During the hardening process the clay layer causes a split second difference in cooling time which creates two different hardness areas in the same piece of steel. The edge cools faster and forms a very hard steel structure called martensite while the body cools slower and forms a very tough steel structure made of ferrite and pearlite. The boundary between these two areas is called hamon and is commonly seen as a frosted line down the length of a polished sword blade.
more about the process of yaki-ire
Nagatsuki Tanto

Nagatsuki (長月, pronounced “nah-gah-tsoo-key”) translates literally as “long moon”. In the ancient calendar it is a poetic name for the time around late September, possibly abbreviated from yonagatsuki meaning “night of the long moon”, or “month of the long night” depending on… Continue reading