On Wed, 27 Jul 2005 15:22:18 -0500
"Bluesea" > wrote:

>
> "Eric Jorgensen" > wrote in message
> news:20050727135836.5893b6ac@wafer...
> > On Wed, 27 Jul 2005 14:26:05 -0500
> > "Bluesea" > wrote:
> >
> > > Iron is better for ...? Hmm...in general, metal dissipates heat
> > > faster than earthenware and some swear that a metallic taste is added
> > > to the brew although I don't know about cast iron in particular. May
> > > need to
> use
> > > a potholder since metal handles are typically significantly hotter
> > > than glass or porcelain.
> >
> <excellent info snipped>
> >
> > Metal does dissipate heat faster. The point of the cast iron pot,
> > however, is that you can pre-heat the pot by filling it with boiling
> > water for a few minutes, then dumping that water, adding the tea, and
> > steeping.
> >
> > Properly pre-heated, tea in my tetsubin stays hot far longer than
> > tea
> in
> > any of my glass pots.
> >
> > - Eric "Cast Iron Chef" Jorgensen
>
> Thanks for the good info.
>
> I thought preheating is SOP for all teapots (except the IngenuiTea) but
> this sounds like you don't preheat your glass pots. Have you compared
> preheated glass to preheated metal? I know that glass doesn't retain heat
> as well as ceramic so wouldn't expect your tetsubin to be better than
> glass.


Ceramic is a better insulator than glass and a better insulator than
iron.

HOWEVER.

Most glass pots are borosilicate, most tetsubin are gray cast iron.

Thermal properties thereof:

Borosilicate Glass:

Mean Specific Heat: 910 Joules per kilogram-kelvin
Mean Thermal Conductivity: 1.3 Watts per Meter-Kelvin
Density: 2.23 Grams per Cubic Centimeter

Gray Cast Iron:

Mean Specific Heat: 440 Joules per kilogram-kelvin
Mean Thermal Conductivity: 80.2 Watts per Meter-Kelvin
Density: 7.87 Grams per Cubic Centimeter

Specific heat is defined as the amount of heat, expressed here in
Joules, per unit of mass required to raise the temperature one degree
kelvin.

With respect to the ability of a given solid to retain heat, it is
generally accepted that solids release heat at the same rate they absorb
it, all things being equal, and assuming spherical horses on an infinite
plane.

Radiation and conduction have their effects, but lets assume for the
moment that the air is still and both pots sit on an asbestos trivet.

Air, treated as either a fluid or a gas, in general, is a slightly
better insulator than glass, it just has wacky-variable density determined
by heat and pressure.

If you want to figure out the convection due to expansion of air heated
both by conduction and radiation, I can give you some numbers to work
with, but you'll never finish working out the math without setting up a
cluster of computers to model the currents of heated air through the
unheated air. And I'm pretty sure that granted the similar surface area of
the pots, the margin of difference in dissipation we're dealing with
renders those results largely meaningless.

Thermal conductivity here is expressed as the heat flow rate (measured
in watts) times the distance divided by an area per temperature gradient,
in this case meters per degrees kelvin. This is how fast heat moves through
a solid.

Density, of course, is expressed here as how much it weighs vs. how big
it is.

So we can see that given an equivalent mass, it takes twice as much heat
to bring the glass pot up to temperature, but it also holds on to that heat
twice as long.

Additionally, an equivalent mass of iron will transmit heat throughout
it's volume more than 60 times faster than borosilicate glass.

The thermal conductivity of the glass does in fact class it as an
insulator where the iron is considered a good conductor of heat.

And you can see that the iron is significantly more dense than the
glass. Moreover, a typical borosilicate pot has very thin walls - 1mm or
less - whereas a typical tetsubin has wall thickness varying from 3 to 5mm.

My 600cc tetsubin - including handle - weighs 1359 grams, without
infuser or lid.

My 600cc borosilicate glass pot, without infuser or lid, but with
integrated handle, weighs 225 grams.

Both pots have a spout roughly 1.5 inches in length, with similar
diameter. The tetsubin is slightly more squat than the borosilicate pot.

For the sake of inquisition, the handle on the tetsubin is a long
front-to-back loop of steel that probably weighs 150 grams or more on it's
own, and the handle on the glass pot is a small side loop that probably
weighs several grams.

So for the sake of argument, lets be generous and say that the tetsubin
weighs 1.2kg.

For what it's worth, the thermal interface between the steel handle and
the cast iron vessel is quite small and quite poor, and the handle never
even gets warm.

THEREFO

Heated to the same temperature, and very roughly speaking, my
borosilicate glass pot is worth about 205 joules, and my tetsubin is worth
about 528 joules.

So, from the numbers above we can infer a few things.

1: A cast iron pot with a mass similar to the borosilicate pot would be
bloody worthless, having quite poor thermal mass and leaking heat to
everything that touches it. It would probably also be quite flimsy, as it
would have to have much thinner walls. Something like an inflated balloon.

2: The truth is that with almost 5 times the mass, even though it's got
half the heat, gram for gram, it's still got more than two and a half
times the stored energy to go around.

3: A borosilicate glass pot with a mass similar to the iron for an
equal interior volume would be superior to the iron, but would have walls
almost 1cm thick, and take a very long time to preheat.

4: A similar mass of aluminum, pre-heated, with a specific heat of 900
k/kg-k would have them both beat. But it would have absurdly thick walls,
and the available array of anodized colors would be no match for the earthy
aesthetics of cast iron with a good patina, or the simple serenity of a
well formed glass semi-sphere.

5: A similar mass of 440 stainless steel, with a specific heat of 460, a
thermal conductivity of 24, and a density of 7.65, would heat slower than
the iron but hold the heat about the same. It would unfortunately be
prohibitively hard to make, and as such most stainless pots have
significantly less mass than their cast iron brethren.


Thus, it is both my considered opinion and my observation that while
preheating a paper-thin borosilicate glass pot is almost entirely
pointless, pre-heating a cast iron pot before steeping is not only
beneficial but required.

Pyrex, fwiw, has a similar specific temperature and poorer conductivity
as compared to borosilicate.

And furthermore, owing to the greater thermal mass in the iron, and
assuming a poor thermal interface with the trivet, tea inside a preheated
tetsubin will stay hot roughly twice as long as tea inside a preheated
borosilicate pot.

QED.

Anybody got numbers for the specific heat, thermal conductivity, and
density of popular ceramics?