A samurai's sword is his most sacred and prized possession. Not just did the samurai depend on his sword to protect him, but spiritually the sword held greater significance as the samurai actually believed his soul inhabited the sword. So that it may come as no real surprise that the same discipline and respect where the samurai wielded his sword, went in to the actual making from the sword itself.
Swords weren't simply 'cast' in a mould after which sharpened. A Japanese samurai sword is made by a complicated procedure for heating the steel, hammering it flat, then folding it, then hammering it flat again, and folding. This process of repeated hammering and folding would be done up to as much as 30 times, or until the maker was satisfied it had been done properly.
There are a number of reasons for this labour-intense procedure. Firstly, any air pockets which might develop during the heating of the steel would be eliminated. Having an air pocket inside a seemingly solid blade would be a weak spot, and then any weak point would be viewed as neglect and any dedicated artisan would produce the best quality blades as though his own life depended upon the very blade he was forging. Secondly, in the repeated folding and hammering, what may be referred to as 'layers' were produced. Have a book and roll up it parallel with the spine, these internal layers would look something like this, almost like the rings of a cross section of a tree trunk. This added much strength towards the blade.Samurai Sword
Also the natural strengthening carbon elements within the steel, as well as the steels impurities could be spread all through the whole of the sword, therefore strengthening it in its entirety. Once the blade came into existence cooled it had not been simply quenched in water, another process was needed first. When steel is been cooled, if it cools from the temperature as a result of cold inside a short amount of time, the metal becomes very hard and brittle. Conversely, if steel is cooled slowly from the lower temperature as a result of cold, the steel assumes more supple, even softer properties. Just because a samurai sword was used primarily as a slicing weapon the blades were put through lots of shock upon effect on the enemy, and so the blade couldn't be produced from the more brittle steel throughout else it would shatter like glass. But the sword needed to retain its sharp edge, so it couldn't be made of softly forged steel throughout else it would simply blunt. So an account balance was struck utilizing a very clever procedure.
Exactly what the Japanese samurai sword makers discovered was by painting on the clay formula onto the blade before quenching, thin amounts onto the cutting edge and thicker amounts to the back, the steel might be designed to take on two completely separate properties, thereby giving the blade hard leading edge it required, and also the more supple back. Due to the different speeds where the two halves from the steel cooled this also formed the beginning of the bend that the sword makers would work to produce the famous curved blade.