Do different clay types retain heat differently?
When I was making a visual piece for a class last week, I did a test of temperature over time for different brewing vessels (https://www.reddit.com/r/tea/comments/4rolqy/when_all_you_have_is_boiling_water_a_pocket_guide/) and I was super surprised by what a difference the clay made, especially on later brews.
I know the different types of clays have different mineral contents that affect the flavors of the teas… but… what other differences do they have that affect brewing?
I don’t have many clay pots for data points, but am curious to learn more about the differences, so next time I have the money for teaware I know what I am looking for. I know thickness would make a difference as well… but does how porous the clay is have a huge effect?
I would agree that different clays retain heat differently but I don’t know the science behind it. There have got to be some good sources out there on this but I fear they may all be in Chinese.
Also, how thick the clay in a teapot is is a factor. But some good thin clay pots will retain heat really well to.
I’d wager that teapots made of the same shape, volume, and wall thickness but from different clays would be so close in thermal conductivity as to be unnoticeable. It would be a good test though.
You can also look to geology for the answer. Well, not from me—I’m not a soft rock expert and could probably only give a few half-hearted guesses about how hand-shaping clay misaligns the platelets allowing for a greater porosity (coupled with the proportions of impurities found in the kaolinite—such as free silica, mica, iron oxide, and on the larger side of particles—zisha has a rather large proportion of silt and sand), and how porosity would break up clay-contact and disrupt or at least change heat retention.
I’d actually be interested in papers concerning the chemical makeup/properties of zisha clays. Unfortunately I haven’t found too many yet that my university actually has access to.
But, there is a somewhat relevant TeaChat thread. If you can get past all the passive aggressive responses (actually, just skip most of the first page), wyardley breaks down one of the papers: http://www.teachat.com/viewtopic.php?f=36&t=18060&start=15
The TLDR of it is:
-Zisha clay is actually /less/ porous than ‘regular’ clay, but still more porous than porcelain; yixing pots are fired at a higher temperature than other clays and porcelain, and the relevant chemical changes that happen at this higher firing temperature are explained in the paper.
-Do to how yixing pots are made, the walls of the pot are actually least porous on the outside, and most porous from the middle of the wall and on towards the inside of the pot
-zisha clay retains heat a bit better than porcelain, but not as well as normal pottery (thermal conductivity can be roughly taken to mean ‘how much heat it loses/how quickly’)
One pitfall is that it only studies one type of zisha clay (albeit many examples of it over many years—part of the paper is to explore how the composition of the clay used has changed with the times). But it does apparently cover how the different construction-types used can change these properties.
Looks like my university’s got access to the full paper, so it’s been added to my list of To-Reads. The rest of the TeaChat thread is seems to be people discussing how they managed to guess the firing temperatures for the clays; if it wasn’t several years old, I’d chime in with the most likely method.
The thing about zisha being fired at higher temperature than porcelain doesn’t quite add up. I know zisha is fired at higher temperatures than most ceramic clays, but porcelain is fired at temperatures up to (and sometimes over) 1400. I believe zisha’s ceiling is 1200? I suppose there could be overlap with the lower end of porcelain and higher end of zisha though. I also don’t claim to be an expert on ceramics or zisha though!
Whoops, that was actually me typing it wrong—I meant to say “yixing pots are fired at a higher temperature than other clays but LOWER than porcelain”. I need to proofread.
All the firing temperatures measured in the paper for zisha are lower than 1200, yes.
To first order, mass will be the determining factor. Heat capacity will be next most significant, though I expect all clays to have similar heat capacity. However, the ability of your tea to heat the mass is IMHO more important than its ability to hold the temperature I prefer light, thin-walled brewing vessels so that the tea stays at high temperature during the steep. If you pre-heat your pot, that isn’t as important.
The type of material does make a difference in cups, for example. Technically, porcelain isn’t clay, it is stone. Porcelain tends to heat up very quickly and get very hot. Those thin cups are also very dense.
I prefer clay for drinking hot tea. There were some hand painted large rooster cups which came out of Asia that were perfect for tea. The density was lower, and the walls of the vessel were thick, making for a ligher cup.
The design of the cup will also affect how quickly it loses heat. After my best cup broke, I use a couple of ceramic mugs. Coffee gets cold too quickly in them. To the extent that I need a cup warmer in the winter.
Remember Newton’s Law Of Cooling. The rate something cools is proportional to the difference in temperatures. So a cup wall that heats up hotter, will also lead to quicker cooling.
I’m not an expert on this subject, but I do like a good cup. :)
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