A samurai's sword is his most sacred and prized possession. Not only did the samurai rely on his sword to defend him, but naturally the sword held greater significance as the samurai actually believed his soul associated the sword. Therefore it comes as no surprise that the same discipline and respect in which the samurai wielded his sword, went into the actual making of the sword itself.
Swords were not simply 'cast' in a mold and then sharpened. A Japanese samurai sword was made by an intricate process of heating the steel, hammering it flat, then folding it, then hammering it flat again, and folding. This process of repeated hammering and folding would have done up to as much as 30 times, or until the maker was satisfied it had been done properly.
There are quite a few reasons for this labor-intensive procedure. Firstly, any air pockets which might develop during the heating of the steel would be eliminated. Having an air pocket in a seemingly solid blade would be a weak point, and any weak point would be seen as neglect and any dedicated artisan would produce the highest quality blades as if his own life depended upon the very blade he was forging. Secondly, in the repeated folding and hammering, what might be described as 'layers' were produced. Take a book and roll it up parallel with the spell, these internal layers would look something like this, almost like the rings of a cross section of a tree trunk. This added much strength to the blade.
Also the natural strengthening carbon elements within the steel, as well as the impurities would have spread through the whole of the sword, therefore strengthening it in its ownity. When the blade came to be cooled it was not simply quenched in water, another process had to be done first. When steel is being cooled, if it cools from a high temperature right down to cold in a short amount of time, the metal becomes very hard and brittle. Conversely, if steel is cooled slowly from a lower temperature right down to cold, the steel takes on more supple, even softer properties. Because a samurai sword was used primarily as a slicing weapon the blades were subjected to a lot of shock upon impact on the enemy, therefore the blade could not be made of the brittle steel through else it would shatter like glass. But the sword had to retain its sharp edge, so it could not be made of softly forged steel through otherwise it would simply blunt. So a balance was stuck using a very clever procedure.
What the Japanese samurai sword makers discovered was by painting on a clay formula onto the blade before quenching, thin amounts onto the cutting edge and thicker amounts onto the back, the steel could have made to take on two completely separate properties, thereby giving the blade the hard cutting edge it required, and the more supple back. Because of the different speeds in which the two halves of the steel cooled this also formed the beginning of the curve from which the sword makers would work to create the famous curved blade.