Dating japanese sword
During much of the 1300's, the Japanese Imperial Court was politically divided into the Southern Court and Northern Court.Most swords will have dates using the nengo of the Southern Court, but occasionally one will be encountered where the Northern Court nengo are used.This makes it easier to decipher many mei knowing little or no Japanese.It is this 'standard' form I am concentrating on here.This rather fluid disparity in Japanese and western dates should be taken into account when reading the archaic date inscriptions on the tangs of Japanese swords.
This probably was not too difficult for the Japanese to deal with, as historically a leap month was added occasionally to ensure that the irregular months maintained alignment with the seasons.
You should also better ask in a Chinese sword forum, e.g. Tim, as per my experience with perisan blades too, it's not safe to use the same approach with swords of different cultures.
I would say that there are no homogeneous criteria to be applied because of the sheer number of variables (steel used, cultural approach to maintenance, period of making, site and type of storage, quality of maintenance in the near past, type of wood of the handle, quality of the craftmenship, ). Sorry the photos were not good enough to make a determination on the tang age.
Japanese swords have been made for over a thousand years. are extremely rare; therefore those nengo have not been included.
Many swords are inscribed with the date they were made. The inscriptions normally read from the top down, nengo (period); nen (number of years into the period); gatsu (month) and hi (day).