James F on mon 28 jul 08
> Think of it this way -- if you
> put a block of ice into a 400 F
> oven=2C what pattern does the melting
> take? The wire rack pattern=2C and
> the ice 'sinks' into it.
>=20
> Likewise the cold food issue -- the
> wall of the piece above the food line
> will have exposure on both sides to
> 400 F hot air. Below=2C it'll be
> one side at 400 F=2C one side at
> room temp (or below=2C if the food
> came from the fridge or freezer).
>=20
First=2C thank you all for the many examples of thermal shock cracking ware=
=2C but the examples seem to be inapplicable. No one has questioned the no=
tion that thermal shock can crack pots. I am only questioning whether baki=
ng in an oven can create enough thermal shock to break a reasonably well po=
tted=2C reasonably thick vessel. Sure=2C pour boiling water into a too thi=
n=2C cold vessel and you can easily crack it. The water has tremendous the=
rmal mass and a very high U value=2C so it transfers heat energy very rapid=
ly to the bottom of the cup or pot=2C more rapidly than the lesser conducti=
ng pot can spread the heat=2C just as with putting the cold raku pot onto a=
hot kiln shelf. Sure=2C if that pot is too thin for it's purpose=2C it cr=
acks. But thin ware is not functional ware=2C is it? Do the same thing to=
a cup or pot that is reasonably thick and I don't think this will happen. =
Sure=2C glaze only one side of a piece=2C and it cracks. But even here=2C=
the lopsided glazing set up the stress. The hot water merely pushed it ov=
er the edge. Also=2C none of these examples apply to the discussion at han=
d=2C as they all involved pouring hot liquid into a cold pot=2C which has n=
ever been at issue.
Steve=2C I too thought about the wire rack from the beginning=2C but my (ag=
ain=2C quite possibly flawed) reasoning lent it a very=2C very low probabil=
ity of being a factor. The wire rack has a very low thermal mass when comp=
ared to the pot full of food. It also=2C being metal=2C has an extremely h=
igh U value=2C approaching infinity=2C with the resultant R value of near z=
ero. I think that initially there may be some tiny hot spots created where=
the wire touches the pot=2C but even if so=2C it seems as though they woul=
d of necessity be extremely fleeting. Rather=2C I believe the very large t=
hermal mass of the pot of food would overcome the much lesser thermal mass =
of the shelf.
I also thought about the 'rim of the pot' scenario=2C but again my reasonin=
g caused me to abandon it. First=2C due to the previously discussed relati=
vely high thermal conductivity of the ceramic=2C especially in comparison t=
o that of the air=2C it does not seem possible for the rim to ever get sign=
ificantly hotter than the rest of the pot. Also=2C this same thermal condu=
ctivity would seem to preclude your hypothesis that the inside wall of the =
pot would be at the near-room temperature of the food while the outside wal=
l would be at 400 degrees. Yes=2C if this were the case=2C I could see the=
pot cracking=2C but my entire point is that from a physics standpoint I do=
not believe this can ever be the case. This line of reasoning=2C outlined=
in my previous post=2C is the basis for my entire contention.
Folks also seem to be laboring under the idea that the heating of an item p=
laced into a hot oven is near-instantaneous=2C or at least very rapid. Aga=
in=2C this is simply not the case. As has been covered earlier=2C air is s=
imply a very poor conductor of heat. You can stick your hand briefly into =
a 350 degree oven with no ill effects=2C but touch a 350 degree pot or meta=
l object and you are burned instantly. This is because it takes a relative=
ly long time for the poorly conducting air to transfer heat=2C while the ve=
ry high U value metal or ceramic transfers it's heat very rapidly.
I am in no way saying I am right=2C but from my admittedly weak understandi=
ng of physics I just do not see a scenario=2C even a contrived one=2C where=
the oven could break the pot. I am just trying to fathom a condition unde=
r which this could happen. If the condition actually exists and can be iso=
lated=2C then it can be overcome by prescription=2C benefiting all future g=
enerations of pot makers and pot buyers. Let's leave aside for a moment th=
e stories we have all heard about this or that happening=2C or something th=
at happened to 'a guy I know'=2C or my cousin's neighbor's friend's uncle's=
professor. Has anyone on the list ever witnessed firsthand a reasonably w=
ell potted 'functional' vessel fail in the oven? Not beginner ware with it=
's too fat bottom and too thin top=2C not very thin ware suitable only for =
decorative use=2C but professionally made=2C reasonably well designed funct=
ional ware that was potted thick enough for daily use? I am starting to fe=
el a bit like Vince in his crusade against the 'invisible craze-line bacter=
ia'.
One last thought: I am all for testing=2C but if=2C as the arguments now s=
eem to imply=2C it is the rather extreme interplay of cold or frozen masses=
of food in a pot being placed into a hot oven that is actually being impli=
cated=2C then I would offer that the standard test of shuttling an empty po=
t back and forth between the freezer and the oven a dozen times fails as a =
reasonable test as it does not replicate nor simulate the conditions suppos=
edly being tested. Wouldn't you have to freeze an actual casserole in the =
pot=2C then move this frozen casserole to the hot oven=2C then repeat for y=
our dozen iterations? The empty frozen pot has only a small fraction of th=
e thermal mass of a pot full of frozen casserole=2C so would not behave in =
an even remotely similar fashion. A much tastier=2C if more time consuming=
test to be sure.
With all due respect...
James
_________________________________________________________________
Stay in touch when you're away with Windows Live Messenger.
http://www.windowslive.com/messenger/overview.html?ocid=3DTXT_TAGLM_WL_mess=
enger2_072008=
Kim Hohlmayer on tue 29 jul 08
Hi All,
Along with "test, test, test." There are all sorts of additives that reduce the risks associated with thermal shock in clay. We can not plan for every eventuality. On all these related threads we as potters are required by law and morality to produce products that we have researched and tested and formulated to be safe under the uses for which we sell it. We are equally obligated to inform and as needed educate our buyers so they know things like not to put a roast in the oven in a raku pot.
On the same level we are not obligated by morality and sometimes not by law to become a parent to each and every person who buys a pot. John
Hesselberth (John, please forgive me if I have misspelled your name. I can't find it anywhere at the moment. I hate doing that to anyone!) says my posts imply that I blame all the victims. I don't. I take my responsibility to anyone who buys my work very seriously. No one should suffer because of my unwillingness to do anything necessary to insure the safest product possible. But if some idiot uses my pot to play catch and it breaks his window I am not responsible for the window!!!
Okay, now I'll quit and go back to quietly reading my email. :^) --Kim H.
--- On Mon, 7/28/08, James F wrote:
> From: James F
> Subject: FW: oven safe?
> To: CLAYART@LSV.CERAMICS.ORG
> Date: Monday, July 28, 2008, 5:37 PM
> > Think of it this way -- if you
> > put a block of ice into a 400 F
> > oven, what pattern does the melting
> > take? The wire rack pattern, and
> > the ice 'sinks' into it.
> >
> > Likewise the cold food issue -- the
> > wall of the piece above the food line
> > will have exposure on both sides to
> > 400 F hot air. Below, it'll be
> > one side at 400 F, one side at
> > room temp (or below, if the food
> > came from the fridge or freezer).
> >
>
> First, thank you all for the many examples of thermal shock
> cracking ware, but the examples seem to be inapplicable. No
> one has questioned the notion that thermal shock can crack
> pots. I am only questioning whether baking in an oven can
> create enough thermal shock to break a reasonably well
> potted, reasonably thick vessel. Sure, pour boiling water
> into a too thin, cold vessel and you can easily crack it.
> The water has tremendous thermal mass and a very high U
> value, so it transfers heat energy very rapidly to the
> bottom of the cup or pot, more rapidly than the lesser
> conducting pot can spread the heat, just as with putting
> the cold raku pot onto a hot kiln shelf. Sure, if that pot
> is too thin for it's purpose, it cracks. But thin ware
> is not functional ware, is it? Do the same thing to a cup
> or pot that is reasonably thick and I don't think this
> will happen. Sure, glaze only one side of a piece, and it
> cracks. But even here, the lopsided glazing set up the
> stress. The hot water merely pushed it over the edge.
> Also, none of these examples apply to the discussion at
> hand, as they all involved pouring hot liquid into a cold
> pot, which has never been at issue.
>
> Steve, I too thought about the wire rack from the
> beginning, but my (again, quite possibly flawed) reasoning
> lent it a very, very low probability of being a factor.
> The wire rack has a very low thermal mass when compared to
> the pot full of food. It also, being metal, has an
> extremely high U value, approaching infinity, with the
> resultant R value of near zero. I think that initially
> there may be some tiny hot spots created where the wire
> touches the pot, but even if so, it seems as though they
> would of necessity be extremely fleeting. Rather, I
> believe the very large thermal mass of the pot of food
> would overcome the much lesser thermal mass of the shelf.
>
> I also thought about the 'rim of the pot' scenario,
> but again my reasoning caused me to abandon it. First, due
> to the previously discussed relatively high thermal
> conductivity of the ceramic, especially in comparison to
> that of the air, it does not seem possible for the rim to
> ever get significantly hotter than the rest of the pot.
> Also, this same thermal conductivity would seem to preclude
> your hypothesis that the inside wall of the pot would be at
> the near-room temperature of the food while the outside
> wall would be at 400 degrees. Yes, if this were the case,
> I could see the pot cracking, but my entire point is that
> from a physics standpoint I do not believe this can ever be
> the case. This line of reasoning, outlined in my previous
> post, is the basis for my entire contention.
>
> Folks also seem to be laboring under the idea that the
> heating of an item placed into a hot oven is
> near-instantaneous, or at least very rapid. Again, this is
> simply not the case. As has been covered earlier, air is
> simply a very poor conductor of heat. You can stick your
> hand briefly into a 350 degree oven with no ill effects,
> but touch a 350 degree pot or metal object and you are
> burned instantly. This is because it takes a relatively
> long time for the poorly conducting air to transfer heat,
> while the very high U value metal or ceramic transfers
> it's heat very rapidly.
>
> I am in no way saying I am right, but from my admittedly
> weak understanding of physics I just do not see a scenario,
> even a contrived one, where the oven could break the pot. I
> am just trying to fathom a condition under which this could
> happen. If the condition actually exists and can be
> isolated, then it can be overcome by prescription,
> benefiting all future generations of pot makers and pot
> buyers. Let's leave aside for a moment the stories we
> have all heard about this or that happening, or something
> that happened to 'a guy I know', or my cousin's
> neighbor's friend's uncle's professor. Has
> anyone on the list ever witnessed firsthand a reasonably
> well potted 'functional' vessel fail in the oven?
> Not beginner ware with it's too fat bottom and too thin
> top, not very thin ware suitable only for decorative use,
> but professionally made, reasonably well designed
> functional ware that was potted thick enough for daily use?
> I am starting to feel a bit like Vince in his crusade
> against the 'invisible craze-line bacteria'.
>
> One last thought: I am all for testing, but if, as the
> arguments now seem to imply, it is the rather extreme
> interplay of cold or frozen masses of food in a pot being
> placed into a hot oven that is actually being implicated,
> then I would offer that the standard test of shuttling an
> empty pot back and forth between the freezer and the oven a
> dozen times fails as a reasonable test as it does not
> replicate nor simulate the conditions supposedly being
> tested. Wouldn't you have to freeze an actual
> casserole in the pot, then move this frozen casserole to
> the hot oven, then repeat for your dozen iterations? The
> empty frozen pot has only a small fraction of the thermal
> mass of a pot full of frozen casserole, so would not behave
> in an even remotely similar fashion. A much tastier, if
> more time consuming test to be sure.
>
> With all due respect...
>
> James
>
>
>
> _________________________________________________________________
> Stay in touch when you're away with Windows Live
> Messenger.
> http://www.windowslive.com/messenger/overview.html?ocid=TXT_TAGLM_WL_messenger2_072008
Steve Slatin on tue 29 jul 08
James --
I don't agree about the idea of
the ceramic having such a high
thermal conductivity. I don't know
what the standard test is for this,
but if you put boiling water into
a mug and wait a minute or two,
a fingertip can readily identify
the level of the water on the
inside from the different in
temperature on the outside.
This difference would be mirrored
in the example of a casserole going
from chilled to above-boiling
temperature. The higher temp part
of the container would (presumably)
have expansion to match.
I can't think of a reason why
adding hot suddenly to the inside
would differ greatly from adding
hot suddenly to the outside. Maybe
you've given this more thought than
I have.
Best -- Steve S
Steve Slatin --
Inanimate! Gooseberry paws two clause
inanimate! (overheard in Pittsburgh)
--- On Mon, 7/28/08, James F wrote:
> First, thank you all for the many examples of thermal shock
> cracking ware, but the examples seem to be inapplicable. No
> one has questioned the notion that thermal shock can crack
> pots. I am only questioning whether baking in an oven can
> create enough thermal shock to break a reasonably well
> potted, reasonably thick vessel. Sure, pour boiling water
> into a too thin, cold vessel and you can easily crack it.
> The water has tremendous thermal mass and a very high U
> value, so it transfers heat energy very rapidly to the
> bottom of the cup or pot, more rapidly than the lesser
> conducting pot can spread the heat, just as with putting
> the cold raku pot onto a hot kiln shelf. Sure, if that pot
> is too thin for it's purpose, it cracks. But thin ware
> is not functional ware, is it? Do the same thing to a cup
> or pot that is reasonably thick and I don't think this
> will happen. Sure, glaze only one side of a piece, and it
> cracks. But even here, the lopsided glazing set up the
> stress. The hot water merely pushed it over the edge.
> Also, none of these examples apply to the discussion at
> hand, as they all involved pouring hot liquid into a cold
> pot, which has never been at issue.
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