Separating three claims
When people say a pot’s material changes the tea they may mean any of three different things. That the vessel holds heat differently, and so the water inside it follows a different temperature curve during the steep. That the vessel’s surface interacts with the liquor — absorbing something, or contributing something. Or that the material imparts a character of its own to any tea brewed in it. The first is ordinary thermal physics and is not in dispute. The second is real for unglazed clay and limited in scope. The third is where the marketing lives. Keeping them apart is most of what it takes to evaluate a claim about a teapot.
Heat retention, the part that is simply true
A vessel’s wall thickness and the material’s heat capacity determine how much heat it takes out of the water on contact and how fast it conducts heat away afterwards. Thin glass and thin porcelain take little and lose it quickly. Thick stoneware and clay take more initially, and then hold what they have taken, which keeps the water warmer through a long steep once the pot itself is warm. Cast iron is the extreme of both. This is why heat-retentive vessels are conventionally paired with roasted oolong and dark tea, which want sustained temperature, and why a preheated pot behaves quite differently from a cold one of the same material.
Porosity: what unglazed clay does
Unglazed stoneware has an open surface that takes up a trace of oil and aroma from the tea passing through it, and releases a little back over subsequent sessions. This is not folklore — it is why such pots are conventionally dedicated to a single tea or a single category, why they are washed with water alone, and why a pot seasoned on aged dark tea is unsuitable for a fresh green one. What is much less settled is the size of the effect on a given cup. It develops over years of use, it is not separable from the vessel’s heat behaviour in any casual comparison, and the confident claims made about how much a seasoned pot improves a tea rest on accumulated practitioner judgement rather than on controlled testing.
The clay-type claims
Within Yixing clay, the trade distinguishes zini, zhuni, duanni and others, and attributes specific flavour consequences to each — one said to round a tea, another to sharpen it. TeaHQ records the names as material categories and does not repeat the flavour attributions, for two reasons. The differences claimed are small relative to the differences between individual pots of the same clay, particularly in wall thickness and firing. And the market for these pots is one where provenance is frequently misrepresented, so a great deal of what circulates as knowledge about a clay type is really knowledge about what a seller said.
Iron, and the claim about softer water
Water boiled in an unlined iron kettle is very widely described as tasting rounder and sweeter, and the practice has a long history in Japan and China. The usual explanation involves the bare iron interacting with the water. TeaHQ has not found a controlled comparison establishing the size of that effect, and notes that in practice an old kettle also carries a mineral layer built up from the local water, so what is being compared is rarely just iron. It is a claim worth reporting and not worth asserting. The enamel-lined pots sold as cast iron teapots have no bare iron in contact with the water at all, so whatever the effect is, they do not produce it.
Shape usually beats material
The properties that reliably change a cup are geometric rather than chemical. Whether the vessel empties completely determines whether the next infusion starts clean. Whether the leaf has room to expand determines whether the water reaches all of it. How wide the vessel is determines how fast it sheds heat. How well it pours determines whether the steep ends when you intended. A porcelain pot that empties cleanly will out-brew a beautiful clay one that dribbles and retains a spoonful, every time, and neither material argument enters into it.
What this means for buying
Choose neutral and well-shaped first — glazed porcelain or glass, pouring cleanly, sized for how much you actually drink. Add a heat-retentive vessel if you regularly brew teas that want boiling water. Consider an unglazed clay pot only once there is a single tea you drink often enough to dedicate a vessel to, because that dedication is the deal the porosity requires. None of this is an argument against buying a pot you find beautiful; it is an argument for knowing which part of the purchase is flavour and which part is pleasure.