Caring for a Nambu cast-iron tetsubin

First, a critical distinction

True tetsubin vs cast-iron teapot. A tetsubin (鉄瓶) is an unlined cast-iron kettle for boiling water over a heat source — gas stove, induction (some models), or traditional charcoal brazier. Modern "Japanese cast-iron teapots" with an enameled interior are a different product — they look similar but are for steeping tea, not boiling water. The enamel coating prevents the iron from reacting with the water.

This guide covers unlined tetsubin only. If your piece has a black or colored enamel interior, follow standard porcelain-care rules instead (hand wash, no abrasives, no stove use).

Initial seasoning (first 7–10 uses)

New tetsubin ship from Iwachu or Oigen with a thin protective oil coating inside. You need to remove that, then build up a mineral patina from boiled water. The process:

1. Day 1: Fill the tetsubin with plain water (tap is fine, filtered is better). Bring to a full boil on your stovetop. Discard the water. Repeat 3 times. This removes the factory protective oil.
2. Day 1 onward: Use the tetsubin every day for at least the first week. Each use, fill with water, boil, pour out into a teapot or thermos, then immediately dry the interior with residual heat by leaving it on the low burner for 30 seconds with the lid off. Do not towel-dry the interior — towel fibers catch and rust.
3. After ~7 days of daily use: You'll start to see white-gray mineral deposits forming on the interior bottom and sides. This is yu-aka — the desired patina. It's mineral scale from your water, not actual rust.
4. After 3–4 weeks of regular use: The interior will be uniformly grey-white and the water inside will taste noticeably softer. The kettle is now properly seasoned.

Daily use, after seasoning

• Always fill before heating. Never heat an empty tetsubin. Cast iron heated dry can crack from thermal shock when water is added.
• Heat gradually. Don't put a cold-from-the-shelf tetsubin onto a high flame. Start on medium-low, build up.
• Don't boil aggressively for long periods. A rolling boil for 2–3 minutes is fine. Boiling at a hard rolling boil for 15+ minutes wastes water and can over-concentrate minerals.
• Empty completely after every use. Standing water in the tetsubin is the main cause of red-rust formation. Even fully seasoned kettles need to be drained.
• Dry with residual heat, not a towel. Leave on low heat for 30 seconds after emptying, with the lid off. The remaining moisture evaporates without touching anything.

What never to do

• Never wash the interior with soap. Soap residue absorbs into the mineral patina and ruins the next several brews. If you must clean the interior, use boiling water and a soft brush only.
• Never put it in the dishwasher. Heat, detergent, and extended water contact all destroy the patina.
• Never store with water inside. Always empty.
• Never scour the interior. Steel wool, abrasive sponges, and metal scrubbers remove the patina you've spent weeks building.
• Don't use it for tea steeping. Tea leaves inside the tetsubin will react with the iron and make the water taste metallic. Boil water in the tetsubin, pour into a separate kyusu or yunomi to steep.

If red rust appears

Some red-orange rust on the interior is normal and recoverable. The tetsubin is not ruined. Process:

1. Fill with water and a handful of green tea leaves (any cheap sencha or hojicha).
2. Bring to a boil, then simmer 10–15 minutes. The tannins in the tea bind to the iron oxide and convert it to a stable iron-tannate compound that's harmless.
3. Pour out, rinse with fresh water, dry over low heat.
4. Repeat once if any orange remains visible.

This tea-tannin treatment is also how new Iwachu and Oigen tetsubin are initially conditioned at the factory before shipping.

Exterior care

The exterior is usually painted with a heat-resistant black or colored coating (Iwachu uses traditional kakubu lacquer-based finishes; Oigen often uses iron-oxide-based finishes). The exterior coating doesn't need seasoning.

Wipe the exterior dry after each use. If water spots form, polish with a soft dry cloth. Don't use scouring pads or steel wool on the exterior — it strips the protective coating. If the exterior coating eventually wears through after many years of use, you can re-paint with a high-heat stove-paint (~$8 from any hardware store) but most users just let the bare iron darken naturally.

Induction-compatibility

Cast iron is naturally magnetic, so traditional tetsubin work on induction cooktops in principle. However, many tetsubin have rounded bottoms that don't sit flat on a flat induction surface — they wobble, and many induction cooktops detect this as "no cookware present" and shut off.

If you cook on induction, look specifically for "IH-対応" (IH-compatible) marked tetsubin from Iwachu or Oigen. These have flat bottoms designed for induction heating. Most tetsubin in our catalog are NOT IH-compatible by default. We label this clearly on each product page.