What the clay actually does
Yixing（宜兴）pots are unglazed stoneware from Jiangsu, and the clay is the whole point.
The ore is mined locally and processed into a body that can be slab-built.
What makes it unusual is that the fired surface is full of tiny pores.
Those pores hold heat, and they hold flavour.
That is why a pot gets seasoned to one category of tea.
Use it for roasted oolong and it will taste of roasted oolong for years.
Whether that is a virtue or a nuisance depends entirely on how you drink.
Anyone who likes switching between green and dark tea should use glass instead.
The walls are thick relative to porcelain, so the pot holds temperature.
A 150 ml pot serves 2 or 3 drinkers and keeps a dark tea hot through a session.
The trade is that you lose the aroma a thin wall would have given you.
Anyone drinking for aroma is usually better served by a gaiwan（盖碗）.
None of that makes Yixing better than the alternatives.
It makes it specific, and specific tools suit specific drinkers.
Decide which kind of drinker you are before spending money.
The three classic clays
Zisha（紫砂）is the purple-brown clay most people mean when they say Yixing.
It is the most porous of the three and the most forgiving to fire.
Zhuni（朱泥）is the red clay, denser, with a much finer surface.
It shrinks more in the kiln, which is why large zhuni pots are rare.
It also rings higher when tapped, and people use that to tell them apart.
The tap test is unreliable, since wall thickness changes the pitch too.
Duan ni（段泥）is the pale yellow-green clay of the three.
It is the least forgiving in daily use, since it shows stains readily.
It suits green and white tea better than the darker clays do.
Colour alone is a weak guide, because one clay fires to several shades.
Additives that brighten the colour are common and rarely declared.
A pot in an unnatural red or blue should be treated as decorative.
Any seller who talks only about colour is not talking about the clay.
Ask what the body is made from rather than what it looks like.
Buy the clay for how it brews, not for how it photographs.
How a pot is made
A Yixing pot is built rather than thrown on a wheel.
The clay is rolled into slabs and beaten into shape.
Full-hand making uses no mould at all, and it takes far longer.
Half-hand making uses a mould for the body and finishes it by hand.
Machine pressing is quicker again, and the result is thinner and more even.
All three produce a pot that works, and the price gap is mostly labour.
The lid fit is the thing to check, whatever method was used.
A well-fitted lid seals properly and still turns freely.
Hold the pot by the handle, cover the spout, and tip it over.
A good lid stays on, and that is a real test rather than a party trick.
Firing runs around 1,100 to 1,200 °C, which partly vitrifies the surface.
Below that the pot stays too porous, and above it the clay loses its texture.
Shapes carry names, from the plain to the deliberately fanciful.
The name matters far less than whether the pot pours cleanly.
Test the pour before you look at the signature on the base.
Buying, seasoning and living with one
The market is full of fakes, and that is the first thing to know.
Clay from outside Yixing is sold as Yixing clay constantly.
A pot stamped with a famous name was almost certainly not made by that person.
Price is a weak guide, since a fake can cost more than a real beginner pot.
Buy from a seller who will say what the clay is and where it came from.
One who cannot answer that has already told you what you needed.
For a first pot, a plain machine-made one is the sensible choice.
Season it by brewing the same tea in it for 10 to 14 days.
Boiling it in tea is advice that circulates and does nothing useful.
Rinse it with hot water, let it dry, and never let detergent near it.
One pot, one category of tea, is the rule most people settle on.
A pot used for everything ends up tasting of everything.
Keep the lid off when it is stored, or the pot will smell stale.
Treat it as a tool that improves with use, and it generally will.
The best pot you own is the one you reach for without thinking.