What is Anko in Japanese Katana terminology?

Anko or 餡子 in Japanese, refers to a characteristic of the ground iron (jigane) of a Japanese sword.

Japanese swords are made using a unique technique called ""Awase Kaji"", which involves forging a softer core iron (shingane) within a harder skin iron (kawagane).

By combining these two types of iron with different hardness, the sword achieves properties of not breaking, not bending, and cutting well. However, as the sword is sharpened over time, the outer skin iron gradually thins, and the inner core iron may appear on the surface. This phenomenon is called ""Anko"". There are various theories about the origin of the name, but it is believed to be named because it resembles the appearance of the bean paste (anko) popping out of a daifuku (a type of Japanese sweet).

This is a characteristic of the Bitchu Aoe school's old swords, and the black spots called Sumihada or Sumitetsu/Sumikane are also a type of Anko. While Anko is not particularly concerned when seen in old swords, it has been a factor in lowering the evaluation of new swords. This is because swords that have shown Anko are considered to be of poor construction, trying to reduce the wear of iron by forging the core iron fewer times than the skin iron.