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.
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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 tended to be lowest in dissolved minerals and volatile aroma compounds.
A six-pot trial found clay pots gave less caffeine and minerals
The 2018 study, run by Liao and colleagues, was published in the Journal of the Science of Food and Agriculture1. The team brewed an oolong in six materials, zisha and zhuni (two Yixing clays), stainless steel, ceramic, glass, and plastic. It ran each infusion through HPLC-UV for catechins and caffeine, GC/MS for aroma volatiles, LC/MS for amino acids, and ICP-MS for minerals.
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. Stainless steel did not separate cleanly from the two clays: the differences among those three pots were not all significant. Total dissolved minerals, potassium among them, and total aroma volatiles tended to come out lowest of the six.
A 2024 trial matched purple sand to oolong and dark tea
A 2024 paper in Plants, by Guo and colleagues2, 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 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, which the paper suggests may be due to its porosity and permeability.
The clay is riddled with pores
A 2021 study of Benshanzhu zisha3 mapped the fired clay's structure with X-ray diffraction and found it built from a specific mineral mix before firing: illite at 24.1 percent by weight, kaolinite 15.7 percent, montmorillonite 9.2 percent, hematite 9.3 percent, and goethite 10.1 percent. After firing, the hematite content rose to about 11.5 percent, higher than the porcelain and pottery it was compared with. The fired body is over 70 percent amorphous phase rather than crystalline mullite.
That composition fires into a body shot through with pores ranging from the nanometer to the micron scale, with many hematite particles in the pores. The paper describes the result as impermeable to water but breathable to gas.
The film on a used clay pot
A 2015 paper in the Journal of Food and Drug Analysis, by Chung and colleagues4, analyzed the waterproof film that builds up on the surface 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, presumably, in the fresh leaf's cell membranes may degrade or oxidize during the leaf's processing into stearic acid.
The standard tasting vessel
ISO 3103, the international standard for preparing tea liquor for sensory judging5, 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. It is loaded at 2 grams of leaf per 100 milliliters of freshly boiling water and, for black tea, steeped for six minutes before the liquor is poured off.