A differential hardening
is a method used in forging swords
to increase the hardness of the edge. To achieve this, the edge is cooled more rapidly than the spine by adding an heat insulator to the spine before quencing. Clay
or an other material is used for insulation.
This technique is mainly used in katana, the traditional Japanese swords and khukuri, traditional Nepalese knifes. Most pieces made with this technique have visible temper line/styles.