What techniques did the swordsmith use to modify the properties of different parts of a katana blade?

Answer 1

In a nutshell, they would build the blade from 2 different iron (steel) alloys. One would be higher in carbon content, and harder to hold an edge. This would be wrapped in an outer layer lower in carbon to give it resiliency so it wouldn't shatter under stress.

They would then also fold these steels and brush them with straw in between foldings to alter the carbon content and remove any impurities. Steel could be folded dozens of times resulting in hundreds of "layers" of steel.

In the end the blade would have it's backing encased in a special clay, and then quenched in a tank of oil or water. The clay would insulate part of the blade, leaving the edge exposed so they would expand and contract at differing rates. This is what gives the katana it's distinctive curve and patterned edge.

Answer 2

The swordsmith used differential hardening techniques like clay tempering to modify the properties of different parts of a katana blade. By applying a clay mixture to the blade before quenching, they would create varying rates of cooling for different parts, resulting in a hard edge and a softer spine for flexibility. This differential hardening process allowed for a sharp, durable edge while maintaining the blade's overall strength.