Motokasane: how important for structural integrity?

[jdawg221]

Hey guys, barely post here, but thought I would start a thread regarding a question that I've been thinking of lately. So I have a modest nihonto collection, mainly of lower end old polish pieces in edo koshirae, some hozon pieces mixed in, and something I have always wondered is in what way does motokasane specifically affect durability. Recently picked up an old koto(presumably bizen) mumei daito in the 68cm nagasa range with a motokasane of 5.5mm, which honestly seems rather narrow for the time. Of course I do not have it in my hands yet so it may have a high shinogi and 5.5mm could just be the mune measurement, but comparing it to my hizen yoshifusa, Noshu ju kanenobu, Fujiwara Yukinaga, even my yoshikatsu signed shoto, all of those are above 7mm for motokasane. So basically what I am wondering is if there was an agreed upon ideal for what a motokasane should be. I frequently see blades with 7mm-7.5mm, so I would assume that was a very common middle ground? 

[Mister Gunto]

For a 68cm Nagasa, if the Motokasane is 5.5mm, it's probably safe to assume the blade has been through several polishes, and is getting thin. In my (limited) experience, usually 9-7mm is more common for katanas. 

 

I can't say if a blade like that would've been considered for combat in the old days. But it'd certainly be fine for Iaido practice. 

[Katsujinken]

5-7mm is average, and strength at the motokasane is very important due to the concentration of stress right at that point when the sword meets resistance or some kind of impact, just above where the hands hold the tsuka. 
 

That said, swords were generally designed for a purpose, for example armored combat (much more kinetic) vs unarmored combat (more of a slicing action needed), so you’ll see variations. 
 

An engineer could certainly explain this better than I can though! 

[Katsujinken]

2 minutes ago, Mister Gunto said:

For a 68cm Nagasa, if the Motokasane is 5.5mm, it's probably safe to assume the blade has been through several polishes, and is getting thin. In my (limited) experience, usually 9-7mm is more common for katanas. 

 

I can't say if a blade like that would've been considered for combat in the old days. But it'd certainly be fine for Iaido practice. 

With respect, I disagree. 9mm would be on the extreme end for a motokasane, far from average. 

Most brand new swords made for iai and tameshigiri are generally 5-7.5mm at the motokasane. 

[jdawg221]

4 minutes ago, Mister Gunto said:

For a 68cm Nagasa, if the Motokasane is 5.5mm, it's probably safe to assume the blade has been through several polishes, and is getting thin. In my (limited) experience, usually 9-7mm is more common for katanas. 

 

I can't say if a blade like that would've been considered for combat in the old days. But it'd certainly be fine for Iaido practice. 

You would think, weirdly enough the blade does not look that tired. Boshi is relatively healthy, pic related. Part of me is curious if it was just measured incorrectly, since I am aware this definitely happens. My kanenobu was advertised as an 8mm motokasane, however upon measuring it its more around 9mm.

[Mister Gunto]

Jonathan, looks like it's still a nice blade. I don't think nihonto are considered tired until they start showing core metal. 

 

Michael, I don't see nearly as many blades at 9mm as I do 8mm and less. But I have seen a few in person and at least listed as such on Ebay. I do have 2 swords in my own collection with 9mm Motokasanes. One's a Wartime Gunto, the other a (possibly) Koto Aoe Tachi.

 

Unfortunately, on the latter, Bubba got to it before I did, and scrubbed the blade, so it's just bright metal at this point. Trying to do more research on it to decide if it's worth the restoration cost. 

[Jacques]

 

There are no standards for the kasane; it depends on several factors: school, era, smith. For example, Kanesada (Nosada) made swords with a thin kasane, contrary to the fashion of the time.

 

Quote

 I don't think nihonto are considered tired until they start showing core metal. 

Provided they have shingane...

[Tohagi]

Hello,

One way to spot if yhe blade is been "over polished" is if a polishing bolt appeard just after the habaki at the begining of the ubu nakago where the blade may be thicker than at the moto. For example motokasane 6mm and "boltkasane" 6,5 or 7mm. The delta is missing steel. 

If you have shallow or no bolt, the blade was probably intended like that.

[Hoshi]

Hello, 

 

The kasane per se does not inform you on the state of the sword. 

 

The delta between the top of the nakago and the kasane is the indicator you need to pay attention to. 

 

There are smith-level trends, school-level trends, and most important: era-level trends on the thickness of the kasane. The most well-known is the transition between Kamakura and Nambokucho, where Nambokucho swords have - on average - a much thinner kasane. 

 

As for the structural aspect, remember that it interacts with the thickness and level of the shinogi, the niku, and the width of the motohaba/sakihaba. Controlling for these factors, a thicker kasane indicates a sturdier sword, all else equal, but it is first and foremost a balance of different geometrical elements. 

 

Also take note that skilled smiths created a balanced descending gradient between the motokasane and the sakikasane which is one of the most important contributor to the feeling of balance. This descending gradient is generally not uniform - the gradient tapering accelerates towards the tip. This type of geometry is often followed by Soshu-Joko smith (Masamune, Norishige, Yukimitsu, Go, Sadamune), leading to an impressive feeling of balance on a robust and masculine sugata, and this generally drives the contrast with shinto/shinshinto smiths, whose blade tend to feel 'tip heavy'. 

 

Koto blades with a thick motokasane (7.5mm+) are considered a plus and can contribute to passing Juyo, and this is generally a strong point. 

• 5.5-7.5mm: normal kasane
• below 5.5mm : thin kasane
• Above 7.5: thick kasane

Best,

 

Hoshi