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Question about phosphorus in historical iron


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I was doing some research on the composition of historical blade materials and I found out that a lot of the iron they used contained a lot of phosphorus, between .4 and 1.4%. It also seems like they used it on purpose for pattern welding.

Here's the links to some articles:

https://www.tf.uni-kiel.de/matwis/amat/iss/kap_b/backbone/rb_3_4.html

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.uni-obuda.hu/journal/Thiele_Hosek_60.pdf&ved=2ahUKEwj3s6rbo43gAhUL54MKHVZ7AEUQFjARegQIBBAB&usg=AOvVaw1XYSF1k9B22QPbps6nWetn

So my question is: wouldn't that much phosphorus make the metal extremely brittle? I've read that phosphorus increases hardness but reduces toughness/ductility but I don't know to what extent. I guess what I'm asking is how a high phosphorus (e.g. 1% )steel or iron would compare to other materials and if it's still strong enough to be useful to a blacksmith.

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High-P iron is indeed a bit brittle compared to low-P iron, but that shows up more during forging than in use.  It tends to snap during forging if too cold or if there are sharp corners.  Where it is useful is that it work-hardens.  That means a cold-hammered high-P blade is substantially harder than plain iron.  That was of great value before the role of carbon in hardening was understood, i.e. the first half of the iron age (technically the role of carbon wasn't understood until the late 18th century, but they were manipulating carbon content deliberately by the first century AD).  It was indeed used in pattern welding, as it etches darker than ordinary iron. There is a lot of information about this on the forum, both here and in the Bloomers and Buttons subforum.

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Interesting... I wonder what the flaw in their theory was. 

Also would you say the ductility/toughness of room temp high P iron is comparable to unhardened high C steel? Sorry if I'm being tiresome, I'm just trying to get a good idea of what it would be like since I am not able to test it myself.

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The other thing to remember is these ancient irons and steels are not directly comparable to modern homogeneous steels.  They are all based on wrought iron, and thus will retain some slag and fibrous structure no matter how well refined.  As Jeroen pointed out, they also have no alloying elements to increase the depth of hardening.  Manganese is the foremost of these in modern steels, and not coincidentally is responsible for modern high carbon steels etching dark.

I believe the academics simply made an assumption (the High-P etching bright is received wisdom from many years ago and I'm not sure who first suggested it) and did not test it.  I, on the other hand, made a blade and etched it to see what would happen.  It's not a huge amount of contrast, but the plain iron stays silvery-grey and the high-P turns a little darker.  Here is that blade:

 

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