What is Ji-nie in Japanese Katana terminology?

"""Ji-nie"" (地沸) refers to the sparkling particles that appear on the surface of a sword blade that has been heated to a high temperature and then rapidly cooled in water. This process causes a physical change in the iron structure of the blade, resulting in the appearance of these glittering particles, which are referred to as ""Nie"". Among these, the ones that appear in the body (Ji) of the blade are called ""Ji-nie"".

This process is a crucial part of Japanese sword making, contributing to both the functionality and aesthetics of the sword. The rapid cooling of the blade in water hardens the edge, enhancing its sharpness and cutting ability. At the same time, the appearance of the ""Nie"" adds a unique beauty to the sword, with the sparkling particles creating a contrast against the steel of the blade. This highlights the high level of craftsmanship and attention to detail in Japanese sword making."