Oigen (及源鋳造) was established in 1852 by Genjūrō Oikawa in Mizusawa, Ōshū City (奥州市), Iwate Prefecture, and is now in its fifth generation. The foundry produces Nambu Tekki (南部鉄器) — the cast ironware native to Iwate that has been designated by METI as a traditional craft of Japan since 1975. Iron casting in this region predates even the Nambu name, stretching back to the twelfth century when the Ōshū Fujiwara clan (奥州藤原氏) flourished in nearby Hiraizumi and commissioned temple bells, armour, and tools from local iron sand.

Oigen's process begins with Mizusawa's fine silica sand, packed by hand into moulds. Molten iron is poured at around 1,400°C by experienced casters whose rhythm and judgement determine each pot's weight and balance — a skill passed through generations. After cooling, each piece is filed, textured, and fired in an airtight kiln to form the distinctive black oxide layer (酸化皮膜) that protects against rust. Their signature patterns — Arare (霰, hailstone dots) and Itome (糸目, thread lines) — both strengthen the surface and create grip. Nambu Tekki is said to require over 60 production steps and 15 years to learn; mastery takes a lifetime.

At iromizu, Oigen's cast iron teapots anchor our Brew Time collection — pots that hold warmth long after you have poured the first cup, with a heft that makes the ritual of brewing feel unhurried and substantial. Traditionally, iron is said to soften and sweeten the water — a small belief worth experiencing for yourself. See the teapots below.