RICHARD ABMROSE: Now if you're serious about cooking then a really sharp knife is a must and if you look after it then it can last you years, but what about a blade that lasts for a thousand years that's as sharp today as it was when it was first made, for that you need to go to Japan.
NARRATOR: The holy grail in sword making is having a steel which is hard enough to hold an edge but flexible enough to survive heavy combat. The Japanese got round this problem by folding the steel up to fifteen times which knocked out all the impurities.
They also often used different grades of steel for different parts of the blade, typically using a hard steel for the cutting edge and a softer more flexible steel for the spine. And finally they'd control the tempering of the blade by covering the hot sword in layers of clay, the less clay the faster it cools and the harder the edge.
JONNY PHILLIPS: Now we've seen in countless martial arts movies how samurai swords hack through limbs as if they are made of butter, but are they really that sharp? We're gonna find out.
NARRATOR: To assist Jonny with this task we've recruited the help of historic weapons enthusiast Andy Dean.
JONNY PHILLIPS: Right so what have we got here Andy?
ANDY DEAN: Well this is a matt soaked... As you feel it's quite damp too, so it's very solid. Basically if you didn't have any peasants or prisoners to practice on this was second best, either that or green bamboo, the idea being... obviously you can see the thickness or it and you can feel the weight of it...
JONNY PHILLIPS: What's that to replicate someone's arm.
ANDY DEAN: Yeah neck arm.
JONNY PHILLIPS: Really.
ANDY DEAN: Yeah. I mean but you'll see that it's not much of a problem either way.
JONNY PHILLIPS: So it's gonna go through that? Because that really does feel quite dense.
ANDY DEAN: With my lack of ability I'm sure the sword will compensate for me and we'll be able to do this.
NARRATOR: Now if you missed that here it is again. Samurai swords wow.
JONNY PHILLIPS: Wow Andy that was really impressive.
ANDY DEAN: Thank you.
JONNY PHILLIPS: That's gone clean through that really is a clean cut.
ANDY DEAN: Well you can see the design the curve of the sword and the steel that's in it makes it a fantastic clinical weapon.