Also known as Jia Gang in Chinese, an insert blade is a resilient blade with a tough edge, made using three different pieces of steel, high carbon and a low to medium carbon steel.
Pure high carbon steel makes an extremely tough edge capable of retaining its edge, though it becomes brittle after the heating from the hardening process and is prone to breaking.
Low carbon steel on the other hand is exactly the opposite; its resilient nature allowing it to bend rather than break, though it is not capable or holding an edge with a consistent hardness.
Therefore, by inserting a high carbon steel edge into a low-medium carbon steel body and forging them together, the sword maker can produce a sword that amalgamates the qualities of both steels – hard (sharp) and soft (yielding).
Such a blade is difficult to make, however, as if any part of the forging process has not been done correctly, the blade will break straight away during the final quenching process. It is for this reason that sword makers will make three blades for one insert blade, in case one should break during the forging process.
After the blade has been finished, it is arduously polished and sharpened together by hand using stones of different grades to bring it to its mirror sheen and razor sharpness. The polishing also accentuates the inserted edge line, where the steels are different along the edge.
While a good sword maker of course can control the appearance of the edge line, it is truly unique and no two edge lines will ever be alike. The uniqueness and beauty of the edge line is considered to be the signature of the sword maker.