Material removal during skillful polishing

[Haon]

Good evening. This question came to my mind a few days ago, when I was researching the measurements of an antique tachi, the Koryu Kagemitsu. As this blade is now 700 years old, I was wondering if the measurements that can be found in magazines and on the Internet are mostly true to the ones it had when it was forged. 

Im aware that the sword was shortened, and I'm also aware that blades with severe damage from fights or accidents would need a different way of polishing than just an "update" of the existing polish.

For example, if the blade has a current motokasane of 7mm and motohaba of 30mm at the hamachi, would it be realistic to assume the original measurements were a motokasane of 8mm and a motohaba of 31 / 32mm?

 

Best regards and a happy new year

 

PS: I'm not sure if this is the correct section to post this question in. If not, move the thread or message me, please.

[ROKUJURO]

Hi Derek,

welcome to the NMB!

Without being able to give exact figures, I think that you are correct in your assumption of material loss through polishing over a long period of time. Of course this is influenced by 'accidents' like KIRI-KOMI, but not all swords were used in combat.

[IanB]

Perhaps less obvious is the change in appearance of a blade. When newly made and given its first polish, the smith would examine it for defects before passing it to its first owner. What they saw would be lost after the next polish which cut away the original surface. Subsequent polishes would gradually reveal what had initially been the interior of the blade.  In other words when we gaze in admiration at the activity in a koto blade, it is a bit sobering to think that we, and the last polisher, are the only people who have seen that incarnation of the blade that probably differs considerably from what its maker saw.

Ian Bottomley

 

[Haon]

Thank you very much for your insight and expertise, I am trying to get a replica blade made of the Koryu Kagemitsu and wanted to have one that micght be handling as the original in its first shape. Of course, exact figures cant be given, as these are propably very small, maybe not even really measureable, if done skillfully. As some people here seem to be really experienced with polishing and nihonto, this forum was one of the first that came to my mind.

[Jussi Ekholm]

Finally found the topic as I remembered it was quite recent. Mark S. documented the changes that came with polishing by Woody. 0,1 cm in length and 5 grams in weight.

 

 

I think one good indicator of how much the sword has changed from original in thickness is the difference between nakagokasane and motokasane.

 

Here is an image of an old tachi from Nanbokuchō period I used to have. Nakagokasane was 8 mm, motokasane was 5,6 mm and motoshinogikasane 6,8 mm. While the picture is not optimal I think it shows the wear quite well.