Creating a sword
Samurai swords were not simply cast in a mould and then sharpened. They were made by an intricate process of heating, hammering, and folding the steel. This cycle of repeated hammering and folding would be done as many as 30 times, or until the maker was satisfied the blade was ready.
This labor-intense annealing-layering process eliminated any air pockets that might have developed during heating of the steel, which would create weak spots in the blade. The process also hardened the carbon in the steel and spread any impurities throughout the sword, strengthening the steel even more.
If steel cools from a high temperature to cold in a short period, the metal becomes very hard, but brittle. Conversely, if it is cooled slowly, it becomes softer and suppler. Samurai swords were used primarily as a slicing weapon, so the blades were subjected to severe shock upon impact. It a blade was brittle, it might shatter on contact. However, if it was made too supple, it would not hold an edge. This created a dilemma for the sword makers.
Japanese samurai sword makers discovered that by painting a thin layer of a clay formula onto the cutting edge, and a thicker layer onto the back the blade, before cooling the blade, the steel would have a hard cutting edge but a supple back. Because of the different speeds at which the two areas of the steel cooled, the blade developed a natural curve that the sword makers then worked to create the famous curved blade of the samurai swords.