I'm currently trying to wrap my head on the differences in steel composition from Koto period swords.
Has there been any metallurgic analysis done to compare steel from different regions?
What I'm basically trying to get at is how much can we say that the technics of a particular, geographically-located lineage of smiths was an adaption to the characteristics of the local iron sand deposits compared to technical aspects that are not material dependant.
We often hear that "steel quality" is the main reason early work cannot be reproduced by Gendai smiths. However, if we had detailed composition data we could in theory reproduce the steel composition and we could see the magnificent koto-style hada and activities that made these swords famous. This is corroborated by the observations of Yasutsugu who could produce superior hamons through re-tempering of old koto blades compared to his own. At least we could take the steel variable out of the equation.
If this is the case then I'm not sure the current centralised "traditional" bloomery steel production in Japan is best way to keep the tradition alive if you're striving for reproducing excellence. If hard data on skin and core steel composition was available we could reproduce the composition of these steels.
Besides, it is well possible that the smiths of old time had a secret formulae for their bloomery steel. Now I'm not a metallurgist but say, in crucible steel production (which happens at a much, much higher temperature) you can infuse elements. (say, vanadium).
Overall my intuition is that technic has been overestimated in the production of these masterworks if technic is understood as an adaptation to raw materials. If we had a time machine and brought Masamune along and gave him the NBTHK Tamahagane he will produce a Soshu-style gendai blade not a Masamune.