Iron and Steel Technology in Japanese Arms & Armors - Part 4: Armor making

Iron and Steel Technology in Japanese Arms & Armors - Part 4: Armor making

A shop of a armor makers in the Edo period, from  職人尽絵.

After discussing how swords and blades in general were made in the last part of my series, in this article I will try to explain how plates for armors were made in the 16th century.
If you are already familiar with my series, you will already know that these posts are exclusively made with the late Muromachi and Azuchi-Momoyama period in mind; in fact despite few scattered evidences, plates weren't used to make armors up until the mid 15th century as I have explained in detail here.

Unlike with Japanese swords, the information on Japanese armor making seems to be quite rare and scarce, and on top of that, very few metallurgical studies exist both in English and in Japanese, so it is fair to said that this field is still a work in progress.
As usual, before I start, here the disclaimer;

I might have missed something here and there, and I might be wrong since I don't have neither the experience nor the academic background to be precisely accurate. Please take these information with a "pinch of salt".

Despite this limitation, I'm using academic references that you can find throughout the article to back up my thesis.

If you haven't done yet, to better understand the topics I will discuss here, I suggest you to read what kind of iron ores were used in Japan and how these ores were turned into steel and iron ingots.


Most if not all of my article will be based on the account described by Sakakibara Kōzan in his masterpiece, "The Manufacture Of Armour and Helmets in Sixteenth Century Japan".


On the making of plates

Kōzan suggest that the best plate used to make armor should be made with an outer surface of high carbon, hard steel and an inner layer of iron (or low carbon steel), the former half the thickness of the latter.
This composition was used later on in Europe (duplex armor) and on steel warships of the 20th century as well.
The various ingots of steel and iron were collected in the armorer shops, where the forging process started, in a similar way to what a bladesmith did.
Kōzan mentions that the best steel came from Harima, while for iron, disused hoes and spades were quite good. He also states that although the two materials have the same origin, they are different in properties. In fact, while extensive forging will make iron stronger, steel on the other hand gets softer and dull. Although the knowledge of material science was incredibly limited at the time, he was right about its statements.

Just like any pre-modern technology, armor making was subjected to trial and error processes in order to figure out the best way to forge the plates, and these tests were documented by the author; this is quite interesting, because it gives us insights on the properties of the armor through the eyes of the ancient makers.
As I have explained before, steel (and iron) needed to be refined and consolidated due to its pre-modern method of production, whether it was obtained by a bloom or by a refined cast iron ingots.
The process is always the same: folding. This was done in order to reduce the number of inclusions, spread them evenly and to homogenize the carbon content.
Folding wasn't a unique feature of Japanese steel, but it was widespread all over the world up until the late 18th century.

Among the various methods of folding the steel studied by the armor makers in the 16th century (and likely before), the ones used for swords, namely masame kitae ( 柾目鍛え,straight grain folding) and itame kitae ( 板目鍛え, board grain folding) were unsuited for armor plates; the former was apt to split while the latter to flake and neither of these two were able to withstand arrows or bullets.
The technique that works best, according to Kōzan, is the jūmonji kitae (十文字鍛え, cross grain folding) which will be explained below.

Hiromichi Miura Sensei hammering a plate for a suit of armor. Taken from this fantastic video.

In fact, it is believed that folding gives unique properties to the grain size and orientation, as it is observed in Japanese swords, because the grain alignment and packing is manipulated much more during the forging process.
The idea probably roots back in the wood technology, where composite materials are created to improve strength and stiffness, by cross-orienting highly anisotropic layers, and is older than the 16th century.
In fact it is possible that the same technique could have been used before in armor making.

As I said in my blade making process article, this properties seems to be observed in Japanese sword but as far as we know with material science, the high temperature that is required to forge weld together the different layers anneals the metal and cancels previous deformations (as preferred grain orientation), determining the impossibility of reproducing a well defined cross-oriented material. In fact, during annealing the microstructure of the deformed metal undergoes a sequence of consecutive and overlapping stages such as, recovery, re-crystallization, and grain growth.

On the other hand however, we have the tests done by Sakakibara Kōzan that states how different types of folding produce different types of grain orientations and thus different properties on the plate which indeed reacted differently against impact.

(References:

 "The Manufacture Of Armour and Helmets in Sixteenth Century Japan" by "S.Kōzan"

"Phase composition mapping of a 17th century Japanese helmet " by "A. Fedrigo, F. Grazzi,  A. Williams, S. Kabra and M. Zoppi" )




Cross oriented folding - Jūmonji kitae (十文字鍛え)

The process starts by selecting the two (or more, depending on the size) different ingots, one of steel and one of iron.
To determine the nature of the material, the ingots are heated and quenched into water, and then hammered in order to consolidate them; this is the same process used for sword making and it is called mizuuchi (水打ち).
Once the harder steel is separated by the softer ones, the latter is further mixed with hardened steel taken from the yakiba (焼刃) of broken swords, or with normal high carbon steel.
After the two materials are ready to forge, they are heated and beaten into two different plates.

Then the same process start for both plates; they are cut into the middle and superimposed with the grains running in the same way. They are then welded and forged again to obtain the same thickness. This process is know as oru (折る) and since this type of forging reduced the carbon in the material, steel shouldn't go further than 5 times according to Kōzan; this means that unlike with swords, in which the amount of carbon was likely to be higher than 1% for the hard steel, a slightly softer carbon steel was preferred with armor, likely around 0.5%. In fact while a hard steel is good for edged tools, it is not suitable for armor plates.

The Oru process explained in a sketch; on the left the plate, on the right the two halves of the plate superimposed with the grains running in the same direction.

Once the oru process is done, the plate is split again in two halves, but this time they are superimposed with the grains in a perpendicular orientation, resembling a cross, hence the name. When the plate is forged, a complex grain structure should be obtained.
The two  finished plates are then forge welded; the surfaces are grooved chequerwise so that in the hammering the lime and the charcoal have avenues of escape.
In welding the plates, only the iron is hammered; being softer, it is beaten wider than the other. The surplus is trimmed off and eventually recycled for another plate.

Another sketch of the procedures involved; on the top left, the plates are superimposed with grains running perpendicular. On the top right, the grain are aligned in a complex matrix. At the bottom there is the final steel&iron plate when the two are welded together "chequerwise".

Kōzan also gives us insights on how to perform heating&hammering; when the steel or iron is heated, the smith has to be careful: too much heating will make the material too soft, while a low temperature won't allow the iron (or steel) to be forged properly.
Regarding the forging, he uses an infallible criterion in order to judge degree of hammering: bending the material. If the fold looks rough further forging is desirable, while if it looks smooth the plate is done.

(References:
"The Manufacture Of Armour and Helmets in Sixteenth Century Japan" by "S.Kōzan"

"Phase composition mapping of a 17th century Japanese helmet " by "A. Fedrigo, F. Grazzi,  A. Williams, S. Kabra and M. Zoppi"

"Neutron diffraction characterization of Japanese armour components" by "Anna Fedrigo and Francesco Grazzi")


Final Conclusions and Analysis

The reason behind a superimposed layered structure of steel and iron is that the former, being harder could prevent penetration of edged weapons like arrows or spears, while the latter deadens the impact which was a very desirable property against bullets; in fact, this was the method to produce bulletproof armor in Japan known as tameshi gusoku (試し具足).
However it is fair to notice that this method was used only with the top quality armor, and not the munition grade ones; in fact, quoting Kōzan, the hammering and tempering employed during the making burns almost 70% of the starting material, and 18 kg of iron and steel are required to make a single breastplate of 5.6 kg.

This structure is observed on plates used to make breastplates, helmets but also other armors components like the lames used to make the shikoro, the nape guard of the Japanese helmets, or the small plates used to assemble the haidate, the thigh guard as well as the lames used to make the tassets, so it's highly possible that entire suits of armor made with such plates were made for the upper class.
The final thickness of the plate as claimed by Kōzan is about 2 mm, but it could be further increased since there are examples of thicker armor.
Until now, the structure described by the author was observed in several armors that presented different hardness value on the inside and on the outside of the plate, but a different combination made with a layer of hard steel wrapped around two layers of softer steels was also discovered recently.

A tameshi kabuto; the "bullet proof" is located in the middle of the forehead as you can see from the mild depression.

As far as the carbon content of Japanese armor is concerned, there aren't plenty of data. Only a tiny fraction of the few armors analyzed presented such structure, and the sample is too low to make any sort of conclusion.
In any case, while the majority of armor had a carbon content of 0.01-0.2%, making the steel used essentially wrought iron, there are some samples which were made with plates that had 0.4-0.8% of carbon.
In particular, two helmets analyzed that were made with the layered structure had respectively 0.8%-0.6% and 0.4% for the harder outer surface, while 0.3-04% and 0.2% for the inner ones.
So just like the European armors of the period, it is fair to assume that medium to high carbon steel was used for the top tier plates, while wrought iron composed the majority of the munition grade armors.
Being made of lower carbon steel isn't necessarily a bad thing; the softer steel reacts by bending after receiving a strong blow, and the force its absorbed by the plastic deformation.

It was also observed that the steel used for the breastplate, helmets and pauldrons was usually harder (0.6-0.8%) on the outside, while the steel used for limbs armor was softer (0.4-0.5%); the latter plates were better at absorbing shocks like the cut of a sword or a hit from a polearm, while the former were much harder to pierce and performed better against arrows, spear thrusts and similar threats.

A close up of a breastplate in the Okegawa style with several bullet dents; again penetration was prevented by the armor's plates.

Regarding hardness instead, all the plates analyzed were hardened through cold working, and it seems that quenching, unlike with swords, wasn't used in when it comes armors.
This is an anomaly, since it doesn't make too much sense: hardening was used in swords, so it should have been used with armors too.

Few studies have been done in this direction, the most famous is the one of A. Williams, although I would argue that the number of armors analyzed was quite low.
However, his conclusions aren't correct in my opinion.
In his paper, he states that since sword masters and armorers were two different types of artisans, there wasn't a shared knowledge on the field of steel science.
Still, we do know that this was not the case, since there are examples of swords makers that became armorers, we have the tests done by Kōzan highlighting that he was well versed with the kitae used in bladesmithing, and most importantly, armorers were also famous to occasionally produce tsuba (鍔) which are the hand guard of the sword.

Although this last detail might seem odd to quote, it is an important fact: a tsuba of the late 16th century was analyzed by the same author, and this example not only stood out for the high quality of the steel, but also because it was quenched in order to harden it.
This fact proves that even the artisans who made tsuba were aware of that process, and we do know that they were linked to armor makers.

It is possible that to withstand bullets from arquebus, a softer material performed better since we see a similar trend in European armors of the 16th century, with hardened pieces diminishing severely.

Another tameshi kabuto.

Despite the study done by A. Williams, at least one example of Japanese armor was hardened; in fact, in a paper in which William took part, a nodowa (throat guard) of the 18th century presented a layer of martensite, something that requires hardening by quenching. While it's clearly two hundreds years younger than the period discussed here, it is highly possible that the same technique would have been known to the armorers of the 16th century, mainly because there wasn't any type of improvements in armor development during the Edo period.
Moreover, it is quite debatable that they were able to establish the date of such piece, since there were no signature on nodowa, and dating of Japanese armors is a topic on its own (spoiler: it's almost impossible to date a Japanese armor with accuracy unless signature is present). Unfortunately no hardness value were given.

Last but not least, Kōzan in his work explicitly says that if the heating is not high enough, the steel "won't stand hammering or hardening".
According to some members of the Japanese armor association, hardening by quenching was done in the eastern provinces of Japan during the 16th century, but more studies on the topic are needed.

All the armors analyzed had a hardness in between 100 and 322 VHP, which is in line with world armor of the period. We could speculate to see a hypothetical quenched pieces with a hardness of 400-500 VHP but no data were given for that specific sample.

While for quality, there are few examples that stood out for their extremely low amount of slag inclusions in the steel: some pieces (the top tier ones) were made with completely reduced plates that had a % of slag under the 1%.
The high quality of the plates is verified by the performance of the armors in the battlefields, which prevented the major threats of the period and saved the warrior's life in many occasions.



Thank you for your time! If you enjoyed this article, please feel free to share it and for any questions don't hesitate to leave a comment below!

Gunbai

(references :

"江戸時代の草摺に用いられた小札の鋼板製作方法"  by " 釘屋奈都子 , 永田和 宏 , 桐 野 文 良 and  北 田 正 弘"

"室町時代末期から江戸時代の鎧に用いられた鉄鋼材料の製作と加飾" by "釘屋奈都子 "

"Japanese arms and armour and their differences from European contemporaries" by "Alan Williams".

"Neutron diffraction characterization of Japanese artworks of Tokugawa age" by "Alan Williams, Francesco Grazzi, and Francesco Civita"

"Ancient and historic steel in Japan, India and Europe, a non-invasive comparative study using thermal neutron diffraction" by "Alan Williams, Francesco Grazzi, and Francesco Civita"

"Neutron diffraction characterization of Japanese armour components" by "Anna Fedrigo and Francesco Grazzi"

"The Manufacture Of Armour and Helmets in Sixteenth Century Japan" by "S.Kōzan"

"Phase composition mapping of a 17th century Japanese helmet " by "A. Fedrigo, F. Grazzi,  A. Williams, S. Kabra and M. Zoppi" )