Although various types of steel, iron, or other materials may be used in the forging of Japanese swords, there are a few dominant techniques used for sword creation that have been passed down for generations. It is the personality of the craftsman forging the weapon, however, which is of specific importance in the tradition of Japanese smithing. As traditional forging techniques eschew the use of modern factory machinery, it can be stated with accuracy that no two authentic Japanese swords are exactly alike. Therefore, where it is often said that the soul of the warrior is encompassed within his blade it can likewise be stated that the spirit of the maker resides there as well.
Years of experience are required for a master Japanese swordsmith to become expert at his craft. It is only through the repetition of forging numerous blades that one can learn what steel/material to use, how long to heat/cool the blade, how to fold the steel, how long to hammer the blade, and so on. Timing and a keen sense of the nature of the metal itself help distinguish a master’s blade from a run of the mill, factory-produced sword.
Many modern smiths employ a method of repeatedly heating, folding and hammering steel for their blades. The makers at Dynasty Forge, for instance, use this process to remove any impurities from the metal of the sword and to highlight the Hada (grain patterning on the blade). Smiths from the Imperial Forge also employ this process with their Imperial Folded Sword collection to eliminate imperfections in the blade while assuring maximum power and flexibility.
Another distinctive technique for the forging of Japanese swords is a process called differential quenching. Clay is used on the back of the sword during the cooling process, causing it to have a softer edge than the blade. This pulls the blade upward, providing the distinctive curved shape of the Katana. Some sword manufacturers, like Dynasty Forge, cover the majority of the blade (for some of their sword lines) in clay during the heating process to focus the force of the fire on the very edge of the sword to produce maximum sharpness. Last Legend also uses clay to create hamon lines on their blades for pattern distinction and aesthetic purposes.
Sword construction can also involve using multiple layers of different types of steel/metal in the forging process. Cold Steel swords, for instance, use three layers of steel in their “San Mai III” blades to allow for maximum flexibility in outer layers (to help keep blade from shattering when struck) while providing for maximum strength at the blade's core. Thaitsuki Nihonto also combines soft iron and steel by hammering them into one piece, again allowing for pliability and strength.
Tradition holds that a famous Japanese sword smith created this traditional forging process (folding, using multiple layers, using clay for differential tempering) after his daimyo (feudal lord) lost a battle using swords he had designed without these traits. The smith realized that he needed to create a blade that was both flexible (to sustain sharp blows of the enemy and remain pliable) and strong. Up until that point in history, swords were largely comprised in a traditional heat tempering method employing only one type of steel/metal. This typically meant swords were both unwieldy (very heavy) or quick to shatter. However, after the master smith created this new brand of sword his Lord won his next and many future battles. It was this smith who realized that combining a softer iron/metal (for durability/lack of shattering) with a harder iron/metal (for cutting power/strength) would provide a weapon worthy of a Samurai.