Clay porosity

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LeoFox
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Sat Jun 12, 2021 9:30 am

We talk a lot about how porous one clay is versus another and make assumptions based on that. A common assumption is that higher porosity means greater rounding of the tea being infused- be it reducing bitterness or trimming flavors and aromas.

There are different ways porosity can be measured. The following is from wikipedia:

https://en.m.wikipedia.org/wiki/Porosity
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure the "accessible void", the total amount of void space accessible from the surface (cf. closed-cell foam).
And some methods:
Imbibition methods,[5] i.e., immersion of the porous sample, under vacuum, in a fluid that preferentially wets the pores.Water saturation method (pore volume = total volume of water − volume of water left after soaking).

Water evaporation method (pore volume = (weight of saturated sample − weight of dried sample)/density of water)

The water evaporation method is also described on this pottery page:

https://digitalfire.com/glossary/clay+body+porosity
Porosity is measured by the weight increase when boiled in water.
A few notable items from the pottery page:
  • X% porosity =X% increase in mass after clay is boiled in water
  • An interesting process in which a potter measures the porosity of a clay under a range of different firing conditions in order to determine the firing parameters that minimizes porosity. A curve can be generated, and the page claims that porosity reaches a minimum as temperature increases but then goes back up again as temperature continues to increase.
  • Not uncommon for earthen ware to have up to 3% porosity
  • The minimum porosity of some clays will remain relatively high irrespective of firing temperature
Based on the above, I feel we can make our own investigations on porosity.

I propose two approaches based on the evaporation test:

Formal method:
1. Measure dry pot weight
2. Take pot and submerge it in boiling water for X minutes
3. Rinse pot 2x with boiling water
4. Shake pot to get water out of nooks and crannies
5. Wait briefly for pot to appear dry visually or pat dry pot with cloth
6. Measure pot weight at time 0
7. Keep measuring pot weight over time until pot reaches dry weight
8. Plot curve of change in pot weight over time

Real-use method:
1. Measure dry pot weight
2. Have a typical session with pot
3. Rinse pot 2x with boiling water
4. Shake pot to get water out of nooks and crannies
5. Wait briefly for pot to appear dry visually or pat dry pot with cloth
6. Measure pot weight at time 0
7. Keep measuring pot weight over time until pot reaches dry weight
8. Plot curve of change in pot weight over time


The formal method will be more objective but the conditions may not be reflective of real life brewing conditions. On the other hand, the real use method introduces a lot of variability and may not translate from person to person.

However, I believe by drawing an evaporation curve, the overall shape of the curve and the rate of drying should be mostly consistent between methods.
Last edited by LeoFox on Thu Jul 22, 2021 4:19 pm, edited 4 times in total.
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LeoFox
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Sat Jun 12, 2021 9:30 am

Aim
Here I describe an informal study on the impact of apparent clay porosity and rounding effects on tea. By rounding, I generally mean a decrease in bitterness, astringency, flavor, body, aroma relative to brewing using porcelain.

Method

I have been performing the real-use method described in the first post on some of my pots as I use them. I will continue to do this over time.

My approach has been to make the first measurement after it is visually dry after a session. I am also only measuring the pot body, not the lid. Finally, I am only doing this after sessions using off boiling water and 7+ infusions.


Pots looked at so far: (will update over time)
Each pot is briefly described, and includes a subjective assessment regarding their impact on tea based on my experience.
  • 70s hong Qing shui ni (HQSN) from @Bok. This is a highly "porous" clay that actually leaks a bit of tea during sessions (spitting black) (105-110 mL). It has a powerful rounding effect on the bitterness of sheng pu erh while preserving much of the aroma. The flavor profile is also dampened a bit. Refer to viewtopic.php?p=34390#p34390
  • Watanabe Tozo's carbonized reduction black mumyoi: very high pitched ring (100 mL). Has some interesting flavor shifting effects and can smoothen the tea. Specifically, it has consistently rounded out the primary flavor and highlight more hidden aspects of a tea. Refer to
    viewtopic.php?p=36762#p36762
  • Watanabe Tozo's Joaka gold mine oxidation mumyoi: my experience is that it does not round at all but pushes the tea to an extreme so that any flaws will be visible if there are flaws (105-110 mL). In particular, aromatics seem to be enhanced. Refer to viewtopic.php?p=34536#p34536
  • Modern zini: a modern ruyi pot that I use for shou pu erh. Although a zini, I find it makes almost no impact on tea and is very neutral (100 mL). Refer to:
    viewtopic.php?p=33784#p33784
  • Yamada Sou wood fired pot (180 ml). It is slightly rounding and smoothing. Seems to make sencha taste sweeter for some reason.
    viewtopic.php?p=34858#p34858
  • Early 70s hong ni (55-60 ml). It has not been seasoned yet. The pot has some subtle rounding effects. Interestingly, I experience a shifting of flavors into the aftertaste.
    viewtopic.php?p=37404#p37404
  • Junzo's kobiwako from hojo (100 ml). This clay greatly rounds tea fragrance and some aromatics. It seems to increase body and darker flavors
    viewtopic.php?p=32418#p32418
  • Modern chao zhou pot (100 mL). This pot seems to round out bitterness significantly, as well as some other flavors while preserving aromatics. Astringency is not rounded as much (unfortunately)
    viewtopic.php?p=34487#p34487

Results:
Of all the pots tested, I only found three with sizable porosity. All other pots had minimal (less than 0.1%) change in mass after an extended session.

Below, I plot the % change in mass relative to dry weight over time in hours after a session.

Note trendlines are moving average.

Note that the joaka and 70s hqsn pots have nearly identical volumes (about 105-110 mL) and have similar mass. The modern CZ pot is closer to 95  mL and is thinner
Note that the joaka and 70s hqsn pots have nearly identical volumes (about 105-110 mL) and have similar mass. The modern CZ pot is closer to 95 mL and is thinner
Untitled.png (136.63 KiB) Viewed 5068 times

Below, I show a matrix of the pots categorized based on subjective assessment of rounding and porosity. Here I focus on rounding of bitterness, astringency and other harsh aspects (smokiness, acrid notes, greenness, harsh aromas, etc)
No clear trend is apparent
No clear trend is apparent
SmartSelect_20210714-174732_Sheets.jpg (196.14 KiB) Viewed 5028 times

Additional matrices focused on aroma, body and aftertaste:
Note that the 70s hong ni is relatively unseasoned. The rounding effects at this point are relatively subtle, but discernable.
Note that the 70s hong ni is relatively unseasoned. The rounding effects at this point are relatively subtle, but discernable.
SmartSelect_20210716-160917_Sheets.jpg (206.31 KiB) Viewed 4991 times
SmartSelect_20210716-160957_Sheets.jpg
SmartSelect_20210716-160957_Sheets.jpg (215.95 KiB) Viewed 4991 times
SmartSelect_20210716-161101_Sheets.jpg
SmartSelect_20210716-161101_Sheets.jpg (222.27 KiB) Viewed 4991 times



Discussion

Both the 70s HQSN and the modern chaozhou pots are porous and round out bitterness and some astringency.

However, the joaka clay has no discernable rounding effects and yet has some porosity

The kobiwako clay can mute the aroma but has no measurable porosity.

The 70s hong ni is a bit rounding relative to the joaka and yet has no porosity

Overall, the data to date do not support a relation between apparent porosity and rounding effects.

One possibilty is that my method is not fit for purpose and my sample size is too low. To confirm my findings, they have been repeated 3+ times and I feel confident of their reproducibility. Ultimately, results from additional pots may be needed, esp pots with higher porosity. It would be very helpful to include, for example, porous clays such as Hojo's shigaraki clay, Hojo's Iga clay, lower fired antique duanni that exhibit tu hei.

Another possibility is that subtle differences in mass change that cannot be measured precisely with my method might make a big difference.

Finally, and perhaps most importantly, additional factors such as surface texture and chemistry may play critical roles as mentioned by @Baisao:

viewtopic.php?p=37603#p37603

By texture, I mean the deviations of the actual clay surface from a smooth idealized average surface; ultimately, just like porosity, this also has to do with accessible surface area.

By surface chemistry , I mean the identity and activity of sites on the clay that could react with tea or catalyze or inhibit chemical reactions that occur during infusion. The density and accessibility of these sites would be important factors.

Follow up discussion:

viewtopic.php?p=38237#p38237

viewtopic.php?p=38239#p38239

viewtopic.php?p=38260#p38260

viewtopic.php?p=38263#p38263
Last edited by LeoFox on Thu Jul 22, 2021 3:52 pm, edited 42 times in total.
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LeoFox
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Sat Jun 12, 2021 9:31 am

In this post I will link to the results from other TF members.

@Victoria
Clays: 2 different shigaraki, kobiwako
viewtopic.php?p=37076#p37076


@faj
Clays: shigaraki from hojo
viewtopic.php?p=37065#p37065
viewtopic.php?p=37123#p37123



Last edited by LeoFox on Mon Jun 14, 2021 5:56 am, edited 5 times in total.
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Bok
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Sat Jun 12, 2021 10:00 am

Nice one! Myself I can’t be bothered to do the same, haha
faj
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Sat Jun 12, 2021 10:24 am

I have a Shigaraki pot that is extremely porous. I actually did a test similar to yours in the past, though not with any standardized method, just out of curiosity. Typically, after a session it weighs about 122g after being heated with boiling water a couple of times, emptied, and left alone for a minute until it seems dry to the eye. Its weight stabilizes around 117g when fully dry, so it absorbs about 4% of its weight in water. Leaving it on a scale with a 0.01g resolution as it dries, I can see the number changing continuously for a good while due to how quick the evaporation is at the beginning.
.m.
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Sat Jun 12, 2021 10:26 am

Interesting experiment. That HQSN sounds like it is seriously underfired. I have LQER export pot in a storage which literally soaks water, kind of like terracotta planters do,. I keep wondering if it could be refired.
However, in most 'normal' pots, i think the water doesn't penetrate deep into the clay, so it would be perhaps more telling to take the absolute change in weight and divide it with the perceived surface area of the interior of the pot (e.g. if all pots are about the same shape, then taking the volume to the power 2/3 would be a relevant quantity proportionate to the surface area). That way the thickness of the walls, the density (relative mass) of the clay, or the weight the handle and spout wouldn't interfere in the data.
In some way, i think porosity should be directly related to the fractality (fractal dimension) of the microscopic texture of the surface.
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Bok
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Sat Jun 12, 2021 11:47 am

There are some specialists who carefully refire such pots, but not sure if they can be found in the West as well. I’ve had an invisible repair done, which also involved a step of firing the pot again.

I’d caution you’d need a highly specialised and competent person to do this risk free.
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LeoFox
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Sat Jun 12, 2021 12:32 pm

Regarding hqsn,

I cant say that it is under fired unless I have a large sampling of hqsn and can draw a distribution of porosity for them.

Although it seems relatively porous, it makes good sheng so it's fine.

Regarding volume corrections, I can look into that as my analysis proceeds. So far all the volumes are similar.
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Victoria
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Sat Jun 12, 2021 1:16 pm

faj wrote:
Sat Jun 12, 2021 10:24 am
I have a Shigaraki pot that is extremely porous.
@faj I’m curious which shigaraki you have that is very porous?
faj
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Sat Jun 12, 2021 1:29 pm

Victoria wrote:
Sat Jun 12, 2021 1:16 pm
faj I’m curious which shigaraki you have that is very porous?
Tachi Masaki Shigaraki rough clay.
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Victoria
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Sat Jun 12, 2021 1:50 pm

When Leo shared with me his idea to measure clay porosity of various pots I was intrigued as I have a few shigaraki kyusu that have varying degrees of porosity. The Tani Seiuemon shigaraki was so porous that it weeped the first few times I used it. I don’t have a scientific mind being an artist, so I approached the test intuitively trying to reduce variables, focusing on a few porous kyusu that I use frequently, and wanting to measure only how much water was being absorbed from inside the kyusu. I’m surprised the Tachi Masaki shigaraki is not as porous as I’d imagined. I had intended on measuring every 4hrs but got sidetracked and forgot, so ended up measuring after 44hrs. Of the three tested the Tani shigaraki is the most porous, and the Junzo kobiwako the least porous.

Shigaraki shiboridashi by Tani Seiuemon 161.4 ml to fill line, water at 140f
177.8 grams empty pot w/out lid
Filled with 161.4 ml 140f water
Wait 4 hours
155.8 ml water poured out (5.6 ml loss)
181.2 grams pot after patting dry with cloth (3.4g heavier)
2.2ml water loss unaccounted for
Wait 44 hrs
177.9 grams empty pot w/out lid

Shigaraki kyusu by Tachi Masaki 140ml to fill line, water at 140f
123.8 grams empty pot w/out lid
Filled with 140 ml 140f water
Wait 4 hours
138.3 ml water poured out (1.7 ml loss)
124.1 grams pot after patting dry with cloth (.3g heavier)
1.4ml water loss unaccounted for
Wait 44 hrs
123.8 grams empty pot w/out lid

Kobiwako by Maekawa Junzo 250 ml to fill line, water at 140f
156.2 grams empty pot w/out lid
Filled with 250 ml 140f water
Wait 4hrs
248.5 ml water poured out (1.5ml loss)
156.3 g grams pot after patting dry with cloth (.1g heavier)
1ml water loss unaccounted for
Wait 44 hours
156.2 grams empty pot w/out lid


Shigaraki shiboridashi by Tani Seiuemon 161.4 ml
7E714D20-F891-4642-9F67-08FB68F125F4.jpeg
7E714D20-F891-4642-9F67-08FB68F125F4.jpeg (288.86 KiB) Viewed 5647 times

Shigaraki kyusu by Tachi Masaki 140ml
BD7564BF-5504-455D-8FC3-80F24DD1F66B.jpeg
BD7564BF-5504-455D-8FC3-80F24DD1F66B.jpeg (374.3 KiB) Viewed 5647 times

Kobiwako by Maekawa Junzo 250 ml
7D47F6EF-B476-4C9F-9567-F4C7D11C6B43.jpeg
7D47F6EF-B476-4C9F-9567-F4C7D11C6B43.jpeg (379.18 KiB) Viewed 5647 times
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Victoria
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Sat Jun 12, 2021 2:28 pm

faj wrote:
Sat Jun 12, 2021 1:29 pm
Victoria wrote:
Sat Jun 12, 2021 1:16 pm
faj wrote:
Sat Jun 12, 2021 10:24 am
I have a Shigaraki pot that is extremely porous.
faj I’m curious which shigaraki you have that is very porous?
Tachi Masaki Shigaraki rough clay.
@faj I wonder if you could re-check your Tachi shigaraki? To see if yours is fired very differently than the one I have, since my results indicated lower porosity than what you measured. Your empty dry to the eye kyusu gained 5 grams in water weight, while mine only gained .3 grams after water was resting inside for 4 hours.
faj
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Mon Jun 14, 2021 5:31 am

Victoria wrote:
Sat Jun 12, 2021 2:28 pm
faj I wonder if you could re-check your Tachi shigaraki? To see if yours is fired very differently than the one I have, since my results indicated lower porosity than what you measured. Your empty dry to the eye kyusu gained 5 grams in water weight, while mine only gained .3 grams after water was resting inside for 4 hours.
All weights measured without the lid.

Dry weight, with the pot having been unused for a few weeks : 117,05g.

I then used your method, put 60C water into the pot, waited about 4 hours, patted dry : 118.13g.

That was much less than I had measured in the past. I continued the test with something more akin to the method I had used. Heated with boiling water, left to wait for about 4 more hours, patted dry : 120.49g.

Still a bit short. I went for an overnight test, which is what I had done in the past. Heated with boiling water, left to wait overnight, patted dry ; 122.45g.

But frankly, the "pat dry" method does not seem right to me, as the surface retains too much water. So I finished with my usual last step. Heated with boiling water a few times, emptied, shaken and left alone until dry to the eye (only a minute or so) : 121.02g.

I would need further testing to see if the difference has to do with the initial water temperature or only with time, but obviously it takes more than 4 hours for the clay to absorb as much was as it can, and a longer test would be required to see if it could absorb even more. One thing to note is that when this teapot has absorbed water and is then filled with boiling water, small droplets form on the outside within a few seconds, as if the hot water is pushing liquid out of the clay.
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Mon Jun 14, 2021 1:42 pm

faj wrote:
Mon Jun 14, 2021 5:31 am
Victoria wrote:
Sat Jun 12, 2021 1:50 pm
Shigaraki kyusu by Tachi Masaki 140ml to fill line, water at 140f
123.8 grams empty pot w/out lid
Filled with 140 ml 140f water
Wait 4 hours
138.3 ml water poured out (1.7 ml loss)
124.1 grams pot after patting dry with cloth (.3g heavier)
1.4ml water loss unaccounted for
Wait 44 hrs
123.8 grams empty pot w/out lid
Victoria wrote:
Sat Jun 12, 2021 2:28 pm
faj I wonder if you could re-check your Tachi shigaraki? To see if yours is fired very differently than the one I have, since my results indicated lower porosity than what you measured. Your empty dry to the eye kyusu gained 5 grams in water weight, while mine only gained .3 grams after water was resting inside for 4 hours.
All weights measured without the lid.

Dry weight, with the pot having been unused for a few weeks : 117,05g.

I then used your method, put 60C water into the pot, waited about 4 hours, patted dry : 118.13g.

That was much less than I had measured in the past. I continued the test with something more akin to the method I had used. Heated with boiling water, left to wait for about 4 more hours, patted dry : 120.49g.

Still a bit short. I went for an overnight test, which is what I had done in the past. Heated with boiling water, left to wait overnight, patted dry ; 122.45g.

But frankly, the "pat dry" method does not seem right to me, as the surface retains too much water. So I finished with my usual last step. Heated with boiling water a few times, emptied, shaken and left alone until dry to the eye (only a minute or so) : 121.02g.

I would need further testing to see if the difference has to do with the initial water temperature or only with time, but obviously it takes more than 4 hours for the clay to absorb as much was as it can, and a longer test would be required to see if it could absorb even more. One thing to note is that when this teapot has absorbed water and is then filled with boiling water, small droplets form on the outside within a few seconds, as if the hot water is pushing liquid out of the clay.
@faj it seems that your shigaraki kyusu by Tachi Masaki is quite a bit more porous than mine (140ml), and slightly smaller (?) since your weight is 6.75 grams lighter. Your kyusu is absorbing after four hours 1.08 ml of water, while mine only absorbed .3 ml after patting dry. I agree patting dry method could be improved by doing what you did and waiting X amount of time before weighing again. I do think initial temperature of water matters so should be consistent, boiling is probably better than 140f (that I used just because that’s temp I had in kettle at the time), and 4 hours I also just randomly chose, so could be expanded to 12 hr or so. When I first got the Tachi shigaraki it also sweat beads of moisture, but only the first few times, after steeping Japanese sencha and gyokuro the sweating mostly stopped. Same thing happened with my other shigaraki shiboridashi by Tani Seiuemon, the sweating stopped after steeping gyokuro in it a few times.
faj
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Mon Jun 14, 2021 1:56 pm

Victoria wrote:
Mon Jun 14, 2021 1:42 pm
faj it seems that your shigaraki kyusu by Tachi Masaki is quite a bit more porous than mine (140ml), and slightly smaller (?) since your weight is 6.75 grams lighter.
It was sold as 110ml. I'd have to measure, but it feels a bit larger than that, but yeah, smaller than yours. And yes, it does seem quite a bit more porous.
Victoria wrote:
Mon Jun 14, 2021 1:42 pm
I do think initial temperature of water matters so should be consistent, boiling is probably better than 140f (that I used just because that’s temp I had in kettle at the time), and 4 hours I also just randomly chose, so could be expanded to 12 hr or so.
Hard to tell for how long the clay can keep absorbing moisture without a lot of testing. In my case, I had the teapot go through many, many consecutive overnight (and "overday") cycles to get mostly rid of brown water oozing out, which I did not like too much. It never measured much above 122g, so probably 12 hours is enough. It may turn out that less porous (but not entirely leak tight) pots might need even longer to reach them maximum absorption, since water has a harder time making its way.
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