After a period of sabbatical and reflection, Amakuni and his son went about developing a superior weapon. The next summer when the Emperor returned from another battle, none of the swords were broken and the Emperor passed and said to Amakuni, “you are an expert sword maker, none of the swords you made failed in battle”. What Amakuni is said to have developed is a style of sword that was hard enough to hold an edge but flexible enough not to break or shatter, the paradoxical combination needed for a good blade.
The Japanese did this by creating a sword from two different types of metal. The core was made of steel with a high carbon content.This makes the steel very hard but very brittle. Folding it over itself over and over again to create steel with up to 30,000 laminations strengthened this hard core. Around this hard core, softer and more flexible steel was wrapped. The two steel were welded together and the edge was tempered (tempering created the Hamon on the blade). The end result was a blade with exceptional edge holding and cutting ability. And with the flexibility to withstand continuous contact with other weapons or armour on the battlefield. This is how the unique nature and shape of the Japanese blade is believed to have been created
The myth of blood grooves.
Why do blades have blood grooves? Answer: – to allow air or blood to escape from a STABBING attack because it will enable you to with draw the blade from the body.
However the samurai sword was made to cut or slash not stab. So what are these grooves for ?
Answer: – The grooves (hi) exist for 2 reasons.
1) To reduce the weight on the heaver blades making them easy to deploy. By using this method the balance was undamaged.
2) Grooves were also used to cut defects out of the blade occurring from the forging process. This could happen to the best sword smith on occasion.
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You said: “Why do blades have blood grooves? Answer: – to allow air or blood to escape from a STABBING attack because it will enable you to with draw the blade from the body.”
Incorrect. “Blood grooves” are properly called “fullers.” They are there to increase the strength of a blade while lightening it – much like an I-beam in construction.
http://en.wikipedia.org/wiki/Fuller_%28weapon%29
So you’re saying the core was hard and brittle, and the metal wrapped around the outside, which is the metal that would form the actual cutting edge, was soft and flexible?
This is basically backwards. The core and spine are made of the more flexible steel; the harder (and therefore more-able-to-take-a-sharp-edge) steel is wrapped around the the core to form the edge.
A fuller, or blood grove, in European swords had nothing to do with preventing suction. The main purpose was to lighten the blade and, more importantly, strengthen the blade by changing the blade’s moment of inertia.
You’ve got it backwards: The hard carbon steel is on the outside, the softer, less-brittle steel forms the core.
Tamahagane is a specific type of steel that the Japanese use for quality swordmaking. They add carbon to it by slow cooking large blocks of the raw iron.
Once the iron is properly carbonized it is separated in to hard and soft batches. Soft batches are used for the core of the blade allowing it to bend without breaking. The harder iron is then used to wrap the softer iron and holds the edge.
The distinctive design on the blades is created from the clay used in the tempering process. The lines represent thickness and heat protection.
The blade shape is caused by the quenching process. The rapid cooling of the blade would cause it to bend back on itself. Unlike European sword makers the Japanese place swords in the water tip first slowly, not the full length of the blade.
The Japanese did not use fullers to decrease the weight of the sword. Rather fullers were added in order to increase the strength of the sword. A proper fuller is compressed into the metal of the blade, reinforcing the back of the blade. A cut fuller weakens the blade. The origins of the fuller in Japan are often disputed, but many believe that it started with swordmakers who did not use tamahagane and spread from there. Often fullers are added to smaller blades simple for looks.
Im pretty sure the hard stuff is on the inside, as that part forms the edge of the blade.