How deep is the hamon?

[Mikaveli]

For Nihonto in general, how deep is the hardened area of the sword?

 

I'm just curious about the potential difference in strength of some of the more decorative / variable hamon I've seen.

 

For example, with all other things being equal, would a straight hamon, a third away from the edge be measurably less brittle than an undulating wave pattern that approaches two-thirds in places?

 

Don't worry, I'm not planning on doing any actual testing - I'm just interested in how much the form would have affected function.

[Katsujinken]

The conventional wisdom is that the harder steel of the hamon is more brittle and therefore a “deeper” hamon is less optimal for combat because it makes the blade more likely to take damage that can’t be repaired or fail entirely. 

The hamon as a statement of art was less of a concern (in general) prior to 1600. That’s not to say it wasn’t something important and purposeful, but it is fair to say that most blades were made for fighting and that consideration extended to the hamon. That’s not the same as saying most blades that have survived today were made for fighting. By definition there is some survivor bias there (plenty of masterpieces came out of the forge and never saw combat, and these may feature “non-combat hamon” more

often), so some nuance is needed here. 
 

Nakamura Taisaburo wrote, “ Between a third and a quarter of the blade’s overall width is a suitable depth for the hamon on a sword for practical use. In general, blades with flowery and deeply applied hamon have a tendency to be brittle and those chip or snap easily.” (The Spirit of the Sword)

[PNSSHOGUN]

Some historical observations:

 

http://www.nihontocraft.com/Suishinshi_Masahide.html

 

http://www.nihontocraft.com/Aratameshi_Nihonto.html

[Mikaveli]

8 hours ago, Katsujinken said:

The conventional wisdom is that the harder steel of the hamon is more brittle and therefore a “deeper” hamon is less optimal for combat because it makes the blade more likely to take damage that can’t be repaired or fail entirely. 

The hamon as a statement of art was less of a concern (in general) prior to 1600. That’s not to say it wasn’t something important and purposeful, but it is fair to say that most blades were made for fighting and that consideration extended to the hamon. That’s not the same as saying most blades that have survived today were made for fighting. By definition there is some survivor bias there (plenty of masterpieces came out of the forge and never saw combat, and these may feature “non-combat hamon” more

often), so some nuance is needed here. 
 

Nakamura Taisaburo wrote, “ Between a third and a quarter of the blade’s overall width is a suitable depth for the hamon on a sword for practical use. In general, blades with flowery and deeply applied hamon have a tendency to be brittle and those chip or snap easily.” (The Spirit of the Sword)

Interesting - is the a trend towards more decorative hamon in the Tokugawa period then?

 

I hadn't heard of Nakamura, but his comments matched my assumption.

[Shugyosha]

Shinto and shin Shinto swords tend towards a deeper hamon (sometimes only as a result of fewer polishes). They can become more elaborate and not aimed purely at creating an efficient and reliable weapon. 

 

There are smiths specialising in sudareba and toranba, and others incorporate designs into the hamon: Kiku sui, a dragon chasing the Pearl of wisdom, moon over Mount Fuji and others. The long peace both reduced the demand for new swords so smiths had to find ways to market their products and fuelled a demand for novelty. 

[ROKUJURO]

From a technical standpoint, it may be of interest that carbon steel of the kind used in Japanese sword blades will harden completely within a depth of 4 mm into the blade from each side. This means that an 8 mm thick blade will (theoretically) harden completely through its cross-section. Bearing this in mind, we can assume that in the usual YAKIIRE process, all blades are hardened completely in the cutting edge area as far as the high-carbon steel is present there.

Now it might be interesting to look at the construction of blades as these are mainly made from different kinds of steel resp. of steel with different carbon content. What is often described as "soft steel" is in fact steel with a very small amount of carbon. Scientifically, steel with less than 0.22 % of carbon will not harden at all. In practice, a minimal carbon content of about 0,30 to 0,35 % will make a steel useful for simple tools, while a cutting tool will retain an edge with a minimal carbon content of 0,4 - 0,5 % of C. For comparison, a good spring-steel will have a carbon content of about 0,55 to 0,7 % of carbon nowadays.

The amount and position of the hardenable steel in the HA will have a great influence on the resilience of a blade, so one has to look at the cross-section of it. It is there where most of the "secrets" of the individual swordsmith are found! Another important factor is the homogeneity of the sword steel and the perfection of the many welds a steel will undergo before being stretched out to form a blade. What we see as WARE or FUKURE in a blade are always imperfections of the welding processes which show up in the blade earlier or later in the polishing/grinding process. These are not KIZU caused by use like SHINAE. HAGIRE on the other side are KIZU caused by unbalanced tensions in a blade, and they can show up directly after finishing a blade or later as result of use.

The last point of resistance of a blade against breaking is NIKU. This aspect of cross-section has great influence on the overall performance of a blade. A high amount of NIKU will produce a good breaking resistance while at the same time prevent it from being a good cutter.

Coming back to Michael's initial question, a lot of factors can influence the breaking strength of a blade. There are theories that SUGU HAMON are not necessarily 'stronger' than O-MIDARE or high CHOJI, as the ASHI e.g. might work as stress relievers. In the end, the inner construction of a blade - especially the thickness of the high carbon steel layer of the KAWAGANE - will have more influence than the pure width of the HAMON.  

[Katsujinken]

Fantastic responses from Jean and John. One important distinction/clarification for Michael from the UK: when Nakamura Sensei and I mentioned "depth" of the hamon, we were referring to its "height" from the ha to towards the shinogi.

 

Other references after that to actual blade construction (i.e. the arrangement of kawagane and shingane, how "deep" the hamon "penetrates" blade structure, etc.) are also extremely important to understand, but these are distinct ideas.

 

 

[DoTanuki yokai]

http://www.ksky.ne.jp/~sumie99/effectofyakiire.html

 

 

[Mikaveli]

On 1/27/2022 at 12:15 PM, ROKUJURO said:

Coming back to Michael's initial question, a lot of factors can influence the breaking strength of a blade. There are theories that SUGU HAMON are not necessarily 'stronger' than O-MIDARE or high CHOJI, as the ASHI e.g. might work as stress relievers. In the end, the inner construction of a blade - especially the thickness of the high carbon steel layer of the KAWAGANE - will have more influence than the pure width of the HAMON.  

 

Interesting, I hadn't really considered the jacket / lamination layers of different steel.

 

So, a deep (wide) hamon on "monosteel" might produce a weak blade, but if only the outer steel is high-carbon, the overall structure isn't measurably weakened.

 

So, next question is are the layering styles documented for the various smiths? I've (so far) only seen Masamune's "Soshu Kitae" style.

[ROKUJURO]

Michael,

this can be found in the internet:

 

Lamination methods (2).doc

[Novak77]

" The sword that has nie (thickly clustered batches of particles of tempered metal in the. blade), and who's tempering along the cutting-edge (yakiha) comes up nearly to the longitudinal ridgeline (shinogi), is a beautiful thing to see, but in practical terms, is easily broken."

 

-Otake Sensei, The Deity and the Sword.

 

My interpretation is, large, and wild hamons look cool, but don't belong on the battlefield.

 

Cheers