a Japanese sword specialist, and 3

3. The Japanese Sword has gone through a lot of characteristic changes throughout the history but there’s very little information to find (at least in English) about changes throughout the forging process and or the relation between forging and characteristic changes of the katana. Has there been a big evolution there as well?
I have not seen much written on it, and I believe that there is no conclusive evidence, but essentially is would seem that the main processes remain relatively unchanged. However, I do know that many of today’s smiths spend quite a lot of time researching jigane through trial and error in order to try to reproduce the similar results as to that of earlier periods or schools. I have also discussed this with some researchers and specialists and the general consensus seems to be that it is thought that originally the billets were formed from small nuggets, or whole chunks of tamahagane. The mizu-heshi process of flattening chunks of tamahagane, breaking it into smaller 2 cm diameter pieces and stacking it, was introduced around the start of the Shinto era of sword making. Also, generally speaking, the number of times the steel was folded (less folds, larger hada) has changed depending on school and era. For instance, early Koto era blades tend to have larger hada, whereas Shin-Shinto blades tend to have a rather tight hada.
Additionally, there are theories that the application of clay in the differential hardening process was a later addition, or that there was a different method. The evidence to support this theory is the continual gradual contrivance of the shape of the hamon from its original natural midare, and the disappearance of utsuri. When blades are quenched using a method that does not include a clay insulator, the chances of producing a form of utsuri increase dramatically. This can be seen in the works of modern swordsmith Sugita Yoshiaki. However, this is not a recent discovery. Smiths have been aware of this for some time, but the objective for them has now become controlling and producing other forms of utsuri like choji, jifu and the like.