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A Katana needs to be made of steel folded 1000 times, according to eHow:

Pound the tamahagane into two separate blades. These need to be heated, pounded by hand, and folded 1,000 times each to remove the impurities. Pound one of them another 1,000 times to make it harder than the other.

But, according to simple mathematics and physics, this would mean that a Katana is made of 21000 layers, or 10 followed by 300 zeros!

Note that this doesn't really mean it was not done, though.

So - were Katanas really made of folded steel? How many times was it typically folded?

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I think they meant 1000 layers which would only mean 10 folds – ratchet freak Feb 5 '12 at 13:10
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It doesn't seem a necessary thing that the every bit of steel is folded in each fold. In fact, that's part of the process - they would have patterns folded into them. Wikipedia suggests up to 65000 layers. – Jivlain Feb 5 '12 at 13:33
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ehow has got to be one of the worst sites on the web. – Sam I Am Feb 5 '12 at 15:04
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I seem to recall that modern metallurgists figure that between 10 and 20 folds is optimum depending on a lot of things (metallurgy being a ridiculously complicated business), and that the best Japanese sword smiths had come very close to the optimum by craft methods. – dmckee Feb 5 '12 at 17:19
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Documentary on Disc or NGC fa few years ago followed Japanese swordsmiths and showed their work. They fold the metal 10-20 times (can't remember exactly, might depend on the smith and/or the customer/purpose of the weapon). Would make this an answer but can't find a video. – jwenting Feb 6 '12 at 7:00
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up vote 36 down vote accepted

According to SamuraiSwords.com, there are two folding steps, in first traverse and longitudinal folding is repeated:

The difference between these two folding procedures lies on that the small hammer is used to fold in transverse folding while the big hammer is used in longitudinal folding. Generally, about 12-15 folings are repeated.

In second (optional one):

another forging step can be added (Age-kitae). Tamahagane is forged again as a shape of stick once more and cut uniformly about 7.5cm long. This age-kitae is an optional step and sometimes omitted. A tight microstrucutre and hada (grain pattern) can be expected after this step and the uniformity of the carbon content is improved.

Likewise the shita-kitae, tamahagane is heated and folded 7 to 8 times.

Of course this is just process of creating 2 or 3 grades of steel (hard, soft and optionally medium). Then they are combined, making greater total number of layers, how many depends on the style:

katana fabrication

As for "1000 folds"...

shita-kitae, is repeated from 8 to as many as 16 times. After 20 foldings, (220, or about a million individual layers), there is too much diffusion in the carbon content, the steel becomes almost homogenous in this respect, and the act of folding no longer gives any benefit to the steel.

(source: Wikipedia, referencing A History of Metallography by Cyril Smith - The MIT Press 1960 Page 53-54)

Also, anything above 25-26 folds would make no sense at all, as by then the layers' thickness would have to be less than radius of iron atoms, which is 126 picometers (= 1.26 × 10-10 meters)

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The argument about how thin the layers get is irrelevant: they don't remain distinct when folded. The whole point of the repeat folding is to homogenize the metal and encourage surface contaminants/additives to migrate into the bulk metal improving its properties. – matt_black Apr 5 '12 at 12:02

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