I am not an engineer of course so engineering is going to be over my punching weight.
Throwing this out there first, Kenji Mishina and I were looking at newly made Juyo some years ago at the Museum and I asked him what swords he preferred and who his favorite smith was and what he thought of Shinto blades. He pointed at a Shintogo and I said "I like this." About the Shinto blades he said that under his stone he could tell that they were different from the koto blades. They were less pliable and he felt harder, with the exception of Sukehiro. He said Sukehiro's blades felt more resilient to him.
About the hamon then, a few things... you need to first think of the sword in terms of a massive investment in resources if you are going to get a master smith make it for you. Same as today.
Now you are going to be handling that thing with a cloth and gloves and you will freak out if someone spits a little drip of spit on it.
Back then you would cut someone in half with it. Through bone and blood and then you'd maybe be looking for someone else to cut in half.
It was a tool.
This is brutal use and they would be damaged.
You need a few things then in the hamon. You need to provide enough hardened steel so that the blade can be polished. Sometimes when Japanese dealers write emails and they are not so good in english they will say even now, the blade is "being sharpened." But that anyway is good to think about because I don't think back then people would be thinking, "Oh, my blade needs a polish" after cutting a couple people in half. They would be thinking, "Oh, my blade is screwed up from smashing and killing people, it needs sharpening."
They were made with a use projection that included being damaged, being repaired, and being damaged again. You cannot do that with a smallish hamon. So in this you end up with, by necessity, a wide hamon. Too wide and you break.
Being able to make the widest hamon then without breaking is going to be a stunt pulled off by the most talented guy. Also it will be the most valuable sword. Not because it's flashy because you can tell your customer, you can get the most sharpenings out of this thing before replacing it.
Like if you buy a new set of tires and they last one week, they are not as valuable to you as tires that last four years. You would pay more for the four year tires unless the one week tires were special race tires or something. Anyway you get the point.
If you could make a wide hamon then and an unbreakable sword it would also behoove you to advertise that and a little bit of flamboyancy never hurt. The thought that the Ichimonji mei meant "Muteki" in that there was no enemy who could stand against it is also in agreement with this thought.
Why make a midare hamon on a sword?
Because you could. Because the other guy can't. His will break. Yours, you can do this and it won't. Because Muteki.
Because you're just better and this is how you show it.
Now suguba was there always and will be there always and there are maybe indeed some functional differences in that it is ideal in some ways. But if you are good enough and your testing shows that you're getting 98% performance with 500% sex appeal, what do you think your marketing department is going to report to the CEO in terms of next year's product design?
It all hinges in your ability to pull off the stunt.
Clearly the Ichimonji blades did this because they were around a long time.
With all due respect to the various theories about the mongols, we keep hearing this from time to time but it is relatively anecdotal.
The Mongol invasions in Japan were 1274 and 1281.
Ko-Ichimonji is a label placed on certain stylistic developments happening in Bizen around 1200 that are taking place in Fukuoka. There is an overlap with the groups that we go on to call the Fukuoka Ichimonji school. Fukuoka Ichimonji goes on from the early 1200s and there are smiths still working in Fukuoka around 1300. Yoshioka Ichimonji branches from Fukuoka and starts around late middle Kamakura. There are often blades attributed to this time period with remarks that the workmanship looks like Yoshioka or Fukuoka work. The Noami Bon says that Yoshioka work peters out at the end of the Nanbokucho so lasts roughly a century, winding up around 1360. Iwato Ichimonji is a thing of the very end of Kamakura and so goes into Nanbokucho as well. Katayama Ichimonji is from around the time of Fukuoka and goes into the Nanbokucho period as well. Katayama work is as flashy as Fukuoka work. Katayama Ichimonji Ietsugu, Fujishiro has him at 1368.
Note in the late Kamakura into the Nanbokucho as well that Aoe made flashy hamon. At the middle and end of the Nanbokucho of course Hiromitsu and Akihiro maxed out the concept of "how far can we go" in terms of hardening the surface of a blade. Chogi and Kencho as well made super flashy flamboyant pieces.
Winding back to the late Kamakura, though Rai Kunitoshi started making calm pieces his very last dated work is 1321 and is done in his Niji Kunitoshi style with a wide hamon and very similar to the Kuniyuki on my site. Rai Kunimitsu made this style from time to time and so did Rai Kunitsugu into the mid Nanbokucho.
There is unbroken evidence of flamboyant midare hamon being in use from the middle Heian all the way through to now.
There is no full stop experience anywhere that made midare hamon no longer be used.
The Ichimonji smiths did not fall because of the Mongol invasions. Yoshioka kept on for almost a century after.
My own personal theory on Ichimonji is that the school spread both because of demand for the product and because of exhausting local resources. If you go to the river for iron sand consistently every day of the work for 40 years you may indeed run out of the best quality of ore you're looking for. If you are already pressed for resources and your school is growing and you have more customers, well you might send the sons out to establish their forge somewhere in the region but not in your back yard. Somewhere they could find again good local resources, and not compete with you for your dwindling supplies. Eventually everyone extinguishes as the best resources are used up and then product quality starts to fail. Sooner or later (this has happened time and again through history, some of us exist here because grandpa and grandma back in the old country bailed the hell out for brighter horizons) you might pack up and move too.
Either to where the better, untapped resources are, or to where the action is for selling your product.
So: honestly, I think this mongol-invasions-killed-midareba holds no water because midareba was not killed, Ichimonji was not killed, and there is no direct evidence for any of that and all the direct evidence is that Ichimonji kept on keeping on. If there is any document from the Kamakura period that says Ichimonji swords are bad because we can't kill mongols with them I would like to see it.
But what does happen is that Bizen remains dominant for 500 years of master smith after master smith after master smith and their reputation is untarnished until today. Midareba is an essential part of what Bizen is. 40% of all Juyo Token are koto Bizen swords. Half of all Tokubetsu Juyo are Koto Bizen Swords! Half of all Jubun and Kokuho are Koto Bizen Swords! This is not a sign of failure, this is a sign of mind blowing success. Midareba was there for all 500 years in Bizen without pause.
This goes for the ikubi kissaki theory that is thrown around a lot as well. No direct evidence and as well, ikubi kissaki as iconic as it is, is not on every blade and chu-kissaki is quite common through all the ages. Chu-kissaki is the suguba of kissaki and people continued to depart from that design only to return to it. I think that is the beginning and end of the story on ikubi-kissaki.
Anyway back to the midareba, I'll just repeat again that basically the fact that your swords could look like that and not shatter is why you would do it.
Anyone here know what a tourbillon is?
In summary it is a super technically difficult thing to make which helps a watch keep better time, and it is also completely unnecessary as well as being completely and incredibly overcomplicated and has extremely low benefit compared to the work and sophistication required to achieve it. Especially with pre-modern era tech. Is is internal guts of the watch.
The tourbillon is considered to be one of the most challenging of watch mechanisms to make (although technically not a complication itself) and is valued for its engineering and design principles.
Think about why you would want to put a tourbillion into a watch. This over engineered, difficult to make thing that may probably not even show noticeable benefit?
Why do any of these things?
Why go to the moon?
Why choose this as our goal? And they may well ask, why climb the highest mountain? Why, 35 years ago, fly the Atlantic? Why does Rice play Texas?
We choose to go to the Moon! ... We choose to go to the Moon in this decade and do the other things, not because they are easy, but because they are hard; because that goal will serve to organize and measure the best of our energies and skills, because that challenge is one that we are willing to accept, one we are unwilling to postpone, and one we intend to win.
If you can make a functional tourbillion, if you can send a man to the moon and bring him home again safe, if you can make a sword that won't bend or break and send the hamon to the shinogi like fire, you are the best. You do it by choice, not because it's easy, but because it's hard. And if you succeed in making something functional, not just a fancy toy, everyone who is anyone will know you are the best at what you do.
When you throw that gauntlet down, you can stand and look around, and very few will pick it up.
That's why midareba hamon.