Their own steel (jika seitetsu, 自家製鉄)

What is known

The inscription

“以自家製鉄鍛之” (jika seitetsu o motte kore o kitau), on the reverse of the family’s blades from 2008 to 2022: it can be read in the shops’ photos HD

What they do

They gather the iron sand themselves and smelt their steel, instead of using the tamahagane distributed by the NBTHK: “殆どの刀工が、日刀保から供給される鋼を使用している現代においても、刀匠自らの手で素材（砂鉄）の採取を行い、自家製鉄によって独自の鋼を生成” (“today, when almost all smiths use the steel distributed by the NBTHK, the smith gathers the material, the iron sand, with his own hands and makes his own steel”), in a shop’s listing AT

Where the sand comes from

From the Kuma river (球磨川, Kumagawa), “one of the three rapid rivers of Japan”, say the shops; the family repeats it AT

Where

A shop mentions a 自家製鉄場 (jika seitetsujō, their own smelting site) next to the forge, the polishing area and the clay area AT

The furnace

It is not on record whether it is a tatara, a small furnace or something else: no public source describes it PC

In the prefecture feature

“From choosing special Kumamoto steel, to wrapping it in mud and wheat, to melding the materials together…”; “up to 15 days” of physical work per sword, and “up to three years” to finish it, in ten steps AT

The construction

本三枚鍛 (honsanmai-gitae: core, edge and back in three pieces) in the joint Sanchōmō of 2019, in the listing of the shop that commissioned it HD

What he says in public

In the 2024 talk he explains the process with tamahagane, at about 1,300 ℃, with several foldings, the kawagane and the shingane separately, and he names the 甲伏せ (kōbuse) and the 本三枚 (honsanmai). He does not talk about his own smelting HD

What can be seen in a blade

In the katana of the 2025 Takamatsu-no-miya Kinenshō, a mokume with very marked chikei, “like a sword of meteoric iron”, in the words of a visitor AT