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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. – Joel Rein 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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    You could fold the metal 1000 times without the result being an unfeasible number of layers: the layers merge slightly on folding and don't remain strictly distinct. This could, in principle result in changes to the alloy crystal structure and chemical makeup. The 1000 folds might be required as the changes only happen slowly. The argument about the resulting number of layers is irrelevant. – matt_black Feb 12 '12 at 16:54
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    @Loren Pechtel: I'm neither a metallurgist nor a swordsmith, but perhaps the repeated folding and stretching alters the microstructure of the metal? As a parallel, consider that when kneading bread I fold it roughly 300 times, a process which forms gluten into long chains, and allows the bread to rise: foodreference.about.com/od/Tips_Techniques/a/… – jamesqf Apr 2 '15 at 18:06
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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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    You could add that folding it 10 times would imply 1000 layers. – ChrisW Nov 25 '14 at 22:12
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    1024, if you're going to be really pedantic. :) – GordonM Apr 1 '15 at 9:50
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    @matt_black While they may not be distinct down to the individual layer, you can get interesting patterns of at least some of the layers from acid etching a folded sword, IIRC. – SGR Feb 24 '17 at 15:26

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