There are few techniques that allow various areas of object to get various heat-treatments. This is known as differential hardening and is common for high quality swords and knives. Its earliest examples include Chinese jian, Japanese katana, Nepalese khukuri etc. differential hardening technique uses insulating layer, for example clay layer for covering areas, required to remain softer. The areas which are required to be hard are left without any coverings. This allows only specific steel parts to completely harden upon quenching. To decide as which areas are to be covered and which are to be exposed, determines the softness and hardness of the final product.