The Pot Is Not Neutral: What Six Teapot Materials Do to the Same Oolong
Researchers brewed one oolong in six vessels, zisha and zhuni clay, stainless steel, ceramic, glass, and plastic, then measured what actually left the leaf. Clay changed the chemistry. Here is what the pore structure explains.
Brew the same oolong in a zisha clay pot and in a glass one, and the two cups are not the same liquid. Researchers at National Chung Hsing University measured it directly: one oolong, six vessels, and a chemical panel run on each infusion. The clay pots came out higher in catechins and lower in caffeine than the ceramic, glass, and plastic pots, and lowest of all six vessels in dissolved minerals and volatile aroma compounds. The pot behaves like an active ingredient in the cup.
A six-pot trial found clay pots hold back caffeine and minerals
The 2018 study, run by Liao and colleagues and published in the Journal of the Science of Food and Agriculture1, is the most direct test on record. The team brewed an oolong in six materials, zisha and zhuni (two grades of Yixing clay), stainless steel, ceramic, glass, and plastic, then ran the same infusion through HPLC-UV for catechins and caffeine, GC/MS for aroma volatiles, LC/MS for amino acids, and ICP-MS for minerals. Every pot got the same leaf, the same water, the same steep.
The clay infusions did not match the rest. EGC, EGCG, and total catechins came out higher from zisha and zhuni than from ceramic, glass, or plastic. Caffeine came out lower. Total dissolved minerals, potassium among them, and total aroma volatiles came out lowest of the six. Nothing about the leaf changed between pots. The vessel did the work.
A 2024 trial narrowed the finding to oolong and dark tea specifically
One study rarely settles a question, and a single result asks to be repeated in another vessel before it is trusted. A 2024 paper in Plants, by Guo and colleagues, ran a wider comparison: five vessel materials (tin, glass, pottery, porcelain, and purple sand, the trade term for Yixing clay) against five fermentation levels of tea, green, white, black, oolong, and dark, with the oolong sample specifically a Tieguanyin.
The result sorted by tea type, not just by pot. Tin suited green and white tea best, pulling out amino acids and theanine for a fresher taste. Porcelain suited black tea, raising polyphenols and soluble carbohydrate for more sweetness and body. Purple sand suited the oolong and the dark tea, the two partly and fully fermented categories, and the paper's own explanation for why points at the material itself: porosity that permits ongoing air exposure during the steep, a property glass, tin, and glazed porcelain do not share.
The clay is riddled with two kinds of pore
Two studies measuring the same effect independently is worth something. What is worth more is a mechanism, and ceramics research supplies one. A 2021 paper in the Journal of Ceramic Science and Technology, examining Benshanzhu zisha specifically, mapped the fired clay's structure with X-ray diffraction and found it built from a specific mineral mix before firing: illite at roughly 24 percent by weight, kaolinite near 16 percent, montmorillonite near 9 percent, hematite near 9 percent, and goethite near 10 percent. After firing, the hematite content rose to about 11.5 percent, the highest of any tea ceramic the paper tested it against, sitting inside a matrix that is over 70 percent amorphous glass rather than crystalline mullite.
That composition fires into a body shot through with pores ranging from the nanometer to the micron scale, clustered densely around the hematite grains. The pores do not run in one direction. Some connect all the way through the wall; others stay sealed inside it. The result is a ceramic the paper describes plainly: impermeable to water, but breathable to gas. Liquid tea stays inside the pot. Air and volatile aroma compounds do not.
That single property is the mechanism Guo's 2024 paper invoked to explain why purple sand suited oolong and dark tea specifically. Both categories are made by holding the leaf's own enzymes and, in dark tea's case, a microbial fermentation, in a controlled oxidative state for an extended period before the leaf ever meets water. A vessel that keeps exchanging a little air through its wall over a six-minute steep is continuing, faintly, the same kind of oxidative environment those leaves were built in. A green tea, killed-green early and never meant to keep oxidizing, has no use for that continuation, and the tin pot that suited it best is airtight, not porous.
The seasoning tradition is measured chemistry
A used zisha pot develops a shine after enough infusions, a polish longtime keepers build by hand with the tea itself rather than with any glaze. Sellers and tea guides call this seasoning, and warn against soap in the same breath: wash a zisha pot with detergent, and the porous body absorbs the chemical along with everything else, permanently.
A 2015 paper in the Journal of Food and Drug Analysis, by Chung and colleagues, put a name to what the seasoning actually is. They analyzed the waterproof film that builds up on the inner wall of used zisha and zhuni pots and identified it as fatty acids and linear hydrocarbons, palmitic acid and stearic acid chief among them. Their account of the mechanism: unsaturated fatty acids present in the fresh leaf's cell membranes degrade and oxidize during the leaf's processing into saturated forms, and steam carries a portion of those lipids to the clay wall on every steep, where the pot's own porosity holds them. The patina is the tea's own chemistry: laid down one infusion at a time, locked into the same pores that let air through.
That mechanism is also the reason a keeper dedicates one pot to one tea, or at most one narrow style of tea, rather than rotating stock through it. Every steep leaves a small deposit behind. A pot seasoned for years on a single roasted yancha carries that specific residue into the next cup, reinforcing it. Brew a green tea in the same pot and the accumulated dark-roast lipid film works against the lighter tea instead of for it.
Evaluation runs the opposite way: the standard vessel is neutral porcelain
The same porosity that suits daily drinking is precisely what a professional tasting panel avoids. ISO 3103, the international standard for preparing tea liquor for sensory judging, specifies a pot of white porcelain or glazed earthenware, holding 150 milliliters (plus or minus 4) or 310 milliliters (plus or minus 8), roughly two-thirds of a cup or one and a third, loaded at 2 grams of leaf per 100 milliliters of freshly boiling water and steeped for exactly six minutes before the liquor is poured off. Nothing about that specification is arbitrary. Porcelain and glazed earthenware are dense and effectively non-porous, so the vessel absorbs nothing, seasons toward nothing, and adds nothing to what the panel tastes. A gaiwan built to the same logic, thin porcelain, no glaze gaps, is why it remains the working tool for judging an unfamiliar tea honestly, distinct from the zisha pot's job of flattering a tea already chosen.
The two jobs do not compete, and the discipline holds both. Judge a new oolong in porcelain or a gaiwan, where the vessel adds nothing of its own. Once it earns a place in the rotation, move it to a clay pot kept for that character alone, roasted yancha to one pot, a green-leaning gaoshan to another if a second pot is owned. There the pot works slowly and by chemistry: it holds back a little caffeine, keeps a little more catechin, and gives back only what earlier steeps of that same tea laid down in its walls.