The object above is not a “tetsubin.” It is also not a “tetsubin teapot, here ” a “tetsubin tea kettle,” or a “cast iron tea kettle.” It is a cast iron teapot.
Japanese cast iron teapots like the one in the photograph are similar in appearance to a traditional Japanese tetsubin, which I suspect is the reason that they are often misleadingly called “tetsubin teapots.” This, in turn, has led to the misapplication of the word “tetsubin” to indicate cast iron, rather than as a reference to a similarity of form with a completely different item of teaware.
Tetsubin (??): An iron kettle with handle and spout. Used on the type of small brazier called binkake for tea procedures employing boxed tea sets (chabakodate), etc.
The definition is from A Chanoyu Vocabulary, Practical Terms for the Way of Tea, which is an excellent reference for Japanese tea terms. The book identifies 1650 terms related to Japanese tea culture, so I think that the desire for clarification of terminology is justifiable. The Japanese are historically quite thorough in the development of appropriate terminology to refer to very specific things. Unfortunately I do not have the knowledge of the Japanese language that would help with my research into cast iron teawares, but every reliable source that I found confirmed that the word “tetsubin” always means a particular type of Japanese water kettle. The illustration below shows the basic shape of a typical tetsubin. To be even more specific, the tetsubin is not the only kind of Japanese cast iron kettle. There is also the kama (sometimes spelled “gama”), which is an unhandled kettle that sits over or into a brazier (furo) for use in the the formal tea ceremony and which has several different types.
There are notable differences between tetsubin and Japanese iron teapots, principle and most obvious among them being that kettles are used for heating water and teapots are used for brewing tea. Japanese cast iron teapots also generally hold considerably less liquid volume and usually have an enamel lining, for the purpose of minimizing rust. Tetsubin, on the other hand, never have linings of any kind as the direct effect of the iron on the water is desirable. A high quality tetsubin will cost more money to acquire, especially one in good, usable condition, as sadly, many of the antique tetsubin available for sale today are ruined due to excessive rust on the inner surfaces.
In spite of a lot of confusing naming and identification, Japanese cast iron teapots are quite useful. I have found that Japanese green teas taste very smooth and sweet when brewed in mine (the one in the photograph above). The iron cups also add a pleasant warmth of character and have a nice hand-feel. I would not use these items for other types of teas for a number of reasons, including incompatibility of iron with black teas and not wanting any darker tea residues to affect green tea brewing. Plus there is generally no cultural cross-pollenization of teas and teawares at my house. I would never use Japanese teaware for Chinese or Korean teas, and certainly not for Indian teas.
For further reading, both Artistic Nippon and Hojo Tea have very good pages on Japanese cast iron teawares. Additionally, Tetsubins: Iron Treasures of Japan is an excellent resource with extensive historical examples.
There is also some beautiful contemporary cast iron teaware in the Masayuki Kurokawa Showroom, and a large number of beautiful antiques on JCollector, which currently also has a couple of lovely cha tansu (Japanese tea cabinets).