Question: what is the wavy line on a katana

title: Japanese swordsmithing
text: To produce a difference in hardness , the steel is cooled at different rates by controlling the thickness of the insulating layer . By carefully controlling the heating and cooling speeds of different parts of the blade , Japanese swordsmiths were able to produce a blade that had a softer body and a hard edge . This process also has two side effects that have come to characterize Japanese swords : 1 . ) It causes the blade to curve and 2 . ) It produces a visible boundary between the hard and soft steel . When quenched , the uninsulated edge contracts , causing the sword to first bend towards the edge . However , the edge can not contract fully before the martensite forms , because the rest of the sword remains hot and in a thermally expanded state . Because of the insulation , the sword spine remains hot and pliable for several seconds , but then contracts much more than the edge , causing the sword to bend away from the edge , which aids the smith in establishing the curvature of the blade . Also , the differentiated hardness and the methods of polishing the steel can result in the hamon 刃 紋 ( frequently translated as `` tempering line '' but better translated as `` hardening pattern '' ) . The hamon is the visible outline of the yakiba ( hardened portion ) and is used as a factor to judge both the quality and beauty of the finished blade . The various hamon patterns result from the manner in which the clay is applied . They can also act as an indicator of the style of sword - making , and sometimes as a signature for the individual smith . The differences in the hardenability of steels may be enhanced near the hamon , revealing layers or even different parts of the blade , such as the intersection between an edge made from edge - steel and sides made from skin - steel .