Unglazed clay
Fired clay with no glaze on the inside, so the body stays porous. Yixing zisha is the famous example, Tokoname and Banko the Japanese ones. This is the only teaware material with a measured, published effect on what ends up in the cup, and also the one carrying the largest pile of claims that nobody has measured.
What it is
Clay fired hot enough to be hard and watertight in practice, but not vitrified, so an open pore network survives. In Yixing zisha that network has been characterised: two size families, one around 0.2 micrometres and one below 10 nanometres, and a water absorption rate for finished bodies of roughly 1.6 to 7.05 per cent, against effectively zero for porcelain. Zisha is unusually high in iron oxide, which is where its colours come from.
What it does to your tea
It changes the chemistry, and this has actually been tested. Liao et al. brewed the same oolong in zisha, zhuni, stainless steel, ceramic, glass and plastic pots and analysed each infusion by HPLC, GC/MS, LC/MS and ICP-MS. The two clay pots gave higher total catechins including EGC and EGCG, less caffeine, the lowest total mineral content and potassium, and the lowest volatile compounds of the six. The pot is doing something and it is not imagination.
What it suits
Black, oolong and dark tea. Thick-walled clay holds heat, and those are the teas brewed hot and resteeped many times. Green and white tea are the ones it suits least, both because of the heat and because their appeal is in delicacy a porous body is most likely to alter.
Care and cleaning
Hot water only. Never detergent, which the pores will hold, and never a dishwasher. Dry with the lid off. One pot for one family of tea, and that rule follows straight from the measured porosity rather than from tradition: whatever the body takes up, it keeps.
What people get wrong
Taking the study's conclusion instead of its numbers. Liao et al. state that clay pots presumably give infusions that are less bitter and more fragrant, and that sentence sits awkwardly against their own data. No sensory panel was run. Catechins are the astringent fraction and they went up. Volatile compounds, which are what fragrance is made of, came out lowest in exactly those pots, and a porous body absorbing aromatics is the straightforward reading of that. Meanwhile the popular claim that Japanese Tokoname or Banko clay adsorbs catechins and measurably reduces bitterness is repeated everywhere and cited nowhere: this study tested Chinese clay, not Japanese, and extending it is an assumption. What is solid is that unglazed clay changes the cup and holds what it absorbs. What that does to the taste is not settled.
Made of this
Sources
- unglazed Yixing clay measurably changes an oolong infusion: higher total catechins, less caffeine, lowest minerals and lowest volatile compounds of six pot materials Liao, Z.-H. et al. Effect of teapot materials on the chemical composition of oolong tea infusions. Journal of the Science of Food and Agriculture, 751-757
- water absorption of Yixing bodies runs roughly 1.6 to 7.05 per cent; the fired body carries pores at about 0.2 µm and below 10 nm Pore structure and crystal-phase analysis of Dicaoqing zisha, Journal of the European Ceramic Society