Comments for What is the difference between katana and shinai cutting?

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Mar 31, 2010
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starstarstarstarstar 		Why layering metal has little connection to curving
by: Z (a Dutch kendoka, medieval european martial artist and aerospace engineer)

On the inside of a katana (or any sword of above average quality for that matter) you have a metal that was heat treated differently than the metal on the outside. The ores for these metals are commonly the same but since the inner metal hasn't been blasted and folded like the outer metal, it's a lot softer then the outer metal. This type of weld for katana is called 'kobuse' (see 2nd link).


This "composite" structure of a good sword ensures that it is flexible yet can hold a good edge. It has less to do with the quenching process. Sure, the inside will cool slower than the outside, but it is still the same type of metal. The only difference between the two is the crystalline structure.


To get the curve in the blade one must force a radically different cooling time across the height of the blade (from edge to back or 'ha' to 'mune'), this is done with clay, and also by having the edge thinner than the back (thick metal cools slower than thin metal obviously).


Having said that, there are also swords (and katana) that are forged out of a single piece of heat treated metal, for katana this structure is called 'maru' (according to the chart linked to below). Katana's made like this are still curved. These basically are the cheapest types of swords since they are the easiest to make as they require no welding of different layers of metal.


The very best swords can have more than 8 pieces of metal of different alloys welded together into a single blade.


an example of a pattern welded knife that consists of many pieces of different metal.


a picture showing various types of layering used to forge katana.

Kendo-Guide.Com: Thanks for the detailed explanaiton.I will study what you wrote here in Japanese. It is much easier now. Again, thank you for contribution!
Mar 24, 2010
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starstarstar 		Curved katana
by: Z (a Dutch kendoka)

The reason a katana is curved has to do with the forging process of the katana.
At some point during the process the smith will apply clay to varying degrees of thickness along the blade with the 'ha' having a different thickness of clay than the rest of the body of the blade. This causes a different cooling time for the ha and the rest which causes the blade to curve when quenched in water.


The reason -to- curve a blade is so that the cutting power will increase. It is easier (to some degree) to cut things with a curved blade than it is with a straight blade. It is also easier to take out of the scabbard or 'saya' single-handedly. It probably doesn't come as a surprise when I say the katana, like most other curved blades, originally was a cavalry weapon. (Which is supported by point 2 of the answer to the question)

Kendo-Guide.Com: Thank you for your comment. Still researching in depth! :)
Mar 24, 2010
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starstarstarstarstar 		Curved Sword
by: John Maisonnueve

Imafuji Hiro recommended a DVD on Japanese sword making, I believe it's the PBS program and it is indeed excellent in explaining how a katana is made. 

The DVD does explain, if I remember correctly, that the curve feature of the blade is caused by using actually two qualities of ore; one is inserted in the inside of the blade and the other is the outside. 

The Japanese sword is recognized as having the both the quality of very sharp and hard and also to a certain degree soft (i.e., it does not shatter when contact is made, if it was only hard). 

Using both ores creates this feature of hard i.e., sharpness and to a certain degree flexible (doesn't shatter like glass).  The inside ore creates this flexible quality.

Now during the process of making the sword, when the sword is heated, it is inserted in cold water.  That's when the curve is created; the inside ore cools at a different degree that the outside ore and hence 'pulls' the entire blade in a curve.

Sorry for the unscientific explanation but that's basically the process that creates the curve.  Again, the DVD I mentioned explains this whole lot better so I highly recommend it. 

Kendo-Guide.Com: Thank you for your comment. I am still researching :)

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