The property that actually varies: conductivity
Silver and copper conduct heat far better than steel, and steel far better than ceramic. A highly conductive vessel heats through almost instantly and sheds heat to the room just as readily, so a thin silver or copper pot is hot in the hand within seconds and cool again quickly. This is why such pots have insulated handles and why they are usually used where the tea is poured out immediately rather than held. Iron is the outlier: it is a modest conductor with a great deal of mass, so a cast iron vessel is slow to heat and slow to lose it, which is a genuinely different behaviour and the one real functional argument for it.
Surfaces, linings and why copper is tinned
Copper reacts with acidic liquids, which is why the interior of a copper vessel meant to hold a drink is traditionally lined with tin — and why a worn lining is the thing to inspect on an old samovar. Enamel is a glass lining on steel or iron and is inert while it is intact and a liability once it is chipped. Stainless steel needs no lining at all, which is most of its appeal. Silver is unreactive but tarnishes, and a heavy sulphide tarnish is detectable. The general rule is that a metal vessel’s behaviour is a property of its surface rather than of the metal named on the label.
The iron claim, and how far it can be taken
Water boiled in an unlined iron kettle is very widely described as tasting rounder and sweeter, in Japan and China alike, and the practice is old and serious. What TeaHQ can say is that the interior is bare iron, that it develops a mineral and oxide layer in use, and that an old kettle’s layer is built from the local water rather than from the iron alone — so the comparison people describe is rarely a clean one. What it cannot say is how large the effect is, because it has found no controlled comparison. And a point of practical importance that the marketing blurs: enamel-lined cast iron teapots have no bare iron touching the water, so whatever the effect is, they are not producing it.
The silver claim
Silver pots are said to brighten, sharpen or clarify a tea. This is asserted constantly and TeaHQ has found nothing behind it. Two things are true and may be doing the work: silver is chemically unreactive, so a silver pot introduces nothing where a scratched or pitted base metal might, and it is thermally fast, so a tea brewed in one follows a different temperature curve than the same tea in ceramic. Both of those are also true of glass, which is a useful comparison to keep in mind when a silver pot’s effect is being described as unique.
Metallic taste, when it is genuinely there
People do sometimes report a metallic character from steel vessels, and the usual explanation is not the steel but the state of it: scratches, pitting, welds and low-grade alloy hold residue and corrode locally. A clean, intact stainless surface is inert. If a steel pot or flask tastes of metal, clean it properly with bicarbonate of soda and hot water, and if the taste survives that, the surface is damaged and the vessel is done.
What to actually choose
For brewing, ceramic or glass, for the reason that they are inert, cheap and hold heat sensibly. For boiling on a hob, stainless or enamelled steel. For a table vessel that must stay hot for an hour, cast iron or a double-walled steel pot. Silver, copper and brass are bought as objects, and that is a perfectly good reason to buy them — it is only worth being clear that the flavour argument attached to them is not one TeaHQ can support.