The Making of Samura Swords (and chisels?)

[Brian Kent]

Last night on the History Channel was a fascinating episode on the metallurgy of making Samurai Swords. Unfortunately, I do not know the title of the episode. Besides being interesting in itself, I was watching with the mind of a woodworker and thinking about the making of Japanese chisels.

The key idea was the combination of softer, more resillient low-carbon steel in the back and core of the blade, with harder, but more brittle high carbon steel on the cutting edge and surface. I was also amazed to see that the curve of the samurai sword blade is not pounded or ground to shape, but comes from the quenching process of putting a 1700° blade into cold water, with the softer steel contracting faster than the harder high carbon.

Somewhere in the past I read that the decline of demand for fine Samurai swords led to the rise in excellence of high end Japanese chisels.

Does anyone know if this is fact or fiction?

Does anyone know what the title of that History Channel episode was?

[Pam Niedermayer]

I saw it on PBS, Nova, called The Secrets of the Samurai Sword. Very interesting and well done, you should be able to catch it on PBS this weekend.

Most sword makers switched to tools when the sword making was outlawed.

Pam

[John Timberlake]

I saw the same episode on the History Channel Tuesday night. Very interesting, especially the part about the curve. On Wednesday I saw an episode of Myth Busters where they tried to break swords. When they hit a Samuria sword with a two handed sword, it only put a knick in the edge. However, the Samuria sword bent into the shape of an S when hit and then came back to its original shape. Hence the flexibility of the low carbon steel along with the hardness of the high carbon.

[Bill Brehme]

I also saw it on Nova. Excellent program... Nova is my absolute favorite program period! Weren't they saying that those hand crafted and forged swords were selling for around $25K ? Or was it $250K? I don't remember... If thats the case, I doubt I could afford a set of chisels. Plus I doubt I could have the patience to polish them w/ a stone the size of a grain of rice!??

[Brian Kent]

Yes, Bill they were talkng about hundreds of thousands of dollars for the best, most perfect swords - probably antique also.

I doubt if there would be the same kind of thrill doing a special on making chisels with the same methods. But if they did, I would love to see it.

[Dave Burnard]

Carrying swords was banned in the 1870's, and swordmaking was banned not long after. This happened around the same time that relatively cheap (compared to sword steel) european steels became available in Japan (at the behest of our gunships BTW). Some swordmakers did make the switch to woodworking tools (and other forms of metal work) more than a century ago.

Swordmaking was revived for WWI, then died again, and was revived again for WWII but by then all but a few actual swordmakers/apprentices were still living. In many ways, the sword making tradition had to be re-discovered when it was revived as a government sponsored art form in the 1950's. On the other hand, the blacksmiths never stopped making planes, chisels, saws, etc.

[Pam Niedermayer]

Hi, Dave, long time no see.

Recently I saw a really good movie called The Twilight Samurai directed by Yoji Yamada (or Yamada Yoji, never know what the correct order is). There are only a couple of fights; but in preparing for the second, Seibei sharpens his short sword before the fight in a somewhat masterly manner.

Pam

[Kyle Kraft]

There was a Modern Marvels episode that I recorded on the DVR recently called "The Sharpest" or something like that. The first story on the episode covered the Samurai Sword manufacturing techniques and the design particulars that made it such an effective killing device, designed to slice a torso diagonally from the neck to the opposite side waist area.

[Ron Petley]

I am finding it hard to believe that the quench would change the shape of the sword, but who am I to question the sectets of the masters. I would think a well normalized blade would be very stable before and after quenching, that is the reason for normalizing. The blade is heated and cooled to release any stress created in the steel from forging. This will hopefully keep the blade from changing shape when quenched, (rapidly cooled to changet the crystal size of the steel).
But again I could be wrong since sword making is a extemly exact process, made by teams, maybe someone else with more knowledge could post on this topic. I will have to catch it on PBS, sounds great. Also it would be great to cut some wood with a chisel you made yourself.
Cheers Ron.

[Marcus Ward]

The reason the quench changes the shape is because the back is wrapped in clay so it doesn't cool as fast as the edge. It's rather ingenious because the curve also makes it cut better. A samurai sword isn't a chopper, it's a slicer. The method of the stroke is designed to take advantage of that and it cuts very clean. I did some tameshigiri (test cutting) on a melon once and was able to achieve that movie-perfect stroke, pause, then a slow slide as the melon slid in half. I've been studying iaido for a couple of years so I get to actually use the swords.

[Ron Petley]

Yes the back is coated in clay, the nature of the clay and its application is very important to the outcome, ageed. Brian mentioned the back is made of diferent steel than the cutting edge and this caused the curve and not a clay coating. Might be both?
My lack of understanding migh be on how much the blade curves as a result of the quench. In my mind something tells me that if the blade curved, the cutting edge would have to get longer, bigger curve longer the edge, I think this is the key to the cutting, longer edge more cutting edge, or this is a element of it anyways. So it would follow that the edge would crack or be streched at the instant of quench. I can not see the blade edge streching as at the heat required to quench it is not hot enough to strech without cracking. A lengething of the cutting edge small enough not to break it would be very very small even over the length of a sword. However this change might be easily noticed under the eye of the master blade maker.
So I am not saying it does not happen, But I am not understanding how it happens, not surprising since I am not a sword maker. Great food for thought though.
Cheers Ron.

[Ron Petley]

Well I lloked up the PBS show,Nova, they clearly stated the quench curves the sword, one in 3 were lost from this process, must be true Nova is not a fiction based show, hence I must eat crow.
Cheers Ron.

[Brian Kent]

Now I want to see it again to listen to what causes the curve.

I think I remember hearing that several things are happening at once. The softer metal is in the core and back - a wedge shape in cross section. The harder metal is in a V that forms the cutting edge and side of the sword. At least two layers of clay are spread - finger painted - on the length of the sword, with the cutting edge not covered.

The entire sword is then heated to 1700 degrees, then quenched in a long tray of cool water. Whether from the clay or the different chemical composition of the two metals, (or both) the back and front shrink at different rates and the sword goes from straight to curved in an instant.

[Marcus Ward]

Ken Meltsner on the oldtools mailing list explained it better than I can paraphrase it so I'll paste it here:

* When quenched in water, the front edge turns into martensite (the hardest form of steel) and expands about 4%. This is OK because the back of the sword is still quite hot and soft. Martensite is only meta-stable, by the way, and forms when austenite is cooled so quickly that the atoms can't shift around fast enough to reach the form stable at lower temperatures. When you temper martensite, some of it transforms to that lower temperature form.

* As the sword continues to cool in the water, the steel continues to transforms towards the back of the sword. As it transforms, it expands. At some point (around the middle), the steel no longer transforms into martensite and changes, instead, into the stable lower temperature form: pearlite (layered ferrite and cementite, IIRC). Pealite is softer and tougher than martensite. The layered structure is really good at stopping cracks from getting larger.

* As the sword cools further, the heat from the back edge of the sword tempers the martensite at the front edge, making it tougher (and a little softer, but not much).

* Since the leading edge of the sword has already cooled and hardened, it's put into compression as the back edge cools and contracts. (thermal expansion in reverse...)

[Ron Petley]

If google Nova PBS and look up the program, click the little launch interactive. I would post the link but I do not seem to be able to.
They show a photo of the blade just before quenching, it is not straight but curved, so it is not going from straight, to quench, to curved. But it must be curving some more without cracking or breaking, amazing feat in its self.
Thank you Markus for the info, clearly you know what is happening in detail.
Cheers Ron.