Was Japanese armour ever case hardened?

[Aaron]

Hi,

In my limited research I have not found a reference to Japanese armour being case hardened. But I am currently limited to research with a phone.

Help?

[Sean M]

Kōzan says that the outer third of an armour plate should be steel, and the inner two thirds should be iron, forge-welded together, because all-steel plate shatters and all-iron plate is easily dented.

[worldantiques]

From the Royal Armouries.

[Aaron]

Thanks!

[Signo]

This does not equate to case hardening or tempering processes, those are just two metals joined together, but there is no info in this text if the steel has been heat treated. As a side note, the very same techique was well known and widely used in medieval europe too, at least since late XIV century.

[Gustovic]

Sandwitching iron and steel for armour? Which are your sources?

[Signo]

Una fonte italiana è una lettera di S. Bernardino da Siena, che è stata trattata anche su questo forum anni fa:

Tra le migliori testimonianze circa l'arte di fabbricare le armature va ricordata quella del senese Bernardino Albizeschi, discendente di un'importante famiglia di fabbri toscani, più noto come San bernardino da Siena:
"Quando io fui a Milano, io la imparai a cognòsciare da uno perfetto maestro, e dissemi la ragione a volerla fare buona, non voleva essere né d'acciaio né di ferro. O di che la faremo dunque? Dissemi che voleva essere fatta in questo modo: che voleva essere da l'un lato d'acciaio e dall'altro di ferro. E volevasi fare in questo modo, che si voleva fare piastre d'acciaio e piastre di ferro puro. Se fusse tutta di ferro, non sarebbe forte, ché 'l guirrettone la passerebbe; e se fusse tutta d'acciaio la percossa della lancia o d'altro la spezzerebbe. E però si voleva fare dell'uno e dell'altro, cioè di fuore l'acciaio e di dentro il ferro, e bàttare insieme l'uno con l'altro, e farne uno corpo, et in quello modo sosterrebbe alla percossa, et anco non passarà mai il ferro: una alteri coniungitur: e così sarà perfetta" .

The underlined part, is a text written by this "Saint Bernardinus from Siena" and his writing say this:

When I was in Milan, I learned from a great master (armourer), that told me that good armour is neither steel or iron. Of what we will do it then? He told me that the armour want to be made in this way: It need to be made of iron from one side and of steel from the other. And this is the way to do it, making steel and iron plates. Because if it was all of iron, it would not be strong, the quarrel would pierce it; and if it was all steel, the lance stroke or other (weapon) would break it. And thus we want to make it from one and the other, out of steel and inside of iron, and hammer them togheter to make it a single part, and in that way it would sustain the hit, and will be weaponproof. and so it will be perfect.

Esistono anche descrizioni simili in area tedesca, tant'è che Maio D'Azzal una volta mi disse anche il nome tedesco di questo processo (impronunciabile)*

Even in Dr. Alan Williams work there are info about that process (always Maio told me).*

* unless memory fail me.

[Sean M]

Una fonte italiana è una lettera di S. Bernardino da Siena, che è stata trattata anche su questo forum anni fa:

Tra le migliori testimonianze circa l'arte di fabbricare le armature va ricordata quella del senese Bernardino Albizeschi, discendente di un'importante famiglia di fabbri toscani, più noto come San bernardino da Siena:
"Quando io fui a Milano, io la imparai a cognòsciare da uno perfetto maestro, e dissemi la ragione a volerla fare buona, non voleva essere né d'acciaio né di ferro. O di che la faremo dunque? Dissemi che voleva essere fatta in questo modo: che voleva essere da l'un lato d'acciaio e dall'altro di ferro. E volevasi fare in questo modo, che si voleva fare piastre d'acciaio e piastre di ferro puro. Se fusse tutta di ferro, non sarebbe forte, ché 'l guirrettone la passerebbe; e se fusse tutta d'acciaio la percossa della lancia o d'altro la spezzerebbe. E però si voleva fare dell'uno e dell'altro, cioè di fuore l'acciaio e di dentro il ferro, e bàttare insieme l'uno con l'altro, e farne uno corpo, et in quello modo sosterrebbe alla percossa, et anco non passarà mai il ferro: una alteri coniungitur: e così sarà perfetta" .

Thanks for that passage! Previously I only had sources from Europe for this technique being used by bladesmiths. Saint Bernardino da Siena was active in the first half of the 15th century.

This does not equate to case hardening or tempering processes, those are just two metals joined together, but there is no info in this text if the steel has been heat treated. As a side note, the very same techique was well known and widely used in medieval europe too, at least since late XIV century.

The original meaning of “case hardening” seems to be baking a ferrous object with carbon in a case so that it would become hard on the surface. Sometimes, as in della Porta, it was further heated and quenched. But today it can refer to many different processes!

As for "no info" ... are you sure? Have you read that passage in Kōzan? It is four pages long and I won't try to summarize it, because I am not a smith, but I see some phrases which could be interpreted in different ways ... and his purpose is not to describe the process in detail, but to explain some technical terms to customers so they can ask good questions and understand their armourers' replies.

"il nome tedesco di questo processo (impronunciabile)" ... that is hitting below the belt, I think!

[Gustovic]

Seems like a very labor intense process, when we know armourers could case-harden armour obtaining basically the same effect.

[Signo]

No, in the most absolute way. Case hardening has a penetration of few hundreds of mm, tenths in the best case, here we are talking whole millimeters, like making a breastplate that has a 4 mm thickness in the center and have it composed of 2 mm mild +2 mm steel sheets. That is another world from case hardening.

[Gustovic]

It would be nice to see some consistent archaeological evidence. You have any?

[Signo]

Evidence? Everytime you see a piece of armour with a delamination exactly in the middle of the thickness, then to me is a solid evidence. I personally examined the famous keyhole Stibbert's museum, at the time I thought it was just the metal that was folded during the production process, but after reading that letter, all made much more sense. I don't really think it's something it must be "proved" as , as I already said, there are multiple sources about it, the fact that I don't have them at hand, doesn't dismiss them.
Sean, sorry, that phrase mean :the same process is described in german, it even has a name, that is obviously impossible to even remember.

[Gustovic]

Cool, but do you have any evidence of a surviving piece that is made of two welded layers? Like x-ray, extensive sampling of both inside and outside, etc...

Because I'm told delamination doesn't necessarily mean forge welding of different layers.

[Sean M]

Cool, but do you have any evidence of a surviving piece that is made of two welded layers? Like x-ray, extensive sampling of both inside and outside, etc...

Because I'm told delamination doesn't necessarily mean forge welding of different layers.

David Sim has an X-ray, VPH, carbon content, and microstructure analysis for a scale from a Roman armour which consists of three layers forged together in JRMES 14/15. de Reuck et al. "Duplex Armour: An Unrecognized Mode of Construction" have a description of the process from the Medici archive. Otherwise, metallurgy of medieval armour is not my thing!

[Sean Powell]

No, in the most absolute way. Case hardening has a penetration of few hundreds of mm, tenths in the best case, here we are talking whole millimeters, like making a breastplate that has a 4 mm thickness in the center and have it composed of 2 mm mild +2 mm steel sheets. That is another world from case hardening.

Yes and no. Case depth penetration is a function of time and temperature. 2 to even 3 tenths of a mm (.008-.012") will see some improvement. Given that highest stress in in the outer surfaces there is a lot of benefit from case-hardening of thinner pieces of armor (1.5mm or .060" or less)

Sean

[Thomas Powers]

Delamination is a "common" issue with real wrought iron that is refined from the bloom by stacking and forge welding often multiple times. If you have a bad weld then delamination can show up later in use. This of course means that delamination does not mean that iron and steel was used; but does indicate that layered materials were used.

(It is interesting to note that every culture I have investigated that used the bloomery process to produce wrought iron also used pattern welding and I lean towards separate discovery of the process as possibly occurring multiple times.)

[James Arlen Gillaspie]

William's massive work is the real go-to for European armour, though I would say that what he really should have is a website where he and others can add examples as they finish them. He hasn't turned up much two layer stuff, but usually several layers, and it seems to be more about adjusting carbon content, not unlike Japanese blade smithing. I have often seen two main layers in my own work, though, but that just seems to be because the two main layers were refined first and then for some reason the forge weld of those two pieces isn't quite as good. I don't know why, though. Japanese practice could well be different.

[Sean M]

Toby Capwell thinks that the Italian cuirass in Norwich Castle Museum is of duplex construction with a steel face welded to an iron back (AotEK vol. 3 p. 47). If I understand correctly, this is his personal observation based on how it is changing as it rusts. I'm not sure how he knows that the face has more carbon than the back, he thinks that the way it cracked suggests a hard face and a soft back.

He estimates that it was made around the year 1440.