Yixing clay that is suitable for yancha

[xiaobai]

I own three pots from EoT (none made of that expensive zini #1 clay though) and never found them to perform exceptionally well with yancha.

I use one of the EoT haunglongshan pots for yancha regularly, it's actually my preferred yancha pot over several others I use occasionally. But it could be due to my personal preference; when drinking yancha I don't care that much about the aroma, its all about mouth feel and body to me. Aside from the wonderful empty cup aroma!

In fact, I must correct what I said yesterday. Today I tried the same DHP with another EoT pot, a PingGai made with the hualongshan clay and the results was much better than with the PanHu yesterday.

Initially, I had obtained bad results using EoT's yancha and a hualongshan
Shi Piao from the same vendor.

The material of the PingGai and the Shi Piao is zini, which appears to me as fired at a higher temperature than the Tian Qing Ni of the PanHu.

The fact that the material of PanHu is probably more porous does not mean that the roasting notes of the DHP get more smoothen out, which probably means that the effect of the clay is secondary (both pots have been used for about the same time, so seasoning should be similar).

In that case, it must be the shape that mainly determines the performance.
In fact, the PingGai has a smaller basal surface than the PanHu, which is much flatter. The volume is slightly different, 80 ml vs 95 ml. But both factors favor a higher heat retention by the PingGai compared to the flatter PanHu.

The above observations would suggest that the thermal properties of the pot are probably the dominant factor and the effect of the clay is secondary as far as those pots are concerned.

By the way: FYI, The comparison described on Stephane's tea masters website was not with zini, but old zisha. I happen to know first hand.

Zini, zisha... Just words.

"ni" means clay, "sha" means sand. They are not the same thing.

[kyarazen]

I think everybody agrees (and the data in your blog shows it) that flatter pots cool faster.

To make a fair comparison we must compare pots of different shapes but equal volume. The reason being that, when hot water is poured in, the heat that is provided is (roughly) proportional to the mass of water held by the pot, that is, the pot's volume in ml = cm^3 = grams.

A spherical or ellipsoidal shape have rather ill defined contact area with the surfaces they sit on (mathematically it is just a point = zero area). I think a better model for a pot is a cylinder of height H and basal radius R.

The volume of the cylinder (pot) is V = pi R^2 H

The surface area of the cylinder is S = 2 pi R (R + H)

If we use the first equation to solve for H, and substitutes in the second
equation, we get the following law for the surface area as a function of the
basal radius:

S(R) = 2 pi R^2 + 2V/R

This a non-monotonic function of the basal radius, R. In particular, it has
a minimum for H = 2R, at which point the surface area equals 6 pi R^2, that is, one and a half times the surface area of a sphere of radius equal to R.

Below is a plot of S(R) normalized to the minimum of the surface area as a function of R divided by the basal radius at the minimum. It shows the non-monotonic behavior mentioned earlier.

The above considerations illustrate the rather simple fact that we can have two different cylindrical-shape pots of the same volume, one flat and one tall.

It then naturally follows that, if the heat loss is radiative and therefore proportional to the total surface area of the pot, then one can have two pots of the same volume but different shape (tall and flat) that will cool at (roughly) the same rate.

IMO, the only way out of this contradiction with the experience is to notice that a large fraction of the initial cooling happens conductively through the base of the pot by releasing heat to the surface it sits on. Since a flat pot has larger contact area with the latter than a tall pot, it should cool faster.

It is also natural to expect that heat conduction occurs dominantly through the bottom of the pot because it is in contact with a solid surface which can conduct heat much more efficiently than the air surrounding the pot. For the surface in contact with the air, the dominant mechanism is radiative, as pointed out above, but this should dissipate less heat for the relatively small temperature differences between the pot and the surrounding air.

a cylinder is not a good representation nor model of a tea pot at all, an ellipsoid is still more accurate in terms of surface area calculations.

that is still rather inaccurate as the base of the pot is a ring, the contact with the table top or plate is not as much. the dimensions of a ring of a flat pot can be about the same contact dimensions as that of a xianpiao.

not many pots are true flat bases which renders the dissipassive loss via the base to be inefficient. most are ring bases.

and it also depends on the material that you sit the pot on, on a coaster in a tray, it insulates, on a wooden tray, it insulates, on a metal plate, you get more efficient heat loss.

[xiaobai]

a cylinder is not a good representation nor model of a tea pot at all, an ellipsoid is still more accurate in terms of surface area calculations.

Let's put it this way: Your pots are ellipsoids. Mine are cylinders

[kyarazen]

a cylinder is not a good representation nor model of a tea pot at all, an ellipsoid is still more accurate in terms of surface area calculations.

Let's put it this way: Your pots are ellipsoids. Mine are cylinders

ok


cheers~

[steanze]

The above observations would suggest that the thermal properties of the pot are probably the dominant factor and the effect of the clay is secondary as far as those pots are concerned.

In my experience qing hui ni/tian qing ni does perform quite similarly to zini so that makes perfect sense with my expectation tian qing ni is a bit too broad of a clay category though, some of it is more porous like duanni. But still the difference is less than with hongni/zhuni. A question is how they compare with the hongni pot you mentioned in your earlier post. In my experience tall and flat zini pots tend to perform more similarly to each other than zini and hongni pots of similar shape

[steanze]

I think everybody agrees (and the data in your blog shows it) that flatter pots cool faster.

I am not sure whether modelling pots as ellipsoids or cylinders makes much of a difference to decide why tall vs flat pots cool faster... if a cylinder works better, as xiaobai mentioned there are two cylinders, a tall one and a flatter one, with the same surface to volume ratio. If an ellipsoid works better, we can put the same ellipsoid with the longest axis parallel to the ground, or with the shortest axis parallel to the ground. Like this:

http://img.tfd.com/ggse/e6/gsed_0001_0030_0_img9466.png
http://help.ultimate3d.org/gfx/CollisionEllipsoid.png

Since it is the same ellipsoid, the surface to volume ratio is the same. But in the first case it is a flat pot, in the second case it is a tall pot. So the surface to volume ratio might not allow us to explain why flat pots cool faster...

[kyarazen]

I am not sure whether modelling pots as ellipsoids or cylinders makes much of a difference to decide why tall vs flat pots cool faster... if a cylinder works better, as xiaobai mentioned there are two cylinders, a tall one and a flatter one, with the same surface to volume ratio. If an ellipsoid works better, we can put the same ellipsoid with the longest axis parallel to the ground, or with the shortest axis parallel to the ground. Like this:

http://img.tfd.com/ggse/e6/gsed_0001_0030_0_img9466.png
http://help.ultimate3d.org/gfx/CollisionEllipsoid.png

Since it is the same ellipsoid, the surface to volume ratio is the same. But in the first case it is a flat pot, in the second case it is a tall pot. So the surface to volume ratio might not allow us to explain why flat pots cool faster...

the sphere has the lowest surface to volume ratio. any deviation from the sphere will allow for increase surface area to volume.

it is not common that pots are made in a vertical standing ellipsoid with the short axis parallel to the ground, but if it is done, like in '82 with that round pancake pot decorated with plum flowers, that should cool pretty fast, and probably not through the base of the pot.

[kyarazen]

In my experience qing hui ni/tian qing ni does perform quite similarly to zini so that makes perfect sense with my expectation tian qing ni is a bit too broad of a clay category though, some of it is more porous like duanni. But still the difference is less than with hongni/zhuni. A question is how they compare with the hongni pot you mentioned in your earlier post. In my experience tall and flat zini pots tend to perform more similarly to each other than zini and hongni pots of similar shape

the earliest record of tianqingni suggests that the best qualities fire to a dark liver color.. which is not too far from some shades of zini..

[BW85]

By the way: FYI, The comparison described on Stephane's tea masters website was not with zini, but old zisha. I happen to know first hand.

Zini, zisha... Just words.[/quote]

"ni" means clay, "sha" means sand. They are not the same thing.

Yes, in a literal sense, of course. But pots sold as zini can be very much sandy. Some people say zini a zisha refer to two different clays, others say zisha is an umbrella term for all yixing clay. I lean toward the latter

[steanze]

I am not sure whether modelling pots as ellipsoids or cylinders makes much of a difference to decide why tall vs flat pots cool faster... if a cylinder works better, as xiaobai mentioned there are two cylinders, a tall one and a flatter one, with the same surface to volume ratio. If an ellipsoid works better, we can put the same ellipsoid with the longest axis parallel to the ground, or with the shortest axis parallel to the ground. Like this:

http://img.tfd.com/ggse/e6/gsed_0001_0030_0_img9466.png
http://help.ultimate3d.org/gfx/CollisionEllipsoid.png

Since it is the same ellipsoid, the surface to volume ratio is the same. But in the first case it is a flat pot, in the second case it is a tall pot. So the surface to volume ratio might not allow us to explain why flat pots cool faster...

the sphere has the lowest surface to volume ratio. any deviation from the sphere will allow for increase surface area to volume.

it is not common that pots are made in a vertical standing ellipsoid with the short axis parallel to the ground, but if it is done, like in '82 with that round pancake pot decorated with plum flowers, that should cool pretty fast, and probably not through the base of the pot.

Yes exactly sphere vs oblongated ellipsoid makes quite a bit of a difference for surface to volume ratio, but tall vs flat not so much

[xiaobai]

Quick update: I made a back of the envelop calculation for the radiative loss assuming a spherical teapot . The conclusion seems to indicate that this mechanism alone would be sufficient to account for the cooling of a teapot in about one minute, consistent with Kyarazen's measurements.

I will post the details of the calculation later today (by the way, the estimation depends very little on the actual shape of the pot, that is, whether sphere, ellipsoid, or cylinder as the input data are just the total surface area and the volume).

What remains to be understood then why Kyarazen's data (and most people's experience) agrees with a faster cooling rate for flat pots and slower cooling rate for tall pots. I believe that the conductive cooling through the base should account for the difference.

By the way, concerning the differences between the EoT Tian Qi Ni PanHu and the Zini PingGai, I need to point out that the former has much ticker walls, which probably affects also its (thermal) properties, probably much more than the type of clay.

[kyarazen]

Yes exactly sphere vs oblongated ellipsoid makes quite a bit of a difference for surface to volume ratio, but tall vs flat not so much

haha. it should be referred to as sphere versus oblongated ellipsoid then.

[kyarazen]

What remains to be understood then why Kyarazen's data (and most people's experience) agrees with a faster cooling rate for flat pots and slower cooling rate for tall pots. I believe that the conductive cooling through the base should account for the difference.

By the way, concerning the differences between the EoT Tian Qi Ni PanHu and the Zini PingGai, I need to point out that the former has much ticker walls, which probably affects also its (thermal) properties, probably much more than the type of clay.

thicker walls matter too, but if the pot is pre-heated, rinsing and discarding the wash, the effect is reduced. all my experiments are performed with the pot simulated with a single wash/rinse. there are some that dont rinse/wash teas. different schools of thought



a nice quick survey if everyone here could participate, how many of you tea pots have really flat bases, and how many have a ring base that reduces contact?

[steanze]

Ok I checked
Flat bottom: 5
Ring: definitely more than 5

[steanze]

Yes exactly sphere vs oblongated ellipsoid makes quite a bit of a difference for surface to volume ratio, but tall vs flat not so much

haha. it should be referred to as sphere versus oblongated ellipsoid then.