Swords were made from selected pieces of a mixture of iron, steel and slag called tamahagane, the product of the blast furnace. This was refined and its carbon content adjusted by repeated hammering, folding and welding. Having prepared the different qualities of steels, they were then welded together to produce a composite billet from which the blade could be forged. Different traditions of sword-making used different constructions. A common system involved wrapping a piece of high-carbon steel around a low-carbon steel core. Another technique, used for cheap swords was to insert a strip of high-carbon steel that would form the cutting edge into a low-carbon steel body. The most complex construction involved separate grades of steel for the core, the edge, the sides and the back of the blade.
The Japanese sword is unique in that only the cutting edge and point are hardened, the body of the blade being softer to support the edge and prevent it cracking and breaking in action. The process involved the blade being coated with a mixture of clay, charcoal and powdered stone, scraped thin along the edge. When heated to a critical temperature and quenched in water, the edge cooled rapidly, and became very hard, while the back, insulated by the thicker clay, cooled more slowly and remained soft. Because the back contracted more than the edge, the sword took on much of its characteristic curve during quenching. When polished, the hardened edge is visible as a misty region whose pattern, hamon, was characteristic of the smith’s tradition.
After hardening, the blade would be polished. In early days this was probably done by the smith himself, but in later years the process developed into a separate and highly skilled craft. The first stage of the process was to define and refine the ridges and planes using a series of stones of gradually increasing fineness. The next stage was to reveal the structures in the blade resulting from the forging and hardening. This was achieved by applying tiny flakes of a series of even finer stones to the blade with the thumb. The back of the blade and the region above the ridge was burnished to a mirror finish with a hard steel needle. The final result was a blade that was highly polished but which revealed all the complex metallurgy that had gone into its production.