Glass and porcelain
Dark tea is the one category where material genuinely divides opinion, and the argument starts with rinsing.
Compressed leaf has to be rinsed before it is brewed, and a gaiwan makes that step quick.
A gaiwan of 100 ml pours out completely in a couple of seconds, and that is what rinsing needs.
Porcelain is also neutral, which matters while you are learning the three technologies apart.
Raw pu-erh, ripe pu-erh and a fu brick taste very different, and a neutral vessel shows all three honestly.
It rinses clean of the fermentation smell, which is the main practical advantage over clay.
Glass works for loose dark tea and shows the liquor, and colour is how a dark tea is judged.
A white interior shows whether the liquor is clear or cloudy, and that is a storage signal.
Glass holds heat badly, though, which is the one thing dark tea cannot forgive.
Water at 100 °C is standard, and a thin walled glass cup loses several degrees immediately.
That loss shows up as a thinner liquor and a slower opening of the leaf.
Porcelain is therefore the better neutral choice, and glass is the one for watching rather than brewing.
Clay
Clay is the traditional material for dark tea, and here the argument for it is strong.
Heat is the first reason, since a thick clay wall holds boiling temperature through a long session.
Dark tea gives out slowly over many infusions, and that needs a vessel that stays hot.
Absorption is the second reason, and it is the one drinkers actually prize.
A zisha（紫砂）pot used on ripe pu-erh builds a layer that rounds the liquor out over a year.
The pores hold oils from the tea, and the effect is a slightly thicker mouthfeel.
That layer is specific to one tea and cannot be moved to another pot.
It also means the pot is finished as a neutral vessel, and it will never be one again.
A ripe pu-erh pot used on a raw cake will blur the two together, and raw pu-erh drinkers notice.
Seasoning takes months of regular use, and a pot sold as already seasoned was seasoned on something else.
Mould is the risk, since a porous pot put away damp will grow it in a warm kitchen.
Dry the pot completely with the lid off, and smell it before every session.
Matching to the tea
Match by technology first, since raw, ripe and basket aged dark tea are almost different drinks.
Raw pu-erh wants a neutral vessel, because its whole value is a delicate bitterness that turns sweet.
Clay would round that off and take away the thing you paid for.
Ripe pu-erh wants clay, since the extra body and heat retention suit it.
A fu brick and Liubao sit somewhere between, and porcelain handles both without complaint.
Match by form second, because compressed leaf needs a rinse and loose leaf does not.
Cake points to a gaiwan or a clay pot, and loose leaf points to porcelain or glass.
Match by age third, since an old cake has more to give and needs the heat.
A young raw cake is harsh in any vessel, and no material will make it taste older than it is.
Fix the dose at 5 grams in 100 ml and weigh it to 0.1 g, since a chunk off a cake is never the dose you think.
Rinse once with boiling water for about 5 seconds, then hold the first infusion for 30 seconds.
Material comes last, and it matters less than whether the cake was stored properly.
Buying advice
Buy a porcelain gaiwan of 100 ml first, and use it for a year before buying anything else.
It costs very little, handles every dark tea, and rinses clean of anything.
Check the glaze is intact, since a crazed surface will start behaving like clay.
If you then want clay, buy one pot and dedicate it to one technology for good.
Avoid anything sold as zisha（紫砂）for under twenty dollars, which is slip cast and tinted.
Real hand built clay costs real money, and a cheap pot claiming to be it is a risk.
Warm the empty pot and smell it, and walk away from any chemical or dusty note.
Check the lid fits and the spout pours, because a cake session means many quick pours.
Buy a pick at the same time, since no compressed tea can be brewed without one.
A pick costs less than a cup of tea and matters more than the material of the pot.
Keep the money in provenance, since a cake with a known history beats any vessel.
Storage and maker first, and the material somewhere well down the list.