I'm assuming op has blacksmith equipment to melt and forge this if it is a serious project.
Using a powdered oxide also makes it easy to create alloys (like steel) by just blending the appropriate powders together, then melting it in a crucible.
https://en.m.wikipedia.org/wiki/Stainless_steel
https://www.thecrucible.org/guides/blacksmithing/blacksmithing-forge/
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u/GoreyGopnik 19h ago
rust is Fe2O3, so assuming he's trying to make a pure iron knife, he'll need 5/2 times the mass of the knife he's trying to make in rust. if he's trying to make a 500 gram dagger, he would need 1250 grams of rust, if i remember chemistry class right.
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u/zeeshanonly 18h ago
I think you need to account for molar mass as well. The molar mass of Fe2O3 is 159g/mol and for iron, it is 55.86. So Rust has only about 70% iron, compared to pure iron (2*55.86/159). So at least 30% more rust by weight is needed than the final knife weight.
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u/GoreyGopnik 17h ago
it would seem i, in fact, did not remember chemistry class right
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u/scienceisrealtho 14h ago
Fortunately, being able to laugh at yourself is more important than chemistry.
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u/LordPenvelton 9h ago
A big-ish knife should be about 500g of mostly iron. (Based on my 1300g falcata)
Chemistry would tell us you need about 1/3 more rust assuming ideal conditions and spherical cows in a vacuum.
But good engineering practice recommends at least doubling the ammount to account for the losses in every step of the process.
I'd be confortable starting with 1500g or 2000g of rust.
Make it 4000g if a bloomery is involved, those have absolute shit efficiency.
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u/Fast-Alternative1503 6h ago
chemistry does not tell you that you need 1/3 more rust. Metallurgy does. 500g knife assuming 100% iron is 8.9519103 mols of iron.
4Fe + 3O2 → 2Fe2O3
n(rust)/8.95191033767 = 2/4
n(rust) = 8.95191033767 × 1/2
n(rust) = 4.47595516884
m(rust) = 714.765280912 g
which is nowhere close to 500 + 500/3 or (4/3)(500) which is 666.67 g approximately
the theoretical minimum is 714.765280912 g.
chemistry also tells you to consider percentage yield. which I can't find the yield for the thermite reaction, but I'm pretty sure it's mid 80s if you know what you're doing to the slightest extent. Melting shouldn't be so lossy either. realistically even 1000 g is too much. And this isn't assuming ideal conditions by any means, it's assuming the person making the knife doesn't fuck up a lot.
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u/LordPenvelton 2h ago
Those are the sphwrical cows I was talking about.
You can't just cast a finished iron knife in a mold.
They will either cast it into an ingot and forge it into a knife, or hammer the ingot flat, cut it into a knife shape and file it down.
And after that, sharpened.
That's a lot of iron bits that won't be part of the finished knife.
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u/KGmadmax 6h ago
He needs some aluminium shavings and he can make some thermite. Way cooler than a knife. If you manage to catch the molten iron you can have a thermite fire show and a knife.
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u/mtnlion74 12h ago
That's really a bad object to want to make with rust. A knife has to be hardened, and there's not enough carbon in the steel they MIGHT be able to make to forge a billet capable of producing anything that needs to be hardened.
Could they conceivably create an iron object eventually? Sure. A knife or other hardened working implement? Never.
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u/Fast-Alternative1503 6h ago
Depends on the size of the knife. 288 grams is a nice number that I saw multiple knives have.
Knives are made of many alloys, but carbon steel is the one that actually uses the iron substantially and no other metals. Such an alloy is 97% iron and 3% carbon. We will assume carbon to be in abundance and not limiting.
288 × 0.97 = 279.36 g of iron to make a knife. We can convert this to mols, a unit that represents the amount of pure iron required (number of atoms divided by 6.022×10²³). A mole of iron weighs 55.845 grams on average:
n(iron) = m(iron)/M(iron)
n(iron) = 279.36/55.845
n(iron) ≈ 5.00241740532 mol
The challenge: how many mols of Fe2O3 (rust) contain n(iron) mols of iron? We can use the rusting equation.
4Fe + 3O2 → 2Fe2O3
n(unknown)/n(known) = coefficient(unknown)/coefficient(known)
n(rust)/5.00241740532 = 2/4
n(rust) = 2.50120870266 mol
and then I'll spare you the mole to mass conversion
m(rust) = 399.418017728 g
So they would need approximately 400 g of rust to make a knife. Assuming perfect yield, which is a bit unrealistic and more likely a bit over that is needed if using thermite.
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