How is a Japanese sword constructed?
Very carefully. Seriously, there are as many as a half-dozen people involved in the construction of a sword. The swordsmith forges the actual blade. He starts usually with a special kind of traditional Japanese steel called tamahagane, and works with hammer and forge to fold it a number of times. There are two processes in general, one to make core steel (shinganae) and the other to make jacket steel (kawagane). Kawagane is folded more times and ends up being harder and less ductile than shinganae. In the most simple construction, a piece of kawagane is folded around a piece of shinganae to form a jacketed core. Thus the shinganae allows the sword to flex instead of breaking on impact, and the kawagane allows it to take the famous razor edge. More complicated construction methods can produce swords made of as many of 5 pieces of steel, all forged differently. The folding process is used to closely control the uniformity and carbon content of the steel. An accomplished smith can tell by
