Cast iron
Heavy, durable and thermally the opposite of what people expect. A cast iron vessel is excellent at staying hot once it is hot, and correspondingly brutal at taking heat out of anything poured into it cold. Whether that helps or hurts comes down entirely to whether you warmed it.
What it is
An iron alloy with carbon and silicon, not pure iron. It is surprisingly brittle for a metal: pockets of iron carbide through the body block stress from travelling, much as the polycrystalline structure does in a ceramic. Its thermal conductivity is higher than either stainless steel or ceramic. In teaware it appears in two forms with opposite jobs, and confusing them damages one of them: the unlined tetsubin, for heating water over a flame, and the enamel-lined tetsukyusu, for steeping leaf and never for direct heat.
What it does to your tea
Through an enamel lining, nothing chemically, which is what the lining is for. Thermally, it dominates. A 1200 g cast iron pot pulled 500 g of boiling water down to 80 degrees Celsius on contact and 76 after four minutes, well below the range for a black tea. Pre-warmed, the same pot started at 90 and lost only 5 degrees over the whole infusion. That gap, 76 against 85, is the largest single effect pre-warming has on anything measured here, and it is a direct consequence of mass.
What it suits
Robust teas brewed hot: black, roasted oolong, dark tea. It suits delicate green and white tea least, both because of the pouring temperature and because a heavy vessel is unforgiving if you skip the warming step.
Care and cleaning
Depends entirely on which object you have. An unlined tetsubin: empty after every use, dry with residual heat rather than a towel, never scrub the interior, and never leave water standing in it. The mineral layer that builds up inside is what keeps it from rusting. An enamel-lined tetsukyusu: rinse and dry, and keep it off the flame, because heat destroys the lining it depends on.
What people get wrong
Two, and both come from the same confusion. Steeping in a bare tetsubin corrodes the kettle and puts metal into the tea. Heating an enamelled tetsukyusu over a flame ruins the enamel. The failure modes run in opposite directions, so this is the one piece of teaware terminology worth being pedantic about.
Made of this
Sources
- 500 g of water at 100 °C fell to 80 then 76 °C in a 1200 g cast iron pot and to 90 then 82 °C in a 400 g glass pot; a double-walled glass pot held 93 then 89 °C Francl, M. (2024). Steeped: The Chemistry of Tea, chapter 7. Royal Society of Chemistry