Aug 13th 16 12:45 pm
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Thermal properties of teapot materials

by absence » Aug 13th 16 12:45 pm

It's a bit difficult to find exact figures for heat capacity and thermal conductivity, but it seems like stoneware, porcelain, and glass are roughly in the same ballpark, at least when compared to metals. Still, most sources online claim that due to thermal properties, glass is terrible for tea, and that porcelain is good for delicate teas like green and oolong, but not black, while stoneware is almost universally acclaimed. If materials with similar thermal properties give so different results, are there other factors in play, like thickness? Stoneware teapots are often thicker than both glass and porcelain, but would a porcelain teapot with the same thickness as stoneware be equally good for brewing black tea?

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Aug 13th 16 1:36 pm
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Re: Thermal properties of teapot materials

by jayinhk » Aug 13th 16 1:36 pm

absence wrote:It's a bit difficult to find exact figures for heat capacity and thermal conductivity, but it seems like stoneware, porcelain, and glass are roughly in the same ballpark, at least when compared to metals. Still, most sources online claim that due to thermal properties, glass is terrible for tea, and that porcelain is good for delicate teas like green and oolong, but not black, while stoneware is almost universally acclaimed. If materials with similar thermal properties give so different results, are there other factors in play, like thickness? Stoneware teapots are often thicker than both glass and porcelain, but would a porcelain teapot with the same thickness as stoneware be equally good for brewing black tea?
Porcelain is fine for all teas. I think glass gives you virtually identical results, but someone will be by shortly to tell you they disagree. :lol:

Aug 13th 16 11:56 pm
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Re: Thermal properties of teapot materials

by steanze » Aug 13th 16 11:56 pm

absence wrote:It's a bit difficult to find exact figures for heat capacity and thermal conductivity, but it seems like stoneware, porcelain, and glass are roughly in the same ballpark, at least when compared to metals. Still, most sources online claim that due to thermal properties, glass is terrible for tea, and that porcelain is good for delicate teas like green and oolong, but not black, while stoneware is almost universally acclaimed. If materials with similar thermal properties give so different results, are there other factors in play, like thickness? Stoneware teapots are often thicker than both glass and porcelain, but would a porcelain teapot with the same thickness as stoneware be equally good for brewing black tea?
Good questions!

1) the capacity of a vessel to keep the water hot depends not only on the material but also on the thickness, the shape of the vessel (more spherical shapes have a smaller surface/volume ratio and keep heat longer), and the size (larger vessel stay hot for longer).
2) the material of a vessel can affect the tea in ways other than heat retention, especially in the case of stoneware.

Porcelain is often thinner than (heat resistant) glass, so porcelain vessels tend to cool down faster and can work better for delicate teas.
I never heard that porcelain is bad for black tea. Do you mean black tea as hongcha or as heicha? That could lead to some confusion. If it's hongcha I'd be surprised about anyone saying procelain does not work well. If it is heicha, then usually clay is preferred to remove some off tastes that can be generated during the pile fermentation (wo dui) process.

I hope this helps!

Aug 14th 16 11:21 am
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Re: Thermal properties of teapot materials

by absence » Aug 14th 16 11:21 am

steanze wrote:Porcelain is often thinner than (heat resistant) glass, so porcelain vessels tend to cool down faster and can work better for delicate teas.
The faster cooling is the reason given for why porcelain isn't as suitable for black (e.g. Assam, not pu erh) tea. Apparently brewing at a high, even temperature is necessary to extract the right flavour components from black tea.

From what you say it sounds like it's mostly a matter of thickness rather than the properties of the material itself.

I guess the faster fall in temperature from a thin porcelain teapot is even good for certain teas? I've read about people brewing certain (green?) teas with the lid off to achieve a similar effect.

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Re: Thermal properties of teapot materials

by steanze » Aug 14th 16 12:33 pm

absence wrote: The faster cooling is the reason given for why porcelain isn't as suitable for black (e.g. Assam, not pu erh) tea. Apparently brewing at a high, even temperature is necessary to extract the right flavour components from black tea.
mmm... where did you find that information? That's what people making British tea usually do, and then they put milk in it. For chinese hongcha (which usually isn't var. Assamica but var. Sinensis), it is quite common to use slightly colder water (at 190-200F/~90C, e.g. https://teatrekker.com/product/yunnan-f ... dian-hong/), because it can be quite delicate tea. It is not mandatory ;) but hongcha is definitely one type of tea for which stable high temperature and heat retaining vessels are not a must (unlike say aged pu-erh or yancha).
absence wrote: From what you say it sounds like it's mostly a matter of thickness rather than the properties of the material itself.
Size and shape are also important for heat retention. The properties of the material are important but not so much for heat retention, mostly because of the interactions that can occur between porous unglazed clays (e.g. yixing or tokoname) and the tea.
absence wrote: I guess the faster fall in temperature from a thin porcelain teapot is even good for certain teas? I've read about people brewing certain (green?) teas with the lid off to achieve a similar effect.
The faster fall in temperature is great for a lot of delicate teas - green teas as well as greener oolongs and IMO also delicate blacks. For those thin porcelain or clay are great, and that's one of the reasons why the eggshell vessels are expensive (they are also harder to make and harder to find). Brewing with the lid off is not exactly the same thing, because without the lid you will also loose some of the aromas. But for delicate teas it is definitely a important to take the lid off right after pouring the water out. Fast cooling is important not only while you are brewing, but also between infusion: keeping the lid closed with a delicate tea will lead to steaming it and if it's a greener tea it will taste like spinach soup :)

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Re: Thermal properties of teapot materials

by Bok » Aug 14th 16 1:57 pm

First time I heard the statement that porcelain is bad for a certain kind of tea...

Can not think of a more neutral, yet efficient material to brew tea.
Some clay may work better for certain teas no doubt, but that does not make porcelain a bad choice, just not the optimum.

Not for nothing that serious tea drinkers first test unknown or new teas in porcelain, defects in the tea can not hide when brewed with porcelain :mrgreen:

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Re: Thermal properties of teapot materials

by absence » Aug 16th 16 8:02 pm

I'm sorry I don't have sources, it feels like I've been browsing the entire web for information about this. :) Some web site here, a forum post there, you know how it is. Anyway, the thing about needing hot water to extract all the flavours from black tea is something I've read several places, and there's no reason to shrug it off as something clueless Brits do to add taste to their milk. There are plenty of good quality black teas from both assamica and sinensis that are deliciously full-bodied and sweet on their own after four minutes with near-boiling water in a pre-heated vessel. But of course don't do that to a first flush Darjeeling or Nepal. :o

Porcelain being less suitable ("bad" is too strong a word) for that kind of brewing follows directly if it can't hold heat as well, and is also something I've seen written explicitly several times. It's interesting to hear people have different experiences though, that's why I asked in the first place. Sounds like the answer is that it depends on the tea and what you want out of it.

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Re: Thermal properties of teapot materials

by jayinhk » Aug 17th 16 1:09 am

absence wrote:I'm sorry I don't have sources, it feels like I've been browsing the entire web for information about this. :) Some web site here, a forum post there, you know how it is. Anyway, the thing about needing hot water to extract all the flavours from black tea is something I've read several places, and there's no reason to shrug it off as something clueless Brits do to add taste to their milk. There are plenty of good quality black teas from both assamica and sinensis that are deliciously full-bodied and sweet on their own after four minutes with near-boiling water in a pre-heated vessel. But of course don't do that to a first flush Darjeeling or Nepal. :o

Porcelain being less suitable ("bad" is too strong a word) for that kind of brewing follows directly if it can't hold heat as well, and is also something I've seen written explicitly several times. It's interesting to hear people have different experiences though, that's why I asked in the first place. Sounds like the answer is that it depends on the tea and what you want out of it.
If you've ever seen a Hario glass teapot, they are so thin that they're almost scary to use! I have one that I'ved used for sencha a few times.

As for porcelain not holding heat well...you could always use a tea cosy, as the British do. The Chinese use padded, cloth-lined baskets for the same effect (to keep the tea hot for longer).

I brewed lovely, all tip-grade dianhong in a large porcelain pot a few days ago and it gave me three days of drinking pleasure. I don't have a Yixing specifically for black tea, and I was still very happy with the results. I have used my tall Jianshui maocha pot for dianhong and loved the results. It's also possible to get thicker porcelain/celadon pots and gaiwans if heat retention is your thing. I have a Ru Kiln travel gaiwan which is very thick walled and works great for pu erh. I use it for all teas. When I was brewing greens in Vietnam, I just used slightly cooler water and left the lid askew between infusions. With more delicate tea you could just leave the lid off entirely.

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Re: Thermal properties of teapot materials

by Bok » Aug 17th 16 1:51 am

Following up what Jay said, one important factor is how well one can brew. Any teaware will produce miserable results if the person brewing does not know what they are doing. So, if one knows what to do:

Good tea will never be ruined by glass
Good tea will never be ruined by porcelain + it smells nice in porcelain
Good tea might be ruined by a clay pot > depends on the type of tea and clay
Good tea can easily be ruined in metal pots
Only very high quality tea can be brewed in silver, all others will most likely be ruined.

We don’t do bad tea.

:mrgreen:

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Re: Thermal properties of teapot materials

by steanze » Aug 17th 16 2:06 am

absence wrote:I'm sorry I don't have sources, it feels like I've been browsing the entire web for information about this. :) Some web site here, a forum post there, you know how it is.
Yes. You will find a lot of contradictory information online, sometimes people have different views, sometimes people write about tea but haven't necessarily explored it much yet. We are all learning and sharing along the way. In this case as in many others I suggest that you experiment with different temperatures and materials yourself and observe the differences.
absence wrote: Anyway, the thing about needing hot water to extract all the flavours from black tea is something I've read several places, and there's no reason to shrug it off as something clueless Brits do to add taste to their milk.
Note that I didn't say is something clueless Brits do. I said that it is what people do when they make tea in a particular style, which is known as British style - regardless of what is their nationality. Brewing strong black tea with boiling water in that case is not clueless, because the tea needs to be strong enough to rise above the milk, and the milk will reduce its bitterness.
absence wrote: There are plenty of good quality black teas from both assamica and sinensis that are deliciously full-bodied and sweet on their own after four minutes with near-boiling water in a pre-heated vessel. But of course don't do that to a first flush Darjeeling or Nepal. :o
When you drink high-quality red tea alone it is quite common to use a lower temperature: red tea is not roasted, and it is quite delicate. See for example these notes published on Cha Dao: http://www.academia.edu/4968550/Notes_o ... 5%E8%8C%B6_ Since you asked, I replied that red tea does not strike me as a tea for which high heat retention is particularly important. However, you should not believe that just because I or the article I linked say so: as above, experiment yourself and decide what you prefer. There is so much misinformation on tea on the internet that you should only trust your observations, and maybe a little bit some people whom you have learned to trust.
absence wrote: Sounds like the answer is that it depends on the tea and what you want out of it.
Yes. In my opinion that is the best answer, and not only for red tea. There isn't only one right way to make a certain tea: depending on the day and the moment, you may want something different. Perhaps if it's cold and rainy you may want a stronger cup, if its a warm summer day a lighter one, or you may want different things depending on your mood. So I think the best advice I can give you is don't follow rigid rules, listen to how you feel, and learn to brew tea that speaks to that.

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Re: Thermal properties of teapot materials

by steanze » Aug 17th 16 2:08 am

Bok wrote:Following up what Jay said, one important factor is how well one can brew. Any teaware will produce miserable results if the person brewing does not know what they are doing. So, if one knows what to do:

Good tea will never be ruined by glass
Good tea will never be ruined by porcelain + it smells nice in porcelain
Good tea might be ruined by a clay pot > depends on the type of tea and clay
Good tea can easily be ruined in metal pots
Only very high quality tea can be brewed in silver, all others will most likely be ruined.

We don’t do bad tea.

:mrgreen:
+1 that sums up well what has also been my experience

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Re: Thermal properties of teapot materials

by Rotor » Feb 1st 18 3:03 pm

This thread is very useful but let involve a bit science.
I am not heat engineering but I think that the most important characteristic concerning the materials heat retention is:
Thermal Conductivity (https://www.engineeringtoolbox.com/ther ... d_429.html)

and not so important for tea brewing:
Specific Heat (https://www.engineeringtoolbox.com/spec ... d_154.html)
Emissivity Coefficients (https://www.engineeringtoolbox.com/emis ... d_447.html)

Thermal Conductivity (lower is better)
Pyrex glass (Borosilicate) - 1.005
Brick, dense - 1.31
Fire-clay brick, 500 oC - 1.4
Porcelain - 1.5

Specific Heat (higher is better)
Porcelain - 0.26
Brick, hard - 0.24
Brick, common - 0.22
Clay (may be raw not fired!?) - 0.22
Pyrex glass (Borosilicate) - 0.2

Emissivity Coefficients (lower is better)
Porcelain, glazed - 0.92
Brick, red rough - 0.93
Pyrex glass (Borosilicate) - 0.85 - 0.95

I am not sure how this source is reliable and accurate but my personal feeling is that the difference between materials characteristic is less than 10 % (except the glass). Far more important characteristic is the wall thickness of the tea vessels. The thickness varies wildly between the tea vessels (one of the reasons is materials density - see the hint below). If the thickness of the vessel walls is doubled the heat retention will be increased twice - 100%. The negative side effect will be that the wight will be increased the same way.

Hint: If you buy online, you could compare the wall thickness of the vessel with similar shape, capacity and material, just comparing the weight. Note that the density of the clay pots vary too much and can be significantly lower than the porcelain/glass density (https://www.engineeringtoolbox.com/dens ... _1265.html)

Regards