What is the difference between a tetsubin and a tetsu-kyusu?

A tetsubin is a kettle with a bare cast iron interior, made to go on direct heat to boil water. A tetsu-kyusu is a teapot with an enamel-lined interior, made only for steeping tea and never for direct heat.

How can I tell which one I have?

Look inside. A glossy black or blue interior is an enamel lining, which means it is a teapot and must not go on heat. A dull grey or brown metal interior is bare iron, which means it is a kettle.

Does Japanese cast iron add iron to the water?

Only an unlined tetsubin does. The enamel lining in a tetsu-kyusu blocks contact between the iron and the water, so a lined pot adds no iron.

What happens if I put a tetsu-kyusu on the stove?

The enamel and the iron body expand at different rates under direct heat, so the lining cracks and can flake. The damage is irreversible.

工芸Kogei What the listing does not tell you

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A Nambu cast iron kettle with hobnail arare texture, a bail handle and a tapering spout. — An AI-generated image, not a photograph of a specific piece.

鉄瓶と鉄急須

A tetsubin and a tetsu-kyusu look like the same object. One of them cracks on the stove.

They look like the same object. One boils water. Putting the other on heat destroys it. Nambu ironware is sold worldwide under one loose word, so people buy a teapot and use it as a kettle. This page is the difference, and how to tell in one look.

Cross-section of two Nambu cast iron walls. Both have the same iron body; the tetsu-kyusu has an additional glassy enamel layer on its inner face, the tetsubin does not. — Both walls are the same iron and the same thickness. Everything that follows — whether it can go on heat, whether it gives you any iron, how much care it needs — comes from that one layer on the inner face.

The testLook inside. Glossy black or blue = enamel lining = teapot, never on heat. Dull grey or brown metal = bare iron = kettle.

Bare cast iron inside

Tetsubin

鉄瓶 — a kettle

A kettle. Goes on direct heat to boil water. Because the interior is unlined, the iron is in contact with the water and leaches into it — this is the only version that measurably adds iron.

Enamel-lined inside

Tetsu-kyusu

鉄急須 — a teapot

A teapot. For steeping only — never on direct heat. The enamel and the iron body expand at different rates, so heat cracks and flakes the lining irreversibly. The lining also blocks iron contact, so it adds no iron.

Why this keeps happening

Most overseas listings show only the outside, where the two are near-identical, and many use "tetsubin" loosely for both. Buyers put an enamel-lined kyusu on a stove, or buy one expecting the iron-supplement effect it cannot give.

What follows from it

✓A tetsubin belongs on direct heat. That is the whole design. Because the interior is unlined, iron meets the water and leaches into it — the reason people seek one out.
✗A tetsu-kyusu must never go on heat. The enamel and the iron body expand at different rates, so the lining cracks and flakes. It is not repairable.
✗An enamel-lined pot adds no iron. If iron intake is the reason for the purchase, a lined teapot cannot deliver it, however traditional it looks.
✓Bare iron must be dried after every use. An unlined interior rusts if left wet. The lined version does not have that problem, which is the trade it makes.

How these pages are put together. Written in Japan, but we do not own or test the pieces described here — the facts are compiled from makers, specialist retailers and published sources, and every one is listed at the foot of the page so you can check it yourself. Makers do change specifications, so confirm against the listing or the manual before buying. If something here is wrong, tell us and it gets corrected.

Where to look

Nambu ironware is made by long-established foundries in Iwate — Iwachu, Oigen and Oitomi among them. Whichever you buy from, the same question applies first: is the interior bare iron or enamel? Listings often show only the outside.

Oitomi — iron kettles ↗Hitachiya USA — Nambu tekki ↗

Two listings that answer the question

Most listings do not say. These two do, which is the only reason they are here — both are made by Iwachu, and each states its own lining on its product page. They are worth reading side by side even if you buy neither, because they show the confusion in its plainest form: the second one is sold under the word 鉄瓶 tetsubin and then tells you not to put it on a flame.

✓The kettle. Its title says No Enamel, Direct Fire, and the material field gives cast iron, black-baked inside and out — bare metal, not a lining. This is the one that boils water and gives you iron.
✗The teapot. Sold as a Tetsubin Teapot, but its own bullets read the interior is coated and avoid use over a direct flame. For steeping only. It will not add iron to anything.

A kettle — listed as unlined, direct heat ↗A teapot — listed as coated, never on heat ↗

Amazon listings change under the same address, and sellers relist. Whatever you are looking at, check the material or interior field on the page in front of you before buying — that field, not the product name, is what tells you which object it is.

Sources The other guides

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