Underfired Zisha Clay
Note: while this chapter is about underfired zisha clay, it touches heavily on yixing-tea interaction, of which we will have much more to say (with experimental results) later in the book.
The terms “high-fired” and “low-fired” are occasionally used in the Western-or-English-speaking tea community as imprecise terms in reference to the maturation of Yixing wares. These terms lead to confusion, as the temperature of the firing alone is indeterminant of a ware’s position on the densification curve[1]; for example, high-fired zhuni teapots are fired at a lower temperature than low-fired zini. In place of these commonly used terms, this book uses the more accurate relative terms “underfired” and “overfired” in reference to a ware’s overall maturation in comparison to its expected level of densification.
Underfired Yixing teapots have been fired above their sintering temperature[2] while remaining below their “maturation target”, yielding a lower level of vitrification and densification. Underfired wares are more porous with a higher ratio of open-to-closed pores (in comparison to a mature firing of the same material) from the reduced formation of amorphous melt. Most moderately underfired Yixing wares are fully usable, as utilitarian ceramics, though they may cause unwanted interaction when used for brewing tea. The decreased vitrification and coarser surface texture generally cause muting effects[3], reducing the intensity of the tea’s aroma and flavor.
Tea in aqueous solution (brewing tea) predominantly interacts with zisha material through adsorption (not absorption[4]), where various molecules “stick” to the surface of the ceramic material via molecular forces. Tea contains a vast array of polyphenols, predominantly in the form of flavonoids[5], ranging from ~15 - 30% of the dry-leaf weight, and 20 – 40% of the brewed tea’s dissolved solids[6]. Polyphenols are largely responsible for the tactile mouth-feel, the bitterness or astringency, and modulation of other flavors within a tea[7]. Select polyphenols may undergo rapid oxidation at the solid-liquid interface of tea and zisha where polyphenols in the aqueous tea soup are adsorbed to iron-oxide nucleation points. Fe2O3 and Fe3O4 act as Lewis acid catalyst sites[8], such that specific polyphenols are oxidized into ortho-quinones[9], theaflavins[10], and thearubigins[11].
Oxidation changes the sensory properties of polyphenolic compounds in distinct ways: theaflavins contribute a clean and tactile quick-drying mouthfeel; when oxidized, this transforms into a heavier, more tannic astringency that coats the entire mouth. Theasinensins, formed through oxidative dimerization[12], provide a refined astringency focused on the front of the tongue; their further oxidation similarly leads to a more diffuse, coating sensation. Thearubigins, while minimally aromatic, contribute to both color and mouthfeel; their oxidation leads to increased polymerization, resulting in stronger astringency and decreased solubility. Catechins provide fresh, green, and vegetal notes; when oxidized to ortho-quinones, they lose these characteristics entirely, developing flat, musty, oxidized flavors with notably reduced aromatic properties. The flavor impact of catalytic oxidation of polyphenols by zisha ceramics generally and under-fired zisha specifically varies from inconsequential to impactful, with other physical attributes of the teapot (such as size, thickness, heat retention, and shape), potentially playing a larger role in the extraction and flavor formation of certain-teas.
The catalytic oxidation of tea polyphenols[13] is similar to well-studied processes in other food products likewise sensitive to oxidation: in wine and olive oil, for example, metal oxide catalyzed oxidation of polyphenols occurs rapidly enough to create perceptible flavor changes within minutes at room temperature. The Arrhenius relationship, an equation to approximate the effect of temperature on the rate of chemical reactions, gives us an approximate 2 – 4x increase in rate per 10°C increment. Given that tea is brewed at near-boiling temperatures, the assumed rate may be as high as 180x faster (an approximate upper-bound) than the room-temperature reaction, allowing for notable flavor changes even during rapid gong-fu style brewing sessions with steeps under 30 seconds, depending on the tea and teapot; the overall impact of this chemical change may be a large increase from a small baseline and the effect can range from inconsequential (no change in flavor) to tastable (with changes below the bound of standard methods of chemical testing such as HPLC) to notable both organoleptically and analytically.
In addition to the flavor changes resulting directly from oxidation of polyphenols, the oxidized theaflavins and thearubigins may undergo further complex interactions[14] with volatile aromatic compounds, forming hydrophobic regions that trap additional aroma compounds, generally or selectively muting the aroma of tea[15].
Finally, the surface texture of the fired zisha material plays a modulating role in the physical chemistry of this process: rougher materials have higher surface area[16] creating more nucleation sites for adsorption; under-fired zisha wares have rougher surfaces, yielding more nucleation sites for (mostly) red-iron oxide interaction[17]. Furthermore, it is theorized that red-iron oxide is more selective in its interaction with polyphenols, while black-iron oxide is both stronger and less selective, oxidizing a larger set of compounds contributing to the flavor changes in the tea from interaction with zisha material.
Less commonly, in severely underfired wares[18] certain molecules (including aromatic compounds themselves or aromatic compounds aggregated with polyphenols) may be absorbed into the pores of the teapot; this is the root cause of related flaws such as “Black Vomiting” (吐黑)[19], and is not the primary mechanism of tea-ceramic interaction. In wares fired to maturation, the proportion of open pores is too small to have a meaningful impact on the absorption and thus flavor profile of the resulting tea[20].
In summary, under-fired zisha teapots (without other flaws) usually have a general-muting effect on the flavor and aroma of the tea, varying in their interaction and intensity based on: the tea itself, the amount and types of iron-oxides within the zisha material, and the surface texture. General muting is considered an undesirable attribute of under-fired zisha clay by most high-level practitioners. While certain sub-types of fully mature (“fully fired”) zisha clay may exhibit partial or selective muting (such as the reduction of bitterness or the reduction of roast notes) and can thus be paired with teas benefiting from a selective rebalancing of flavor-and-aroma attributes, an overall reduction in tea-flavor intensity (inclusive of aroma) is rarely desirable[21].
[1] See previous chapter Book 2 – Chapter 4, Section 4: An Introduction to the Production of Clay Ceramics: Firing (Sintering and Vitrification) for a discussion of densification; See previous chapter Book 2 – Chapter 10, Section 2: Firing of Yixing Teapots: Mature Firing of Zisha Clay for a discussion of maturation.
[2] And will thus not slack in water – wares fired to only sintering temperature are bonded yet not vitreous.
[3] Muting in this context indicates the tea’s flavor and aroma are reduced in intensity; muting often carries the adjacent connotation of “rounding”, where the flavors and aromas present are more diffuse, blending into one another instead of sharp and defined, and are thus more difficult to identify. A teapot can be either muting, rounding, or both muting and rounding. Descriptive terms for zisha clay’s interaction with tea and their definitions will be discussed in detail in subsequent chapter: Book 2 – Brewing Theory
[4] It is commonly (and mistakenly) believed that Yixing teapots absorb the flavor of tea; the similarity of these terms, “adsorption” and “absorption”, has likely caused some of the prevalent confusion found amongst tea practitioners on the form of sorption taking place when brewing tea in a Yixing teapot.
[5] A category including catechins, theaflavins, and thearubigins; all flavor-active bioactive organic compounds that form and modulate the flavor-profile of tea. Each of these sub-categories contains hundreds of specific compounds, of which many have yet to be characterized.
[6] Calculated from Astill, Conrad, Mark R. Birch, Clive Dacombe, Philip G. Humphrey, and Philip T. Martin. "Factors affecting the caffeine and polyphenol contents of black and green tea infusions." Journal of agricultural and food chemistry 49, no. 11 (2001): 5340-5347. Other research gives similar values, though most research concentrates on teabag tea.
[7] The flavors of various polyphenols are cited from Obanda, Martin, P. Okinda Owuor, and Richard Mang'oka. "Changes in the chemical and sensory quality parameters of black tea due to variations of fermentation time and temperature." Food chemistry 75, no. 4 (2001): 395-404; there are hundreds of other papers with similar findings.
[8] This finding is arguably theoretical, though these interactions are cited in the literature across a range of disciplines. This section cites Xin, Yunzi, Sota Shido, Kunihiko Kato, and Takashi Shirai. "Glazes induced degradation of tea catechins." Scientific Reports 13, no. 1 (2023): 10507. and Ouyang, Qiong, Fangying Kou, Pokeung Eric Tsang, Jintao Lian, Jingyi Xian, Jianzhang Fang, and Zhanqiang Fang. "Green synthesis of Fe-based material using tea polyphenols and its application as a heterogeneous Fenton-like catalyst for the degradation of lincomycin." Journal of Cleaner Production 232 (2019): 1492-1498. Let it never be said that I do not suffer for you dear reader – for I have read the Journal of Cleaner Production on your behalf to find information on iron-oxide catalyzed tea polyphenol oxidation.
[9] Highly reactive forms of oxidized catechins; likely to rapidly form new compounds in solution during tea brewing. These compounds are generally bitter and astringent.
[10] Compounds responsible for the red coloration of tea; generally contribute to high-tone astringency and mouth-feel; not necessarily a negative contribution to the flavor profile.
[11] Compounds responsible for the brown coloration of tea; generally contribute to complex malty flavors.
[12] Dimerization is the oxidative coupling of two catechin molecules, which changes the flavor and aroma of the joined compound.
[13] For clarity: this catalytic oxidation of tea polyphenols is distinct from the polyphenol oxidation pathway (via polyphenol oxidase) of tea processing, despite both reactions oxidizing polyphenols.
[14] Specifically, polymerization and aggregation.
[15] This is similar to how aromatic compounds in wine can be bound and "released" by tannins through reversible hydrophobic interactions and hydrogen bonding. A similar mechanism occurs in tea: the oxidized polyphenol groups can capture and release volatile aromatics, modulating both their concentration and timing of release into the tea’s soup and headspace.
[16] Technically: “specific surface area” though that specification is not particularly important here. This sidenote is defensive writing against fellow pedants.
[17] The Kiln-type also plays a role in the formation of surface texture, a point that has been obliquely referenced in multiple chapters and will be discussed in detail in a later section of this book.
[18] Determinable by the presence of additional flaws; zisha underfired to the point of being absorptive is bound to develop other more serious flaws than the merely-underfired teapots that are the focus of this chapter. For example, in addition to stained zisha teapots, Yixing teapots with a distinct muddy or pond-like aroma from the clay may be suffering from severe under-firing.
[19] Discussed in subsequent section of this chapter, Section 3 – Part 1: Post-Firing Material Flaws: Black Vomiting (吐黑).
[20] This is evidenced by the lack of tea or degraded tea aroma in well maintained antique and contemporary Yixing teapots (those without an inner patina (茶山) of degraded tea material), which smell like ceramic or sand, and not degraded tea compounds; this lack of aroma is evidence of little to no organic compound absorption in a fully fired ware.
[21] Teapots that are generally muting are occasionally referred to as “Shou Pots” in the Western-and-English-speaking tea community, in reference to their suggested “only suitable use” in brewing shou pu’er tea. This is a humorous if not totally fair assessment of the qualities of good shou pu’er.