Habaxi

What is the minimum and the maximum carbon content in tamahagane steel? and what is the optimal carbon ratios for a sword made of tamahagane? Also how did ancient swordsmiths know and determine the other metals content in the steel to ensure their swords were functional?

I've seen some people use paper shims in the shape of seppa but slightly smaller, placing them between the seppa and the tsuka to improve the fit of their katana, so If a katana is already stable with no noticeable looseness, does adding paper shims still provide any benefit? Or is it only useful when there is some slight movement that needs to be corrected? Would it enhance overall stability or have no real impact in a well-fitted katana?

Thank you for your effort. English is not my first language, and I thought that "katana" referred to the blade itself without considering the other parts (that's why I said "folding a katana"). Thank you again for correcting me.

Do you know any techniques that a swordsmith can do to reduce the damage when folding a katana made of modern steel?, as you know modern steel is already pure and dont need folding, the folded ones are only for Aesthetics like (hada) appearance or just Japanese traditions, so folding it reduce the carbon content as the result of repeated heating which is also known as (annealing the steel), i heard that you can fold it but with certain techniques that reduce the loss of carbon, i already know about the tatara furnace but modern steel katanas mostly isnt made in it cuz its an Expensive and time consuming method for a modern katana.