Making a Masterpiece
Curving the blade
Next, the smith pulls the katana from the fire and plunges it into a
trough of water in a rapid cool-down process called "quenching."
Because the sword's back edge and inner core contain very little
carbon, they can contract more freely than the high-carbon steel at
the front edge of the blade. The difference in both the degree and
speed of contraction between the two forms of tamahagane causes the
sword to bend, creating the distinctive curve. This is a tricky
stage, in which as many as one in three swords is lost.