KATHI LESUEUR on thu 24 jul 08
Last week was the Ann Arbor art fair and lots of potter friends were
around for the show. One of them until recently has made only
decorative pieces. During a conversation he said something about "the
baking dish that woman bought". Which caused me to ask him if he'd
tested his new line to see that it WAS oven, microwave, and
dishwasher safe. Had actually baked anything in his baking dishes?
Had he put them into his dishwasher? Had he used them in a microwave?
The answer to all of these questions was "no". "But" he said, " I'm
using those glazes from Mastering Cone 6 Glazes so I know they are
alright."
Another example of someone who just assumes that if he uses glazes
that someone else uses with success everything will work. Not
understanding that the compatibility of clay and glaze is essential
to the performance of the piece and that not all clays can be used
successfully in an oven. I strongly urged him to do testing before
he offered anymore "oven safe" work to the public.
Unfortunately, I don't think this is an unusual story. People put
items out for sale without doing the homework to make sure that they
will perform as represented. They just assume they can make a baking
dish and it will be fine in the oven. If it isn't, if it cracks, it
affects all of us offering bake-ware. People will question all of us
and hesitate to buy another piece.
I can't urge other potters strongly enough that if you are going to
offer items to be used in the oven, microwave, and dishwasher you
better know that they can actually be used successfully.
Kathi
Lee Love on sat 26 jul 08
On Thu, Jul 24, 2008 at 9:53 PM, KATHI LESUEUR
wrote:
> I can't urge other potters strongly enough that if you are going to
> offer items to be used in the oven, microwave, and dishwasher you
> better know that they can actually be used successfully.
I have noticed that most of the potters I admire also cook and bake.
( Even the guys.) They often brew their own beer and make their own
wine or keep bees, etc. It makes sense to me, because in the use of
their work, they recognize how well it functions for the intended
purpose.
I usually use other people's work in my home. If you only use
your own work, it is like talking to yourself. But when you use the
work of others, you have a dialog with that maker. Also, my view is
that a functional piece of pottery is "completed" when the owner puts
it to use. Your patrons are not just consumers, but participants in
your creativity.
The exception I make in using my own work, is for testing it
when something is new. You can also have good patrons test things
for you, especially if they are put to use in a way that you normally
don't use them.
Greg Crowe from Perth is the new McKnight resident at Northern
Clay Center, the one following me. He fired the NCC woodkiln with me,
Steve and my studio mate Andy, up at Jeff Oestrich's. We finished
last night at 10pm
We had a chat about our work. He said he didn't have a dishwasher
or a microwave, because he thought that a dishwasher kept you from a
very important aspect of using functional ware: getting to know the
pot really well while you are washing it. I explained that I had
neither a microwave nor a washer in Japan, but here in Minneapolis, I
inherited both with the house. So I just use them for testing.
We both agree that people can live better if they get out of the
"wash and wear" mentality.
It was great to have dinner at Jeff's. We brought chicken and
fixings to grill. Jeff cooked up a pot wild rice and made Steve a
birthday carrot cake with cream cheese frosting!
Jeff's home is incredible. It is filled with old and new
potter. The food was great and the pots we ate one were even better!
And the dishes were washed by hand! I watched Greg enjoy that. We
unload on Tuesday. I go back to Japan for August on Thursday.
--
Lee Love in Minneapolis
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
James F on sun 27 jul 08
"the
> baking dish that woman bought". Which caused me to ask him if he'd
> tested his new line to see that it WAS oven=2C microwave=2C and
> dishwasher safe. Had actually baked anything in his baking dishes?
I accept on your collective word that not all pottery is oven-safe=2C and I=
don't make functional ware per se anyway=2C but it seems illogical. I hav=
e a dumb but honest question=2C perhaps for the scientists on the list: If=
a pot=2C with whatever it's peculiar combination of clay and glaze=2C just=
survived baking and cooling in a 2400 degree oven=2C why is it not a safe =
assumption that it would by definition be able to survive baking and coolin=
g in a 400 degree oven? If one of the chairs I build holds a 200 pound per=
son=2C I assume without testing that it will also hold an 80 pound person. =
Does the logic not hold?
Ceramic folk routinely re-glaze and re-fire pieces in various states of vit=
rification without incident. Fine china is often high-fired to vitrificati=
on without glaze=2C then glazed and fired to lower temperatures. I recall =
reading somewhere that Chinese potters did this also. Raku ware is pulled =
from an oven in excess of 1500 degrees=2C waved about in the cool air=2C du=
mped into trash bins=2C plunged into cold water=2C all without incident. H=
ow could a 400 degree oven hurt the stuff?
Thanks in advance for any insight or elucidation you can provide.
...James
_________________________________________________________________
Time for vacation? WIN what you need- enter now!
http://www.gowindowslive.com/summergiveaway/?ocid=3Dtag_jlyhm=
Lois Ruben Aronow on sun 27 jul 08
It's not really the heating up of the pot that's the concern; it's the
cooling down. Taking any pot out of a 500 degree oven would cause cracking.
Also bear in mind that the pot would be filled with food, adding additional
heat and hot spots.
Thin ware would likely break if it bears the weight of food, especially hot.
The repeated heating and cooling of ware puts a lot of stress on it over
time, too. Hell, I've shattered vintage Pyrex when removing it from a hot
oven.
I tell people they can certainly heat stuff up in mine, but don't bake in
it, and don't remove it from an oven hotter than 300 degrees. Keeping the
stuffing warm on Thanksgiving is not a problem, but don't bake the yams in
it.
This isn't even taking into consideration that some glazes might change at
high oven temps. I'm guessing some might leach.
There are reasons god invented La Creusette.
KATHI LESUEUR on sun 27 jul 08
On Jul 27, 2008, at 9:18 AM, James F wrote:
> "the
>> baking dish that woman bought". Which caused me to ask him if he'd
>> tested his new line to see that it WAS oven, microwave, and
>> dishwasher safe. Had actually baked anything in his baking dishes?
>
>
> I accept on your collective word that not all pottery is oven-safe,
> and I don't make functional ware per se anyway, but it seems
> illogical. I have a dumb but honest question, perhaps for the
> scientists on the list: If a pot, with whatever it's peculiar
> combination of clay and glaze, just survived baking and cooling in
> a 2400 degree oven, why is it not a safe assumption that it would
> by definition be able to survive baking and cooling in a 400 degree
> oven? If one of the chairs I build holds a 200 pound person, I
> assume without testing that it will also hold an 80 pound person.
> Does the logic not hold?>>
That is one of the fallacies of bake-ware. If it survived a firing to
2400 it can take anything. A mug goes from room temp to 190 in
seconds when coffee is poured into it. same with a teapot. Even
though many potters tell their customers to pre-warm their teapots
and put baking dishes in a cool oven, it's not always going to
happen. Then what. The potter needs to know. this was brought home to
me when I was in school. My professor made these wonderful mugs out
of a new clay body. When he poured coffee into them all cracked in a
spiral pattern around the mugs. Test, test, test. I can't stress it
enough.
Kathi
>
>
John Hesselberth on sun 27 jul 08
Hi James,
Good question. Think about it this way. Suppose that dish contains a
casserole which has been stored in someone's freezer. When it is put
in the oven and begins to heat up the edges of the dish are up to
oven temperature while the center is still 32F. That puts a whole lot
more stress on the clay/glaze combination that the relatively uniform
heating/cooling an empty pot gets in a kiln. Same thing happens in
reverse when you take it out of the oven.
It gets even worse if there is any cristobalite in the clay body
(there usually is not at cone 6 and below, but there can be at cone
10 if the body was not formulated well). Somewhere between 428F and
536F cristobalite undergoes a 3% change in volume. That means it can
happen in a hot oven. If one part of the pot goes through that
before another part--well, pots are not very elastic.
Regards,
John
On Jul 27, 2008, at 9:18 AM, James F wrote:
> If a pot, with whatever it's peculiar combination of clay and
> glaze, just survived baking and cooling in a 2400 degree oven, why
> is it not a safe assumption that it would by definition be able to
> survive baking and cooling in a 400 degree oven? If one of the
> chairs I build holds a 200 pound person, I assume without testing
> that it will also hold an 80 pound person. Does the logic not hold?
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Lee Love on sun 27 jul 08
On Sun, Jul 27, 2008 at 2:40 PM, John Hesselberth
wrote:
> Hi James,
>
> Good question. Think about it this way. Suppose that dish contains a
> casserole which has been stored in someone's freezer.
John, you don't actually cook casseroles, do you? I have never heard
of a casserole going from the freezer to a hot oven.
> 10 if the body was not formulated well). Somewhere between 428F and
> 536F cristobalite undergoes a 3% change in volume. That means it can
> happen in a hot oven. If one part of the pot goes through that
> before another part--well, pots are not very elastic.
Most things are not baked at these temps.
"If" we want to talk "hypothetical", self cleaning ovens reach a
temperature of 900*F. Door stays locked until it cools to 600*F
Maybe someone will put your pot in the oven when it is being cleaned,
take it out at 600*F. Heck, some folks might put Tupper ware in the
oven. No accounting for Idiocy.
--
Lee Love in Minneapolis
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
Kim Hohlmayer on mon 28 jul 08
OUCH! This is the reason that I still make mostly decorative ware. I was raised to believe that I should NEVER assume anything except that I will be held accountable if I screw up! It sounds cynical but as a potter it is a phylosophy that seems to work.
A great example of this on the commercial level was about seven or so years back when Laguna came out with new cone6 glazes that were meant to craze. A friend and I tried them only to open the kiln and find that the glaze had fired then as it cooled the fit was so bad that the glaze simply fell of in shards. We took it back to our local supplier who gave us our money back but couldn't convince Laguna there was something wrong with the glaze. Turns out that it fit western clay bodies but nothing else. Eventually the guys at Laguna realized that they needed to revamp the glaze and broaden the bodies on which it would work. Now that glaze is fine on almost any body. However, I am sure the experience cost them. --Kim H.
--- On Thu, 7/24/08, KATHI LESUEUR wrote:
> From: KATHI LESUEUR
> Subject: oven safe?
> To: CLAYART@LSV.CERAMICS.ORG
> Date: Thursday, July 24, 2008, 10:53 PM
> Last week was the Ann Arbor art fair and lots of potter
> friends were
> around for the show. One of them until recently has made
> only
> decorative pieces. During a conversation he said something
> about "the
> baking dish that woman bought". Which caused me to ask
> him if he'd
> tested his new line to see that it WAS oven, microwave, and
> dishwasher safe. Had actually baked anything in his baking
> dishes?
> Had he put them into his dishwasher? Had he used them in a
> microwave?
> The answer to all of these questions was "no".
> "But" he said, " I'm
> using those glazes from Mastering Cone 6 Glazes so I know
> they are
> alright."
>
> Another example of someone who just assumes that if he uses
> glazes
> that someone else uses with success everything will work.
> Not
> understanding that the compatibility of clay and glaze is
> essential
> to the performance of the piece and that not all clays can
> be used
> successfully in an oven. I strongly urged him to do
> testing before
> he offered anymore "oven safe" work to the
> public.
>
> Unfortunately, I don't think this is an unusual story.
> People put
> items out for sale without doing the homework to make sure
> that they
> will perform as represented. They just assume they can make
> a baking
> dish and it will be fine in the oven. If it isn't, if
> it cracks, it
> affects all of us offering bake-ware. People will question
> all of us
> and hesitate to buy another piece.
>
> I can't urge other potters strongly enough that if you
> are going to
> offer items to be used in the oven, microwave, and
> dishwasher you
> better know that they can actually be used successfully.
>
> Kathi
Randy O'Brien on mon 28 jul 08
Hi James,
Maybe some examples of types of ware that did not survive heat shock
from my past experience might help.
Some were more personally painful than others.
Iron bearing clay bodies that are over-reduced can be very brittle. I
once over reduced a round vase form
that was very thin. 3 weeks after it had been fired, it spontaneously
broke into a dozen pieces while I was
standing next to it. The shards could be snapped as easily as a potato
chip. Ovenware in this category can
easily crack.
About 20 years ago before I was familiar with how different earthenware
is from stoneware, I made a
series of cone 1 glazes that I used on a white earthenware. The
earthenware was right at the edge of
where it would start to slump. It didn't occur to me at the time to
test for thermal shock, and every
teapot and pie pan I made in the 2 months that I used this body was
returned cracked. Unfortunately
they didn't start coming back for 2 months, otherwise I would have
stopped earlier. At the time I
attributed the cracking to overfiring the earthenware, but now I think
it could have been a glaze fit
problem.
Which leads to the next example - glaze fit. A glaze with a thermal
expansion that differs markedly
from the clay body weakens the ware. This is more critical with
earthenware than stoneware, but
still can lead to failure with stoneware/porcelain. Try glazing the
inside only of a large, thin porcelain
bowl with a shino that crazes. Most likely when you open the kiln, what
you will find is a pile of
shards where the bowl used to be, the result of severe dunting. Glaze
fit is very important with
earthenware. I've been working on a series of earthenware glazes for
the past couple years for
functional ware. The test I use for heat shock is to glaze the inside
only of a mug with the test
glaze. I then pour boiling water into the cup after it has been in the
freezer. A pretty severe test,
but there are glazes that pass. Firing temperature also affects the
glaze fit. I have a glaze that
works fine if it is fired to cone 05 on a particular body bisque fired
to cone 03. But if the clay
body is bisqued to cone 05, it cracks. Or if the glaze is fired to cone
03, it cracks.
Raku pieces generally survive extreme temperature changes in the firing
and cooling, but
raku is not vitreous. It is basically stoneware bisque, so it can give
a bit with the
uneven expansion without cracking. Try raku firing a piece that has
been bisque
fired to its maturation temperature and it will most likely crack.
There is a reason why
underfired stoneware is used for raku and not earthenware or cone 5
stoneware bisqued to
cone 5, or cone 10 stoneware bisqued to cone 10.
Sorry this isn't more chemically specific regarding the technical
explanations for cracking
due to thermal shock. I'm a bit rusty in that department.
Best regards,
Randy
http://www.dakotacom.net/~rdobrien
James F wrote:
> John, et al...
>
> Thanks for your replies both on and off list. As I stated, I am certainly not questioning anyone's statement that they have witnessed a pot fail in an oven. I am, however seeking to understand how this could happen. I had considered all of the factors mentioned before posing my question, but none of them (with the possible exception of cristobalite, on which I shall ponder further) seems able, by my quite possibly erroneous reasoning, to cause the reported failures. I am trying to understand how the failures could possibly be attributed to the composition of the ceramic materials.
Steve Slatin on mon 28 jul 08
James --
Interesting thoughts, but you may
have left out one element in the
mix -- that there will always be
parts of the pot getting more heat
transmission than others.
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. That's
a part of the high conductance
issue you raise. So contact points
on the foot ring of the piece (or 'lines'
on the flat base, if there's no foot
ring) will get more heat transmission
-- and more thermal expansion -- than
the rest.
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).
That should account for plenty enough
differences in thermal expansion --
above and below the food line, direct-
contact heat transmission surfaces vs.
air conduction of heat, and inside vs.
outside the pot -- to create problems.
I always tell my clients that the
work isn't designed for baking, but
if they want to, to start cold-cold-
cold (food, pot, oven) and bring it
all up to the target temp together.
I also break my own rule and bake
cold food in room-temp casseroles
that I sling into a hot oven. And
I've never lost a piece, but I've
heard of it happening.
Steve Slatin --
--- On Sun, 7/27/08, James F wrote:
> From: James F
> Subject: Re: oven safe?
> To: CLAYART@LSV.CERAMICS.ORG
> Date: Sunday, July 27, 2008, 6:38 PM
> John, et al...
>
> Thanks for your replies both on and off list. As I stated,
> I am certainly not questioning anyone's statement that
> they have witnessed a pot fail in an oven. I am, however
> seeking to understand how this could happen. I had
> considered all of the factors mentioned before posing my
> question, but none of them (with the possible exception of
> cristobalite, on which I shall ponder further) seems able,
> by my quite possibly erroneous reasoning, to cause the
> reported failures.
KATHI LESUEUR on mon 28 jul 08
On Jul 28, 2008, at 5:37 PM, 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.
Why do I think that there are casseroles and baking dishes out there
that can't take use in the oven? Because people come up to me at
shows with their cracked piece made by someone else and ask if it is
common for pieces to crack in the oven. The conversation goes
something like this, "I bought this baking dish and the first time I
used it in the oven it cracked. I showed it to the potter who said,
'yeah, that happens sometimes.'" No offer to replace it. No offer to
refund the cost. Just "oh yeah, that happens sometimes."
So, there they are standing in my booth wanting a baking dish from me
but really worried about being taken twice. As I said, such an
attitude affects all of us. There are pots that won't hold up in the
oven and it is the responsibility of the potter to make sure those
pots are not theirs.
Kathi
>>
Lois Ruben Aronow on mon 28 jul 08
They DON'T?!?!??!?! Wow, I do.
> Ric, learn to read. The previous person said from the FREEZER. If
> you don't cook, you might think folks put things frozen solid
> into a 500*F oven to bake, but that is simply a lack of
> knowledge about baking.
Ric Swenson on mon 28 jul 08
Hey... Lee
get real....people do take things from the fridge to the hot oven......not smart .......but people are in such a rush to watch JEOPARDY!!!
Some folks do NOT understand that ceramics cannot go from freezing to 400 degree F. oven preheated temp.
Go Figure!
We know............ but the general population is competely clueless.
Casseroles taken from frozen....or even from the fridge.... to 400 Degrees can indeed dunt.....Fact of life.
The edges of the ceramic cannot take the temp change wit the base..........the food stays too cold for too long....even in the micro.
Is this news ?
Ric
"...then fiery expedition be my wing, ..." -Wm. Shakespeare, RICHARD III, Act IV Scene III Richard H. ("Ric") Swenson, Teacher, Office of International Cooperation and Exchange of Jingdezhen Ceramic Institute, TaoYang Road, Eastern Suburb, Jingdezhen City.JiangXi Province, P.R. of China. Postal code 333001. Mobile/cellular phone : 86 13767818872 < RicSwenson0823@hotmail.com> http://www.jci.jx.cn/http://www.ricswenson.com
> Date: Sun, 27 Jul 2008 17:18:18 -0500> From: togeika@CLAYCRAFT.ORG> Subject: Re: oven safe?> To: CLAYART@LSV.CERAMICS.ORG> > On Sun, Jul 27, 2008 at 2:40 PM, John Hesselberth> wrote:> > Hi James,> >> > Good question. Think about it this way. Suppose that dish contains a> > casserole which has been stored in someone's freezer.> > John, you don't actually cook casseroles, do you? I have never heard> of a casserole going from the freezer to a hot oven.> > > > 10 if the body was not formulated well). Somewhere between 428F and> > 536F cristobalite undergoes a 3% change in volume. That means it can> > happen in a hot oven. If one part of the pot goes through that> > before another part--well, pots are not very elastic.> > Most things are not baked at these temps.> > "If" we want to talk "hypothetical", self cleaning ovens reach a> temperature of 900*F. Door stays locked until it cools to 600*F> Maybe someone will put your pot in the oven when it is being cleaned,> take it out at 600*F. Heck, some folks might put Tupper ware in the> oven. No accounting for Idiocy.> > --> Lee Love in Minneapolis> http://mashikopots.blogspot.com/> http://claycraft.blogspot.com/> > "Let the beauty we love be what we do.> There are hundreds of ways to kneel and kiss the ground." --Rumi
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pdp1@EARTHLINK.NET on mon 28 jul 08
Hi James,
My understanding...
If any part of the Pot is at a different temperature than the rest of it,
this will be stressful for the Pot.
If a Pot is heated or cooled too rapidly for the internal temperatures of
it's 'parts' and shape and sections to enjoy uniform expansion or
contraction, with respect to how the shape interferes with or promotes
uniformity of either movement, assuming uniform temperature through-out, it
will be stressful for the Pot.
Stresses which exceed the elastic limits of the material in any given shape,
will tend to express themselves as cracks, breakage or shattering,
depending.
I will guess that any Pot which has been Fired, would do just fine in a
Convection Oven, so long as it and the Oven start out at the same
Temperature, AND, both are very very slowly, VERY slowly, brought up to
whatever Heat one likes, with no opportunities of differential 'cooling'
allowed...and or, if the Pot-in-Oven were allowed to cool very very slowly,
together,back to room Temperature, before the Pot is removed.
And no 'peeking'..!
Lol...
Phil
l v
----- Original Message -----
From: "James F"
John, et al...
Thanks for your replies both on and off list. As I stated, I am certainly
not questioning anyone's statement that they have witnessed a pot fail in an
oven. I am, however seeking to understand how this could happen. I had
considered all of the factors mentioned before posing my question, but none
of them (with the possible exception of cristobalite, on which I shall
ponder further) seems able, by my quite possibly erroneous reasoning, to
cause the reported failures. I am trying to understand how the failures
could possibly be attributed to the composition of the ceramic materials.
Let's take the most extreme example of the frozen casserole going into the
oven. You have 400 degree air trying to heat the pot, and a 32 degree
frozen mass trying to keep it cold. Air is a very poor thermal conductor
(high R value, low U value), so you have 400 degree air doing a poor job of
trying to make the pot hot. The frozen food is a very good thermal
conductor, so it does a very good job of trying to keep the pot cool. Now,
it seems to me that the ceramic, also a very good thermal conductor, can
never get much hotter than the food that is touching it. Wouldn't the cool
mass of highly conductive food prevent the pot from heating at the same rate
it might if empty? Wouldn't the very high thermal conductivity of the
ceramic prevent the outside of the wall from reaching a temperature much
different from the inside of that same wall? Both the pot and the food have
a tremendous (cold) thermal mass that has to be overcome by the relatively
poorly conducting and low thermal mass air. This explains why you can put a
frozen roast in a hot oven for a long time, and it will still be frozen in
the center. Given all of this, I don't understand how the air can win.
Think of a double boiler used to melt wax or chocolate. The relatively cool
212 degree water with it's high thermal mass prevents the wax or chocolate
from exceeding this temperature despite the 3500 degree flame beneath. I
just can't see how the ceramic pot could heat much more quickly than the
portion of food that is immediately adjacent to it from a physics
standpoint. Even if it could, I don't see how it could have enough thermal
differential at only 400 degrees minus the temperature of the food at the
food/pot juncture to cause any damage. It seems like something else must be
at work.
Now, if there were an undetected hairline crack in the pot, I could perhaps
see moisture seeping in, turning to steam at a relatively early point in the
baking, and, due to it's powerful expansion in the crack, tearing the pot
asunder. Perhaps water from the food at the very edge where the frozen mass
touches the pot could turn to steam and exert enough pressure between the
wall of the pot and the frozen mass of food to cause a fracture. This
doesn't seem too likely, but does seem possible. Even if this were the case
though, the actual cause of the resulting fracture would then be the
structural weakness of the pot (too thinly potted) rather than the
composition of the ceramic material per se.
Someone mentioned the example of putting a cold pot into a hot raku kiln and
watching the bottom pop off. I offer that this is not the same thing, and
in fact reinforces my thoughts. The hot kiln shelf is a very good thermal
conductor and has a very high thermal mass. The empty pot has a much lower
thermal mass which is quickly overcome via conduction from the hot shelf.
If the hot air in the kiln conducted heat as well as the shelf does, the pot
wouldn't break. Someone else mentioned preheating a ceramic teapot. I
suggest that this, too, reinforces my thoughts. One preheats a ceramic
teapot by pouring in boiling water. This is not done to prevent shock to
the clay, as the preheating water is the same temperature as that from which
you will brew the tea. Rather, the purpose is to overcome the (cold)
thermal mass of the clay so it does not suck the heat out of your tea.
Now I know that anecdotal evidence does not carry any weight (except perhaps
in the Nytal trial). Consider, however, that I cannot possibly be the only
person who impatiently pulls pots from a 350 degree kiln and sets them on
the concrete studio floor to cool. Who among us hasn't crash-cooled a kiln
to make a deadline? Who hasn't plunged a hot raku pot into cold water? I'm
absolutely not questioning that pots have broken in the oven. I am only
questioning whether it is a function of the composition of the ceramic
materials, or whether these seemingly isolated incidents may have another
physical cause unrelated to the clay and glaze composition. Perhaps I am
just dense, but the ceramic material per se seems to be amongst the least
likely causes. What am I missing here?
With all due respect...
James
Lois Ruben Aronow on mon 28 jul 08
I cook and freeze - mostly in the winter. I only thaw meat, not pre-cooked
or par-cooked food.
I have broken Pyrex when removing it from the oven, but mostly, it is made
specifically for baking. Glass, on the other hand, is not ceramic. Pyrex,
in particular, differs from "regular" glass by being tempered for better
thermal resistance. If it breaks, it will break into tiny little pieces,
like your windshield.
The Pyrex I broke was the vintage, and presumably old borosilicate, kind.
...Lo
making an effort to be more eco-conscientious by not using plastic.
> I can't be the only one to prepare two lasagnas (or baked
> ziti) and freeze one and cook it later? Cook books tell you
> all the time to prepare up to step (whatever) and freeze for
> later - add a little extra time for cooking. I never used a
> ceramic dish, I have a couple of pyrex ones. I've been
> cooking for over 30 years. I haven't always had a microwave
> or the brain to put it in the microwave to partially cook it
> first before I put it in the oven. Usually, I threw it in
> the oven right out of the freezer. To thaw it first would be
> bad I think.
>
> If you say oven safe - that's what folks will remember. Best
> to make sure.
>
> nancy g.
pdp1@EARTHLINK.NET on mon 28 jul 08
Hi James, all...
My understanding...
If any part of the Pot is at a different temperature than the rest of it,
this will be stressful for the Pot...owing to how some parts will be
expanding or contracting at different rates.
If a Pot is heated or cooled too rapidly for the internal temperatures of
it's 'parts' and shape and sections to enjoy uniform expansion or
contraction, with respect to how the shape interferes with or promotes
uniformity of either movement, assuming uniform temperature through-out, it
will be stressful for the Pot.
Stresses which exceed the elastic limits of the material in any given shape,
will tend to express themselves as cracks, breakage or shattering,
depending.
I will guess that any Pot which has been Fired, would do just fine in a
Convection Oven, so long as it and the Oven start out at the same
Temperature, AND, both are very very slowly, VERY slowly, brought up to
whatever Heat one likes, with no opportunities of differential 'cooling'
allowed...and or, if the Pot-in-Oven were allowed to cool very very slowly,
together,back to room Temperature, before the Pot is removed.
And no 'peeking'..!
Lol...
Phil
l v
----- Original Message -----
From: "James F"
John, et al...
Thanks for your replies both on and off list. As I stated, I am certainly
not questioning anyone's statement that they have witnessed a pot fail in an
oven. I am, however seeking to understand how this could happen. I had
considered all of the factors mentioned before posing my question, but none
of them (with the possible exception of cristobalite, on which I shall
ponder further) seems able, by my quite possibly erroneous reasoning, to
cause the reported failures. I am trying to understand how the failures
could possibly be attributed to the composition of the ceramic materials.
Let's take the most extreme example of the frozen casserole going into the
oven. You have 400 degree air trying to heat the pot, and a 32 degree
frozen mass trying to keep it cold. Air is a very poor thermal conductor
(high R value, low U value), so you have 400 degree air doing a poor job of
trying to make the pot hot. The frozen food is a very good thermal
conductor, so it does a very good job of trying to keep the pot cool. Now,
it seems to me that the ceramic, also a very good thermal conductor, can
never get much hotter than the food that is touching it. Wouldn't the cool
mass of highly conductive food prevent the pot from heating at the same rate
it might if empty? Wouldn't the very high thermal conductivity of the
ceramic prevent the outside of the wall from reaching a temperature much
different from the inside of that same wall? Both the pot and the food have
a tremendous (cold) thermal mass that has to be overcome by the relatively
poorly conducting and low thermal mass air. This explains why you can put a
frozen roast in a hot oven for a long time, and it will still be frozen in
the center. Given all of this, I don't understand how the air can win.
Think of a double boiler used to melt wax or chocolate. The relatively cool
212 degree water with it's high thermal mass prevents the wax or chocolate
from exceeding this temperature despite the 3500 degree flame beneath. I
just can't see how the ceramic pot could heat much more quickly than the
portion of food that is immediately adjacent to it from a physics
standpoint. Even if it could, I don't see how it could have enough thermal
differential at only 400 degrees minus the temperature of the food at the
food/pot juncture to cause any damage. It seems like something else must be
at work.
Now, if there were an undetected hairline crack in the pot, I could perhaps
see moisture seeping in, turning to steam at a relatively early point in the
baking, and, due to it's powerful expansion in the crack, tearing the pot
asunder. Perhaps water from the food at the very edge where the frozen mass
touches the pot could turn to steam and exert enough pressure between the
wall of the pot and the frozen mass of food to cause a fracture. This
doesn't seem too likely, but does seem possible. Even if this were the case
though, the actual cause of the resulting fracture would then be the
structural weakness of the pot (too thinly potted) rather than the
composition of the ceramic material per se.
Someone mentioned the example of putting a cold pot into a hot raku kiln and
watching the bottom pop off. I offer that this is not the same thing, and
in fact reinforces my thoughts. The hot kiln shelf is a very good thermal
conductor and has a very high thermal mass. The empty pot has a much lower
thermal mass which is quickly overcome via conduction from the hot shelf.
If the hot air in the kiln conducted heat as well as the shelf does, the pot
wouldn't break. Someone else mentioned preheating a ceramic teapot. I
suggest that this, too, reinforces my thoughts. One preheats a ceramic
teapot by pouring in boiling water. This is not done to prevent shock to
the clay, as the preheating water is the same temperature as that from which
you will brew the tea. Rather, the purpose is to overcome the (cold)
thermal mass of the clay so it does not suck the heat out of your tea.
Now I know that anecdotal evidence does not carry any weight (except perhaps
in the Nytal trial). Consider, however, that I cannot possibly be the only
person who impatiently pulls pots from a 350 degree kiln and sets them on
the concrete studio floor to cool. Who among us hasn't crash-cooled a kiln
to make a deadline? Who hasn't plunged a hot raku pot into cold water? I'm
absolutely not questioning that pots have broken in the oven. I am only
questioning whether it is a function of the composition of the ceramic
materials, or whether these seemingly isolated incidents may have another
physical cause unrelated to the clay and glaze composition. Perhaps I am
just dense, but the ceramic material per se seems to be amongst the least
likely causes. What am I missing here?
With all due respect...
James
Bill Merrill on mon 28 jul 08
It sounds more like your bottle broke because the glaze did not fit the
bottle. If the bottle had a small neck and you didn't glaze the inside
, that could be another problem causing the bottle to break. It sounds
more like a compression issue with the glaze. I reduce from cone 010
until cone 9 1/2 in reduction. If an oxygen probe was reading the
reduction, it would read .65. My clay body has iron in it from Neuman
Red (15-20%) and there are traced of iron in the other clays in the
body, so over reducing may not be the issue with your bottle breaking.
Since you said the pieces from the bottle "snapped" like a potato chip,
it is undoubtedly a compression issue with the glaze. The glaze is
compressing more than the clay body is shrinking. By the way, does your
glaze gave lithium in it? Lithium is in spodumene, so it doesn't have
to be lithium carbonate. Petalite and=20
LEPIDOLITE also have lithium in them. Just some things for you to think
about.
Keep testing and be consistent with everything from your clay and glazes
th firing, cooling etc.
Regards, Bill
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Randy
O'Brien
Sent: Monday, July 28, 2008 5:33 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: oven safe?
Hi James,
Maybe some examples of types of ware that did not survive heat shock
from my past experience might help.
Some were more personally painful than others.
Iron bearing clay bodies that are over-reduced can be very brittle. I
once over reduced a round vase form
that was very thin. 3 weeks after it had been fired, it spontaneously
broke into a dozen pieces while I was
standing next to it. The shards could be snapped as easily as a potato
chip. Ovenware in this category can
easily crack.
About 20 years ago before I was familiar with how different earthenware
is from stoneware, I made a
series of cone 1 glazes that I used on a white earthenware. The
earthenware was right at the edge of
where it would start to slump. It didn't occur to me at the time to
test for thermal shock, and every
teapot and pie pan I made in the 2 months that I used this body was
returned cracked. Unfortunately
they didn't start coming back for 2 months, otherwise I would have
stopped earlier. At the time I
attributed the cracking to overfiring the earthenware, but now I think
it could have been a glaze fit
problem.
Which leads to the next example - glaze fit. A glaze with a thermal
expansion that differs markedly
from the clay body weakens the ware. This is more critical with
earthenware than stoneware, but
still can lead to failure with stoneware/porcelain. Try glazing the
inside only of a large, thin porcelain
bowl with a shino that crazes. Most likely when you open the kiln, what
you will find is a pile of
shards where the bowl used to be, the result of severe dunting. Glaze
fit is very important with
earthenware. I've been working on a series of earthenware glazes for
the past couple years for
functional ware. The test I use for heat shock is to glaze the inside
only of a mug with the test
glaze. I then pour boiling water into the cup after it has been in the
freezer. A pretty severe test,
but there are glazes that pass. Firing temperature also affects the
glaze fit. I have a glaze that
works fine if it is fired to cone 05 on a particular body bisque fired
to cone 03. But if the clay
body is bisqued to cone 05, it cracks. Or if the glaze is fired to cone
03, it cracks.
Raku pieces generally survive extreme temperature changes in the firing
and cooling, but
raku is not vitreous. It is basically stoneware bisque, so it can give
a bit with the
uneven expansion without cracking. Try raku firing a piece that has
been bisque
fired to its maturation temperature and it will most likely crack.
There is a reason why
underfired stoneware is used for raku and not earthenware or cone 5
stoneware bisqued to
cone 5, or cone 10 stoneware bisqued to cone 10.
Sorry this isn't more chemically specific regarding the technical
explanations for cracking
due to thermal shock. I'm a bit rusty in that department.
Best regards,
Randy
http://www.dakotacom.net/~rdobrien
James F wrote:
> John, et al...
>
> Thanks for your replies both on and off list. As I stated, I am
certainly not questioning anyone's statement that they have witnessed a
pot fail in an oven. I am, however seeking to understand how this could
happen. I had considered all of the factors mentioned before posing my
question, but none of them (with the possible exception of cristobalite,
on which I shall ponder further) seems able, by my quite possibly
erroneous reasoning, to cause the reported failures. I am trying to
understand how the failures could possibly be attributed to the
composition of the ceramic materials.
Lee Love on mon 28 jul 08
On Mon, Jul 28, 2008 at 4:21 AM, Ric Swenson wrote:
> Hey... Lee
>
> get real....people do take things from the fridge to the hot oven......not smart >......but people are in such a rush to watch JEOPARDY!!!
Ric, learn to read. The previous person said from the FREEZER. If
you don't cook, you might think folks put things frozen solid into a
500*F oven to bake, but that is simply a lack of knowledge about
baking.
I live in hot dish land, where stoneware casseroles are made for
all sorts of things, including dishes that will hold large fishes like
Walleye.
> Some folks do NOT understand that ceramics cannot go from freezing to 400 degree F.
>oven preheated temp.
It is OUR job to educate our patrons. It is our job to make
our casseroles so they can do their intended job. Your clay, your
methods and careful attention to firing are all essential.
There is a high level of ceramic education in these parts thanks to
people like Warren MacKenzie. Folks know how to treat ovenware.
Have Courage!
--
Lee Love in Minneapolis
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
James F on mon 28 jul 08
John=2C et al...
Thanks for your replies both on and off list. As I stated=2C I am certainl=
y not questioning anyone's statement that they have witnessed a pot fail in=
an oven. I am=2C however seeking to understand how this could happen. I =
had considered all of the factors mentioned before posing my question=2C bu=
t none of them (with the possible exception of cristobalite=2C on which I s=
hall ponder further) seems able=2C by my quite possibly erroneous reasoning=
=2C to cause the reported failures. I am trying to understand how the fail=
ures could possibly be attributed to the composition of the ceramic materia=
ls.
Let's take the most extreme example of the frozen casserole going into the =
oven. You have 400 degree air trying to heat the pot=2C and a 32 degree fr=
ozen mass trying to keep it cold. Air is a very poor thermal conductor (hi=
gh R value=2C low U value)=2C so you have 400 degree air doing a poor job o=
f trying to make the pot hot. The frozen food is a very good thermal condu=
ctor=2C so it does a very good job of trying to keep the pot cool. Now=2C =
it seems to me that the ceramic=2C also a very good thermal conductor=2C ca=
n never get much hotter than the food that is touching it. Wouldn't the co=
ol mass of highly conductive food prevent the pot from heating at the same =
rate it might if empty? Wouldn't the very high thermal conductivity of the=
ceramic prevent the outside of the wall from reaching a temperature much d=
ifferent from the inside of that same wall? Both the pot and the food have=
a tremendous (cold) thermal mass that has to be overcome by the relatively=
poorly conducting and low thermal mass air. This explains why you can put=
a frozen roast in a hot oven for a long time=2C and it will still be froze=
n in the center. Given all of this=2C I don't understand how the air can w=
in. Think of a double boiler used to melt wax or chocolate. The relativel=
y cool 212 degree water with it's high thermal mass prevents the wax or cho=
colate from exceeding this temperature despite the 3500 degree flame beneat=
h. I just can't see how the ceramic pot could heat much more quickly than =
the portion of food that is immediately adjacent to it from a physics stand=
point. Even if it could=2C I don't see how it could have enough thermal di=
fferential at only 400 degrees minus the temperature of the food at the foo=
d/pot juncture to cause any damage. It seems like something else must be a=
t work.
Now=2C if there were an undetected hairline crack in the pot=2C I could per=
haps see moisture seeping in=2C turning to steam at a relatively early poin=
t in the baking=2C and=2C due to it's powerful expansion in the crack=2C te=
aring the pot asunder. Perhaps water from the food at the very edge where =
the frozen mass touches the pot could turn to steam and exert enough pressu=
re between the wall of the pot and the frozen mass of food to cause a fract=
ure. This doesn't seem too likely=2C but does seem possible. Even if this=
were the case though=2C the actual cause of the resulting fracture would t=
hen be the structural weakness of the pot (too thinly potted) rather than t=
he composition of the ceramic material per se.
Someone mentioned the example of putting a cold pot into a hot raku kiln an=
d watching the bottom pop off. I offer that this is not the same thing=2C =
and in fact reinforces my thoughts. The hot kiln shelf is a very good ther=
mal conductor and has a very high thermal mass. The empty pot has a much l=
ower thermal mass which is quickly overcome via conduction from the hot she=
lf. If the hot air in the kiln conducted heat as well as the shelf does=2C=
the pot wouldn't break. Someone else mentioned preheating a ceramic teapo=
t. I suggest that this=2C too=2C reinforces my thoughts. One preheats a c=
eramic teapot by pouring in boiling water. This is not done to prevent sho=
ck to the clay=2C as the preheating water is the same temperature as that f=
rom which you will brew the tea. Rather=2C the purpose is to overcome the =
(cold) thermal mass of the clay so it does not suck the heat out of your te=
a.
Now I know that anecdotal evidence does not carry any weight (except perhap=
s in the Nytal trial). Consider=2C however=2C that I cannot possibly be th=
e only person who impatiently pulls pots from a 350 degree kiln and sets th=
em on the concrete studio floor to cool. Who among us hasn't crash-cooled =
a kiln to make a deadline? Who hasn't plunged a hot raku pot into cold wat=
er? I'm absolutely not questioning that pots have broken in the oven. I a=
m only questioning whether it is a function of the composition of the ceram=
ic materials=2C or whether these seemingly isolated incidents may have anot=
her physical cause unrelated to the clay and glaze composition. Perhaps I =
am just dense=2C but the ceramic material per se seems to be amongst the le=
ast likely causes. What am I missing here?
With all due respect...
James
> Date: Sun=2C 27 Jul 2008 15:40:06 -0400
> From: jjhesselberth@GMAIL.COM
> Subject: Re: oven safe?
> To: CLAYART@LSV.CERAMICS.ORG
>=20
> Hi James=2C
>=20
> Good question. Think about it this way. Suppose that dish contains a
> casserole which has been stored in someone's freezer. When it is put
> in the oven and begins to heat up the edges of the dish are up to
> oven temperature while the center is still 32F. That puts a whole lot
> more stress on the clay/glaze combination that the relatively uniform
> heating/cooling an empty pot gets in a kiln. Same thing happens in
> reverse when you take it out of the oven.
>=20
> It gets even worse if there is any cristobalite in the clay body
> (there usually is not at cone 6 and below=2C but there can be at cone
> 10 if the body was not formulated well). Somewhere between 428F and
> 536F cristobalite undergoes a 3% change in volume. That means it can
> happen in a hot oven. If one part of the pot goes through that
> before another part--well=2C pots are not very elastic.
>=20
> Regards=2C
>=20
> John
> On Jul 27=2C 2008=2C at 9:18 AM=2C James F wrote:
>=20
> > If a pot=2C with whatever it's peculiar combination of clay and
> > glaze=2C just survived baking and cooling in a 2400 degree oven=2C why
> > is it not a safe assumption that it would by definition be able to
> > survive baking and cooling in a 400 degree oven? If one of the
> > chairs I build holds a 200 pound person=2C I assume without testing
> > that it will also hold an 80 pound person. Does the logic not hold?
>=20
> John Hesselberth
> http://www.frogpondpottery.com
> http://www.masteringglazes.com
_________________________________________________________________
Stay in touch when you're away with Windows Live Messenger.
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enger2_072008=
Lee Love on mon 28 jul 08
On Mon, Jul 28, 2008 at 1:26 PM, Bill Merrill wrote:
> It sounds more like your bottle broke because the glaze did not fit the
> bottle. If the bottle had a small neck and you didn't glaze the inside
> , that could be another problem causing the bottle to break.
Bill, from my cooking/baking experiences, I think most pots break in
the oven because they were damage outside of the oven undetected.
Then they fail when they are used in the oven.
--
Lee Love in Minneapolis
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
James F on tue 29 jul 08
Having just completed a plumbing project=2C I am reminded of another exampl=
e which would seem to bolster the notion that the outside of a pot-full-of-=
food in an oven can never get much hotter than the food inside regardless o=
f oven temperature.
Have you ever tried to solder a fitting onto a pipe that still contained wa=
ter? It cannot be done. To solder a joint=2C the metal to be joined is he=
ated to a point above the melting temperature of the solder being used=2C t=
hen the solder is melted by the hot metal and wicked into the joint. Most =
conventional solders melt in a range from approximately 400-450 degrees F. =
If there is water in the pipe to be soldered=2C the metal never gets hot e=
nough to melt the solder despite having a 3500 degree torch flame trained d=
irectly on it. The water in the pipe cannot get any hotter than 212 degree=
s (at atmospheric pressure)=2C and it's thermal mass combined with the very=
high thermal conductivity serves to prevent the metal pipe surface from ev=
er getting hot enough to melt the solder in spite of the massive amount of =
heat being directly applied.
Another example from kitchen physics: If you place an empty aluminum pan o=
n a lit stove burner you can easily melt the pan. This is because the stov=
e flame is in excess of 3500 degrees=2C while aluminum melts at about 1200 =
degrees. With food in the pan=2C however=2C the pan does not melt despite =
the 3500 degree flame. This again is because the thermal mass of the food =
combined with the very high thermal conductivity of the pan prevents the su=
rface of the pan from ever going much beyond the temperature of the food=
=2C again despite a 3500 degree flame playing directly against it's surface=
.
I still do not see any way an oven=2C even in combination with a freezer=2C=
can ever set up enough stress to break a sound=2C reasonably well designed=
=2C reasonable well potted clay vessel full of food.
With all due respect...
James
_________________________________________________________________
Use video conversation to talk face-to-face with Windows Live Messenger.
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John Hesselberth on tue 29 jul 08
On Jul 28, 2008, at 2:17 PM, Lee Love wrote:
> Ric, learn to read. The previous person said from the FREEZER. If
> you don't cook, you might think folks put things frozen solid into a
> 500*F oven to bake, but that is simply a lack of knowledge about
> baking.
Hi Lee,
And I think you need to reacquaint yourself with what people do in
this country. Lots or prepackaged oven dishes sold in grocery stores
have instructions saying to put them directly from the freezer into
the oven. Regardless of what you tell people when they purchase your
pot, some will do that with a casserole they have made at home and
stored frozen for a while. The fact that you have never heard of it
doesn't mean it doesn't happen regularly.
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
John Hesselberth on tue 29 jul 08
Hi James,
It is thermal stress combined with a pot that is already under a high
level of stress from clay glaze mismatch or a high level of
cristobalite. Ever heard of the straw that broke the camels back?
That stress that builds with a clay/glaze mismatch does not go away
unless something breaks to relieve it. I just holds the pot at the
edge of disaster. Poorly matched pots can even break sitting on the
kitchen counter months after they were made. To think this can't
happen when they get a little added stress in an oven is not
reasonable. Well made/matched pots can endure the thermal shock of
being put in an oven (and they don't break sitting on the kitchen
counter either). Poorly made/matched pots often can't.
Regards.
John
>
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
KATHI LESUEUR on tue 29 jul 08
On Jul 29, 2008, at 12:16 PM, James F wrote:
>> 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
The examples people have brought to me of casseroles failing did not
involve frozen items. Rather they were making simple casseroles from
scratch. The old "take noodles, add Campbell's soup of choice,
chicken, cheese, etc." The food was slightly warm and put in a cold
oven. I know all of this because I asked. I'm sorry that you just
can't imagine common stoneware failing in the oven, but it does. As
far as I can tell, not infrequently. I suggest you take your clay
and make a few baking dishes. See if they survive use. Try cold food
in a cold oven. Warm food in a cold oven. Warm food in a hot oven.
Then, you may have an idea if YOUR clay and glazes can take the oven.
Kathi
>
> ________________________________
>
Lee Love on tue 29 jul 08
On Tue, Jul 29, 2008 at 5:51 AM, John Hesselberth
wrote:
> stored frozen for a while. The fact that you have never heard of it
> doesn't mean it doesn't happen regularly.
Hi John,
Visa versa is true. In the book, The Science of Fear, the
author explains how we are biologically over influenced by anecdotal
reports over empirical evidence. We are still biologically apes,
and when we hear about a child abduction on the T.V., we inordinately
fear that our children will be abducted. That is because, for most of
our history, what we see is what we knew. Now, we see on the T.V.,
or from email list reports, about the occurrence of certain fearful
situations and regardless of the empirical evidence, we become
fearful, simply because we are told a story. A little part of the
brain, the amygdala, overpowers our reason. But because we are
unconscious of this effect, we don't know the gravity of its
influence.
Check the book out. I am hoping I can find a copy at the airport
on my way to Japan:
http://www.amazon.com/Science-Fear-Shouldnt-Ourselves-Greater/dp/0525950621
There will always be idiots, but we can't cater to them. You
have to focus on the greater population.
When I left America, it was known as "The Home Of The Brave." I
came back to help it get back on course.
--
--
Lee Love in Minneapolis
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
Steve Slatin on tue 29 jul 08
Hank -- the roast also has
low R and high U -- but it
doesn't heat up equally and
simultaneously throughout.
Neither does the pot. That's
where I presume the thermal
stress comes from.
Best -- Steve Slatin
--- On Tue, 7/29/08, Hank Taylor wrote:
> James ,
>
> your theory might work if the heat transfer was
> limited to convection air only. The inside of the hot oven
> would have hot walls and shelves that could conduct higher
> heat to the pot, this localized heat could easily crack a
> casserole. The thermal conduction of the ceramic is less
> than you would think at temps higher than boiling water.
> Also you have heat transfer by radiation from the elements
> or burners in a oven that would raise the temps to well
> over 400 degrees F.
> The reason that the chocolate or wax doesn't heat
> higher than 212 is not because the water has a high thermal
> mass, but that the water uses up the heat to change phases
> from liquid to a gas (steam). If some one put a ceramic pot
> on the stove eye with water in it , it would burst in a
> heart beat, even pyrex cannot handle that temprature
> difference (don't ask how I know this, hard lessons are
> learned well).
>
> a.. Let's take the most extreme example of the frozen
> casserole going into the oven. You have 400 degree air
> trying to heat the pot and a 32 degree frozen mass trying
> to keep it cold. Air is a very poor thermal conductor
> (high R value, low U value) so you have 400 degree air
> doing a poor job of trying to make the pot hot. The frozen
> food is a very good thermal conductor so it does a very good
> job of trying to keep the pot cool. Now it seems to me that
> the ceramic also a very good thermal conductor can never get
> much hotter than the food that is touching it. Wouldn't
> the cool mass of highly conductive food prevent the pot from
> heating at the same rate it might if empty? Wouldn't
> the very high thermal conductivity of the ceramic prevent
> the outside of the wall from reaching a temperature much
> different from the inside of that same wall? Both the pot
> and the food have a tremendous (cold) thermal mass that has
> to be overcome by the relatively poorly conducting and low
> thermal mass air. This explains why you can put a frozen
> roast in a hot oven for a long time and it will still be
> frozen in the center. Given all of this, I don't
> understand how the air can win. Think of a double boiler
> used to melt wax or chocolate. The relatively cool 212
> degree water with it's high thermal mass prevents the
> wax or chocolate from exceeding this temperature despite
> the 3500 degree flame beneath. I just can't see how
> the ceramic pot could heat much more quickly than the
> portion of food that is immediately adjacent to it from a
> physics stand=
> point. E
Hank Taylor on tue 29 jul 08
James ,
your theory might work if the heat transfer was limited to =
convection air only. The inside of the hot oven would have hot walls =
and shelves that could conduct higher heat to the pot, this localized =
heat could easily crack a casserole. The thermal conduction of the =
ceramic is less than you would think at temps higher than boiling water. =
Also you have heat transfer by radiation from the elements or burners in =
a oven that would raise the temps to well over 400 degrees F.
The reason that the chocolate or wax doesn't heat higher than 212 is =
not because the water has a high thermal mass, but that the water uses =
up the heat to change phases from liquid to a gas (steam). If some one =
put a ceramic pot on the stove eye with water in it , it would burst in =
a heart beat, even pyrex cannot handle that temprature difference (don't =
ask how I know this, hard lessons are learned well).
a.. Let's take the most extreme example of the frozen casserole going =
into the oven. You have 400 degree air trying to heat the pot and a 32 =
degree frozen mass trying to keep it cold. Air is a very poor thermal =
conductor (high R value, low U value) so you have 400 degree air doing a =
poor job of trying to make the pot hot. The frozen food is a very good =
thermal conductor so it does a very good job of trying to keep the pot =
cool. Now it seems to me that the ceramic also a very good thermal =
conductor can never get much hotter than the food that is touching it. =
Wouldn't the cool mass of highly conductive food prevent the pot from =
heating at the same rate it might if empty? Wouldn't the very high =
thermal conductivity of the ceramic prevent the outside of the wall from =
reaching a temperature much different from the inside of that same wall? =
Both the pot and the food have a tremendous (cold) thermal mass that =
has to be overcome by the relatively poorly conducting and low thermal =
mass air. This explains why you can put a frozen roast in a hot oven =
for a long time and it will still be frozen in the center. Given all of =
this, I don't understand how the air can win. Think of a double boiler =
used to melt wax or chocolate. The relatively cool 212 degree water =
with it's high thermal mass prevents the wax or chocolate from exceeding =
this temperature despite the 3500 degree flame beneath. I just can't =
see how the ceramic pot could heat much more quickly than the portion of =
food that is immediately adjacent to it from a physics stand=3D
point. Even if it could, I don't see how it could have enough thermal =
differential at only 400 degrees minus the temperature of the food at =
the food/pot juncture to cause any damage. It seems like something else =
must be at work.
Hank Taylor
Woodshed Pottery
hanktaylor@dtcweb.net
Steve Slatin on tue 29 jul 08
Good point, Kathi -- and those were the
willing purchasers of hand-made ware who
weren't so turned off by the bad experience
they just decided to give up on buying
craft goods.
Every time someone unloads an unreliable
product on an unsuspecting client, they're
taking a good shot at ruining that person
as a client for many other products, and
many other craftspeople, in the future.
Last fall I sold a half-dozen tumblers
to a guy who sent them as a Christmas
present to a lady friend in Alaska. Two
broke in transit. I gave him two more,
just to make sure he felt OK about buying
pottery. So far he's still buying.
Best - Steve Slatin
--- On Mon, 7/28/08, KATHI LESUEUR wrote:
> Why do I think that there are casseroles and baking dishes
> out there
> that can't take use in the oven? Because people come up
> to me at
> shows with their cracked piece made by someone else and ask
> if it is
> common for pieces to crack in the oven. The conversation
> goes
> something like this, "I bought this baking dish and
> the first time I
> used it in the oven it cracked. I showed it to the potter
> who said,
> 'yeah, that happens sometimes.'" No offer to
> replace it. No offer to
> refund the cost. Just "oh yeah, that happens
> sometimes."
>
> So, there they are standing in my booth wanting a baking
> dish from me
> but really worried about being taken twice. As I said, such
> an
> attitude affects all of us. There are pots that won't
> hold up in the
> oven and it is the responsibility of the potter to make
> sure those
> pots are not theirs.
>
> Kathi
> >>
Edouard Bastarache on tue 29 jul 08
Hello all,
35 years ago the lady next door bought a large bean crock
from me and recently told me she has used it more then 500
times and it is still going well.
I used a Plainsman's dark reduction clay (Tony Hansen),
probably the equivalent of the current H-440 that I use these days.
Another lady, a medical secretary, did the same and she is still using
it.
Go figger !!!
Gis la revido
Edouard Bastarache
Spertesperantisto
Sorel-Tracy
Quebec
Canada
http://www.flickr.com/photos/30058682@N00/
http://pagesperso-orange.fr/smart2000/livres.htm
http://www.flickr.com/photos/potier/20321056/
http://myblogsmesblogs.blogspot.com/
Nancy Guido on tue 29 jul 08
I can't be the only one to prepare two lasagnas (or baked ziti) and freeze one and cook it later? Cook books tell you all the time to prepare up to step (whatever) and freeze for later - add a little extra time for cooking. I never used a ceramic dish, I have a couple of pyrex ones. I've been cooking for over 30 years. I haven't always had a microwave or the brain to put it in the microwave to partially cook it first before I put it in the oven. Usually, I threw it in the oven right out of the freezer. To thaw it first would be bad I think.
If you say oven safe - that's what folks will remember. Best to make sure.
nancy g.
John Hesselberth on tue 29 jul 08
On Jul 29, 2008, at 10:52 AM, Kim Hohlmayer wrote:
> 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.
Kim, you did fine on my name, but you missed the person. I didn't
even come close to what you say I implied. It was someone else.
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Bill Merrill on tue 29 jul 08
Lee,
I thought the "bottle" broke into pieces like potato chips, not a baking
dish or casserole. That sounds like a compression issue. Putting the
piece ( a bottle) in an oven doesn't make sense. Pots break in an oven,
or when taken out of an oven. I find pots don't break into potato chip
size pieces and are not easily broken into pieces by your fingers when
coming out of an oven.
Bill
---Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Lee Love
Sent: Monday, July 28, 2008 5:43 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: oven safe?
On Mon, Jul 28, 2008 at 1:26 PM, Bill Merrill
wrote:
> It sounds more like your bottle broke because the glaze did not fit
the
> bottle. If the bottle had a small neck and you didn't glaze the
inside
> , that could be another problem causing the bottle to break.
Bill, from my cooking/baking experiences, I think most pots break in
the oven because they were damage outside of the oven undetected.
Then they fail when they are used in the oven.
--
Lee Love in Minneapolis
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
Karin Givon on tue 29 jul 08
OH, so this is interesting , with James talking about (plumbing and)
the melting point of aluminum...I was getting interested in aluminum
foil saggars and the cool effects you can get...but they melt at 1200
degrees? Farenheit? SO does anyone know how this newspaper
(andwhatever) and foil-wrapped saggar works? Not with other pots, I'm
gathering. I think I read about this method on Ceramic Daily news
listserv.
Thanks ,
Penelope Peeorplethorpe
James F on tue 29 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.
>
> 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).
>
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
________________________________
________________________________
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Kim Hohlmayer on wed 30 jul 08
Sorry, John,
Well, at least I got your name right. I keep trying to be perfect but so far I am sadly behind in said effort. Again, apologies. --Kim H.
--- On Tue, 7/29/08, John Hesselberth wrote:
> From: John Hesselberth
> Subject: Re: oven safe?
> To: CLAYART@LSV.CERAMICS.ORG
> Date: Tuesday, July 29, 2008, 2:50 PM
> On Jul 29, 2008, at 10:52 AM, Kim Hohlmayer wrote:
>
> > 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.
>
> Kim, you did fine on my name, but you missed the person. I
> didn't
> even come close to what you say I implied. It was someone
> else.
>
> John
>
> John Hesselberth
> http://www.frogpondpottery.com
> http://www.masteringglazes.com
Arnold Howard on wed 30 jul 08
From: "Lee Love"
The Science of Fear, the
> author explains how we are biologically over influenced by
> anecdotal
> reports over empirical evidence.
That theory makes sense. For the last two weeks, I have been
oblivious to current events by "unplugging" from the news.
It feels nice to tune out.
Sincerely,
Arnold Howard
Paragon Industries, L.P., Mesquite, Texas USA
ahoward@paragonweb.com / www.paragonweb.com
Lee Love on wed 30 jul 08
On Tue, Jul 29, 2008 at 8:18 PM, KATHI LESUEUR
wrote:
> The examples people have brought to me of casseroles failing did not
> involve frozen items. Rather they were making simple casseroles from
> scratch.
Kathi,
I believe Jim is right, that frozen food in the casserole would
help protect it from heat shock.
--
Lee Love in Minneapolis
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
James F on wed 30 jul 08
I apologize in advance for the length of this post=2C but there are many is=
sues to address. I know folks are starting to get bored with this thread=
=2C so I fully intend this to be my last post on the matter=2C then I will =
go away. Thanks for bearing with me.
To this point=2C we have been discussing hypotheticals. I maintained that f=
rom my understanding of physics=2C an oven could not possibly stress a reas=
onably suitable pot enough to break it. Most of the respondents=2C both on =
and off list=2C have maintained that from their understanding of physics=2C=
an oven could easily do so. Rather than simply restating the same theories=
back and forth ad nauseum=2C I decided to conduct actual experiments. Here=
are my actual findings. I don't want to pick on anyone=2C so have omitted =
names=2C and have used posts that were representative of common objections =
to my hypothesis.
First=2C a poster offered:
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=2C
but if you put boiling water into
a mug and wait a minute or two=2C
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.
This morning as I prepared my tea=2C I noticed that indeed one could easily=
detect a temperature difference between the part of the mug below the wate=
r line and that above=2C but the difference seemed very small. I therefore =
decided to run a test. I filled a mug (a well-potted=2C but butt-ugly slip-=
glazed=2C soda fired atrocity=2C if it matters) with cold water and placed =
it in my microwave. After 2 minutes=2C the water was simmering=2C and the t=
emperature differentials were as follows: water to mug=2C 52 degrees=3B mug=
to rim=2C 9 degrees (progressively cooler=2C as one would assume). I then =
emptied the mug=2C cooled it back down under cold running water=2C refilled=
it with fresh cold water=2C and nuked it for 3 minutes. The water was at a=
full boil=2C and the temperature differentials were as follows: water to m=
ug=2C 51 degrees=3B mug to rim=2C 6 degrees. I repeated the entire procedur=
e once more=2C but nuked the mug for 4 minutes. Again=2C the water was at a=
full boil=2C and the temperature differentials were as follows: water to m=
ug=2C 51 degrees=3B mug to rim=2C 4 degrees. Very small differentials to be=
sure=2C and the longer the mug was heated=2C the lower the differentials b=
ecame. I did not repeat this particular experiment in a conventional oven d=
ue to the rather extreme amount of time it would take to repeatedly heat an=
d cool an entire oven=2C but since an oven heats much more slowly than a mi=
crowave=2C I believe it is reasonable to conclude that the differentials wo=
uld be even smaller.
Next=2C another poster offered:
your theory might work if the heat transfer was limited to convection air o=
nly. The inside of the hot oven would have hot walls and shelves that could=
conduct higher heat to the pot=2C this localized heat could easily crack a=
casserole. The thermal conduction of the ceramic is less than you would th=
ink at temps higher than boiling water. Also you have heat transfer by radi=
ation from the elements or burners in a oven that would raise the temps to =
well over 400 degrees F.
The reason that the chocolate or wax doesn't heat higher than 212 is not be=
cause the water has a high thermal mass=2C but that the water uses up the h=
eat to change phases from liquid to a gas (steam). If some one put a cerami=
c pot on the stove eye with water in it =2C it would burst in a heart beat=
=2C even pyrex cannot handle that temprature difference (don't ask how I kn=
ow this=2C hard lessons are learned well).
First=2C some observations=2C then the experimental data: On your idea tha=
t IR radiation from the walls or elements of the oven could cause breakage =
even if convection cannot=2C my contention is that no matter how the heat g=
ets from the oven elements to the pot=2C it cannot be rapid enough to cause=
much stress. On your boiling water point=2C whether the factor at play is=
thermal mass=2C phase change=2C or a combination of the two=2C if the boil=
ing water moderates the temperature of the chocolate pan that it touches=2C=
then the food in the pot-in-the-oven=2C especially considering the water c=
ontent of the food=2C would have to do the same thing to the pot. Regardle=
ss of which mechanism is at play=2C you have in fact bolstered my argument=
=2C not countered it. Lastly=2C to the flame. First=2C direct impingement=
of a flame which is in excess of 3500 degrees onto very small and static p=
oints on a pot could=2C of course=2C break the pot. This has never been at=
issue. We are here discussing only the relatively even heating an oven pr=
ovides (relative to a direct static flame trained on a tiny percentage of t=
he pot's surface). Even in your example=2C move your pot continuously over=
this flame=2C and the (reasonably well-made) pot will not break. To test =
this hypothesis=2C and to attempt to put to rest ALL of the oven scenarios=
=2C I conducted the following experiment=2C designed to RADICALLY exceed an=
ything that an oven could do to a pot:
As a test victim=2C I selected the most crazed and stressed piece in my col=
lection=2C a wheel-thrown tea bowl made of porcelainous stoneware=2C dipped=
in Malcolm's Shino then rolled in raw wood ash=2C placed on it's side atop=
a catcher=2C piled high with more wood ash=2C then fired to a hard cone 10=
in a gas reduction kiln. This bowl is warped from the side-firing=2C is c=
oated with a thick=2C uneven=2C heavily crazed layer of melted wood ash com=
mingled with shino (very high sodium=2C very high expansion)=2C and has eve=
n thicker spots of melted ash where it sat on the seashells. I filled this=
bowl 2/3 full of cold water from my well (55 degrees)=2C placed it atop a =
banding wheel=2C and rotated it slowly by hand as I heated it with a propan=
e torch held about an inch from the bowl=2C the 3500 degree flame fully pla=
ying against it's surface (I wore safety glasses in case I was wrong. I'm =
stubborn=2C but not stupid). In a bit over two minutes the water was at a =
full rolling boil. Even after this exceptionally rapid heating in a direct=
3500 degree torch flame=2C FAR more rapid=2C violent=2C and uneven than an=
y kitchen oven will EVER be capable of=2C the temperature differential betw=
een the water and the bowl was only 61 degrees=2C and there was no measurab=
le differential between the bowl and the rim. There was absolutely no dama=
ge to the bowl from heating=2C nor from the rapid cooling it underwent afte=
r I dumped the water.
Before anyone brings it up=2C yes=2C I am pretty sure the thermometer was a=
ccurate=2C and since we are talking about temperature differentials and not=
about absolute temperatures=2C it needn't be accurate anyway=2C merely con=
sistent.
Finally=2C my conclusion=2C after which I shall shut up and leave you all a=
lone:
The previous thought experiments suggested that a pot full of food=2C place=
d in a kitchen oven=2C could never get much hotter than it's contents. The=
above experiments support and confirm this notion.
The thought experiments suggested that the rim of said pot-of-food-in-the-k=
itchen-oven could never get much hotter than the rest of the pot. Again=2C=
the experiments support and confirm this notion.
The thought experiments suggested that there would never be a significant t=
emperature differential between either the pot-of-food-in-the-kitchen-oven =
and it's contents=2C nor between various parts of the pot itself. Again=2C=
the experiments support and confirm this notion.
If you read my original posts=2C you will see that I NEVER denied that pot=
s have broken in the oven. I merely suggested that the oven was likely not=
the cause=2C as it seemed impossible from a physics standpoint for it to c=
ause either rapid heating or wide temperature differentials. Since this ha=
s now been demonstrated experimentally=2C and the physical principles to su=
pport it have been identified (to the best of my limited abilities)=2C I do=
not see how anyone could reasonably deny it.
So what causes pots to fail=2C if indeed they do? Only one person on the l=
ist offered a firsthand experience with a pot breaking in the oven=2C and t=
hat was an earthenware vessel=2C not particularly known for strength. Othe=
r than that=2C we have only hearsay. A potter on the list suggests that it=
is entirely possible that the broken pot possessed any number of undetecte=
d flaws prior to it's fatal use. These may have been present in the pot fr=
om the beginning=2C or may have occurred much later at the hand of the user=
. If they were present from the beginning=2C then a baking test might have=
uncovered them. To be certain=2C the potter would have to test every piece=
=2C or at least a statistically significant sample. Testing only one examp=
le of a given piece may uncover a defect in materials common to all of your=
pieces=2C but would not detect a flaw in forming any vessel other than the=
one you specifically tested. If the defect occurred at the hands of the u=
ser and went undetected until the fatal use=2C perhaps a hairline crack=2C =
it is easy to understand how that user might conclude that the last use cau=
sed the break and that the pot was fine before that use. Another on the li=
st suggested that if food were frozen in a pot=2C then expansion=2C even mi=
nor=2C could easily cause a hairline fracture which could subsequently be f=
orced open by steam or other factors in the oven. This=2C to me=2C seems e=
ntirely reasonable=2C but then the freezer was actually the cause of the fa=
ilure=2C not the oven=2C and I personally have never heard any potter claim=
that their pots are freezer-safe. Lastly=2C a list member suggested that =
it would be possible for a clay and glaze=2C or the combination thereof=2C =
to be so unsuitable for their purpose as to set up extreme stresses within =
the vessel to the point where the vessel is right on the edge of self destr=
uction. The poster contends that such a theoretical vessel could easily be=
pushed over the edge by an oven=2C or even from nothing at all just sittin=
g on a shelf. I have no reason to doubt this=2C but would offer that=2C ag=
ain=2C in such a case one can hardly blame the oven=2C when in fact such a =
vessel would not be ANYTHING-safe. The oven=2C a microwave=2C a tiny tap a=
gainst the sink=2C pouring a cup of coffee=2C would all likely set off such=
a vessel=2C and of the causes listed=2C the oven is probably the gentlest=
=2C and is certainly far from the most violent. Again=2C if a mere 51 degr=
ee temperature differential can push a pot over the edge=2C then that pot h=
ad way more issues than oven-safety. And if a 3500 degree blowtorch cannot=
rapidly heat a full pot=2C then a 400 degree oven certainly can't. So=2C =
by all means test your pots to make sure they are not junk. Use an oven=2C=
a microwave=2C a pot of boiling water=2C a tap from a hammer=2C whatever=
=2C to make sure you have a reasonably well made=2C functional piece. No o=
ne has ever suggested otherwise. There is still no evidence=2C however=2C =
to counter the theory that any reasonably well-made pot is oven-safe.
And to those who suggested both on and off list that I am thinking too much=
=2C I thought that was the entire point of Clayart.
My best to all.
With respect=2C
...James (who is now ducking and covering his head)
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Lee Love on thu 31 jul 08
On Wed, Jul 30, 2008 at 4:13 PM, Arnold Howard wrote:
> That theory makes sense. For the last two weeks, I have been
> oblivious to current events by "unplugging" from the news.
> It feels nice to tune out.
As primates, the part of our brain responsible for fright/flight
believes everything we see is in our immediate environment. It is why
the T.V. is so powerful.
We often think the modern world has "become" an unsafe place, but
that is because the news chases the stories that push these buttons
and we hear about things, we would have had no information about until
recently. Or by the time it reached us, it would have been pretty old
news.
I think the important thing is not to be manipulated by these things.
--
Lee Love in Minneapolis
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"Let the beauty we love be what we do.
There are hundreds of ways to kneel and kiss the ground." --Rumi
Ron Roy on fri 1 aug 08
Hi Kathi,
You would think it would be standard practice - to use your own work - what
better way to make sure it does what it's supposed to do.
It does not mater if the glazes are perfect - it's the combination of clay
and glazes that is the true test in the end.
RR
>Last week was the Ann Arbor art fair and lots of potter friends were
>around for the show. One of them until recently has made only
>decorative pieces. During a conversation he said something about "the
>baking dish that woman bought". Which caused me to ask him if he'd
>tested his new line to see that it WAS oven, microwave, and
>dishwasher safe. Had actually baked anything in his baking dishes?
>Had he put them into his dishwasher? Had he used them in a microwave?
>The answer to all of these questions was "no". "But" he said, " I'm
>using those glazes from Mastering Cone 6 Glazes so I know they are
>alright."
>
>Another example of someone who just assumes that if he uses glazes
>that someone else uses with success everything will work. Not
>understanding that the compatibility of clay and glaze is essential
>to the performance of the piece and that not all clays can be used
>successfully in an oven. I strongly urged him to do testing before
>he offered anymore "oven safe" work to the public.
>
>Unfortunately, I don't think this is an unusual story. People put
>items out for sale without doing the homework to make sure that they
>will perform as represented. They just assume they can make a baking
>dish and it will be fine in the oven. If it isn't, if it cracks, it
>affects all of us offering bake-ware. People will question all of us
>and hesitate to buy another piece.
>
>I can't urge other potters strongly enough that if you are going to
>offer items to be used in the oven, microwave, and dishwasher you
>better know that they can actually be used successfully.
>
>Kathi
Ron Roy
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Ron Roy on fri 1 aug 08
Hi James,
Sorry to be so late with this - the way it is these days up here.
Heat and cooling shock leading to failure of ware is directly related to
expansion during heating and contraction during cooling.
If you want to minimize the risk of ware cracking in an oven then make sure
your clay has lower rather than high expansion.
What happens with high expansion clay is - one side gets heated faster and
expands - while the cooler side does not. It happens with clays with lower
expansions as well but not as much.
Flame ware is made to have very low expansion - one part of the pot is
heated but does not expand and so no cracking happens.
By the way - cristobalite raises the expansion of clay dramatically at
cooking temperatures. My dilatometry charts show it happening between 100C
and 200C (212F to 392) - exactly where you don't want it in a baking
situation.
Low expansion clay is hard to fit glazes to - glazes have to have a low
expansion to not craze for instance - and crazing weakens ware making it
more susceptible to heat shock.
The glaze must be right as well - it should not be too low in expansion
because it would then be adding stress in a situation when stress is a
constant problem.
You can see that happening sometimes - when you pour hot water into a tea
pot - if the glazes is already to big for the clay - the hot water will
heat it an instant before the clay and expand it a bit more - sometimes
enough to break the pot.
The form of the casserole is important - flat bottoms and straight sides
concentrate any stress at the bottom corners - sometimes the bottom falls
off. A rounded form is much better - the stress in not able to concentrate
but is distributed over the whole pot.
Let me know if you would like to see a dilatometer chart - you will see
exactly why cristobalite is the last thing you want in ovenware.
RR
> "the
>> baking dish that woman bought". Which caused me to ask him if he'd
>> tested his new line to see that it WAS oven, microwave, and
>> dishwasher safe. Had actually baked anything in his baking dishes?
>
>
>I accept on your collective word that not all pottery is oven-safe, and I
>don't make functional ware per se anyway, but it seems illogical. I have
>a dumb but honest question, perhaps for the scientists on the list: If a
>pot, with whatever it's peculiar combination of clay and glaze, just
>survived baking and cooling in a 2400 degree oven, why is it not a safe
>assumption that it would by definition be able to survive baking and
>cooling in a 400 degree oven? If one of the chairs I build holds a 200
>pound person, I assume without testing that it will also hold an 80 pound
>person. Does the logic not hold?
>
>Ceramic folk routinely re-glaze and re-fire pieces in various states of
>vitrification without incident. Fine china is often high-fired to
>vitrification without glaze, then glazed and fired to lower temperatures.
>I recall reading somewhere that Chinese potters did this also. Raku ware
>is pulled from an oven in excess of 1500 degrees, waved about in the cool
>air, dumped into trash bins, plunged into cold water, all without
>incident. How could a 400 degree oven hurt the stuff?
>
>Thanks in advance for any insight or elucidation you can provide.
>
>...James
>
>_________________________________________________________________
>Time for vacation? WIN what you need- enter now!
>http://www.gowindowslive.com/summergiveaway/?ocid=tag_jlyhm
Ron Roy
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Ivor and Olive Lewis on sat 2 aug 08
Following on from what Ron Roy has told you, it is necessary to
consider the whole story, to get as much information as you can about
the materials you deal with. Some of those minerals we use do not have
constant values for Thermal expansion over the heating and cooling
range, nor may their CoE be constant in all directions
If Free Quartz remains in a body or is not taken into solution in a
glaze then you are going to get problems. Quartz had two values for
its CoE. In one direction it will be 8 x 10minus 6 per Degree C, and
perpendicular to that 14 x 10minus 6 per Degree C. I suppose any
effect will depend upon particle size and orientation. But it would
seem to be prudent not to use large grained silica sand as a grog in a
clay body mix.
Always remember that Sodium monoxide has a designated CoE that is
greater than that of Potassium Monoxide. Since Na2O seems to expand
30% more than K2O over a temp range of room temperature to 600 deg C
choosing felspars that are very low in Na2O and avoiding Nepheline
Syenite as a body flux may be the wise way to go. Limiting or
eliminating free Silica in a clay body seems to be paramount. Using
the finest mesh possible in a Glaze may encourage the formation of a
silica rich solution which mimics Fused Silica in its thermal
behaviour.
I cured my crazing problems by incorporating a proportion of Petalite
at a rate of 20% of the total Felspar content in my base glaze. This
provided two proven ingredients that are known to alleviate
differential CoE problems, Lithium monoxide and Silicon dioxide.
Pages 93-95 in "Glazes for the Studio Potter" by Cooper and Royle,
ISBN 0-7134-3291-8 is useful reading.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Eleanor on wed 14 dec 11
Long ago, in the 50's, When I was learning clay, the instructor, a =3D
knowledgeable, respected clay artist, when asked whether fired (^4) pots =
=3D
were oven-safe, replied, "of course, they were fired at over 2000=3DB0, =3D
they certainly can survive home oven temperatures."
Recent Clayart discussion concluded, yes, but start in a cold oven. My =3D
50's teacher never said anything about a cold oven.
My friend, a Clayarter (hi, Marcey!) who makes beautiful pie dishes, =3D
tells her customers to start cold.
In the latest Clay Times (pre-holiday 2011) there is an article by Bill =3D
van Gilder on how to make a ceramic muffin pan followed by a recipe for =3D
muffins to be baked at 375=3DB0. No mention in the article or the recipe =
=3D
about a cool oven.
I use commercial glass or metal ware for my own baking and I don't make =3D
pots for oven use so I have never test, test, tested this.=3D20
I can understand that freezer-to-oven might suffer a shock, but has =3D
anyone ever had a cool room-temperature pie or casserole explode or =3D
crack when put into a hot oven?=3D20
I fire ^6 electric oxidation.
I am confused.
Eleanor Kohler=3D20
Centerport, NY
Merry Christmas to all to whom it applies and Happy Whatever to everyone =
=3D
else.
I don't much go for PC
"reality is a great teacher. fantasy is the enemy"
mel jacobson, potter, Clayart moderator
gsomdahl on wed 14 dec 11
Even starting with a cold oven is not always safe. Several years ago my
wife was roasting a large piece of meat in a stoneware platter bowl. She
had the meat covered with foil and there was about a half inch of liquid
that she used to baste the meat. She started the oven cold and rapidly
raised the temperature to I think 350F. The rim of the platter went to
the oven temperature but the liquid stay at its boiling point.This
caused the platter to crack in a ring completely around the
circumference at the top of the liquid. A real mess in the oven. We
won't do that again.
On 12/14/2011 07:29 AM, Eleanor wrote:
> Recent Clayart discussion concluded, yes, but start in a cold oven. My 50=
's teacher never said anything about a cold oven.
>
> My friend, a Clayarter (hi, Marcey!) who makes beautiful pie dishes, tell=
s her customers to start cold.
--
This is a post only account. Send replies to "gene" at my ".com" domain nam=
ed "somdahl".
Lis Allison on wed 14 dec 11
On December 14, 2011, Eleanor wrote:
> .....
> I can understand that freezer-to-oven might suffer a shock, but has
> anyone ever had a cool room-temperature pie or casserole explode or
> crack when put into a hot oven?
>
Crack, yes, explode, no.
I think that in Britain, people know how to cook/bake using pottery, but
in Canada they mostly do not. I almost don't make pie plates and
casseroles because I am so afraid someone will have an accident with one.
People will put a small frozen something in the middle of a pie plate and
pop it into a 400F oven. C-r-r-a-a-a-c-k. My card tells them to make sure
the whole surface of the dish is covered and to put it into the oven while
it is still relatively cool, but nobody reads the card. Plus they forget
once they've had the pie dish for a year or two.
Casseroles are less of a problem, luckily. The ones I've had to replace
have all been because they put it on a burner. Again, my card tells them
never to do this but they do.
So far, knock wood and cross all fingers, none have actually split apart.
I once, years ago, took a Pyrex dish out of the oven and had it shatter
before it cleared the oven door. Blazing hot lasagna all over everything
but luckily very little on me. What did hit my knees made a nasty burn but
very little did. So I know first hand how awful that can be.
This is a huge issue. I'd love to be able to offer pie plates and
casseroles in my usual colours. But, is ovenware feasible at cone 6?
Lis
--
Elisabeth Allison
Pine Ridge Studio
website: www.pine-ridge.ca
Pottery blog: www.studio-on-the-ridge.blogspot.com
Garden blog: www.garden-on-the-ridge.blogspot.com
Lee on wed 14 dec 11
On Wed, Dec 14, 2011 at 7:29 AM, Eleanor wrote:
>
> "reality is a great teacher. fantasy is the enemy"
>
Imagination is where creativity springs from. Literalists rarely
understand symbolism and metaphor.
Eleanor,
First, EVERYTHING depends on craftsmanship, design,
construction, firing and materials. With Donabe pots that are used on the
flame, you are always cautioned not to put flame to it empty.
It is the same with ovenware. If the work is filled, the food
in the pot accepts some of the heat and the shock is not as great.
--=3D20
--
Lee Love in Minneapolis
http://mingeisota.blogspot.com/
"Ta tIr na n-=3DF3g ar chul an tI=3D97tIr dlainn trina ch=3DE9ile"=3D97tha=
t is, "T=3D
he land
of eternal youth is behind the house, a beautiful land fluent within
itself." -- John O'Donohue
KATHI LESUEUR on wed 14 dec 11
On Dec 14, 2011, at 8:29 AM, Eleanor wrote:
> Long ago, in the 50's, When I was learning clay, the instructor, a =3D
knowledgeable, respected clay artist, when asked whether fired (^4) pots =
=3D
were oven-safe, replied, "of course, they were fired at over 2000=3DB0, =3D
they certainly can survive home oven temperatures."...
>=3D20
> Recent Clayart discussion concluded, yes, but start in a cold oven....=3D=
20=3D
> In the latest Clay Times (pre-holiday 2011) there is an article by =3D
Bill van Gilder..... No mention in the article or the recipe about a =3D
cool oven....
>=3D20
> I can understand that freezer-to-oven might suffer a shock, but has =3D
anyone ever had a cool room-temperature pie or casserole explode or =3D
crack when put into a hot oven?>>>=3D20
>=3D20
It always amuses me when someone says a piece can survive the oven since =
=3D
it survived the heat of the kiln. The problem is not the temperature but =
=3D
the heat shock. Cool to hot. That's why people advise to start in a cool =
=3D
oven. I advise this but in my own home never do it. On the other hand, =3D
I'm usually starting with something that is already warm, like a noodle =3D
casserole where the noodles have just been boiled. I've never had a =3D
piece returned because it failed in use. I did have a customer tell of =3D
her experience with the microwave. The lid and bottom formed a vacuum =3D
when they microwaved it and they couldn't get the lid off. The were =3D
going to a concert and had just a little time for dinner so they didn't =3D
want to wait for it to cool down. They put the casserole in ice water. =3D
It didn't break and the vacuum didn't release. They finally took a screw =
=3D
driver to the lid to pry it off and broke the lid. She wasn't =3D
complaining. Just though I should warn people to cock the lid a little =3D
so that steam could escape.
My advise is always test. For a number of years I poured boiling water =3D
into one piece from every firing. Nothing ever broke and I finally quit =3D
doing it. But, know your clay and what it can handle.
KATHI LESUEUR
http://www.lesueurclaywork.com
Steve Slatin on wed 14 dec 11
Eleanor --=3D0A=3DA0=3D0AFundamentally, if someone asks the question "is th=
is ite=3D
m oven-safe" the best answer could be=3D0A"oven-safe in what sense?"=3DA0 O=
f co=3D
urse this can be taken as hostile, so it might not be good in=3D0Aa retail =
co=3D
ntext ...=3D0A=3DA0=3D0AI have never made a pot that you could, for example=
, whac=3D
k with all your strength onto an oven=3D0Awithout probably damaging pot, ov=
en=3D
, or both.=3DA0 Of course few people would use that as a model=3D0Afor dete=
rmin=3D
ing oven-safety.=3DA0 (Or have any cause to do the whacking.)=3D0A=3DA0=3D0=
AI'd ima=3D
gine that your instructor was thinking in terms of the question "is an oven=
=3D
temp of perhaps=3D0A425 Degrees F a threat to the structural integrity of =
a =3D
pot" and the answer to this in most cases=3D0Ais going to be no, it's not a=
t=3D
hreat.=3D0A=3DA0=3D0ABut suppose your question is coming from a person who'=
se thi=3D
nking of using the pot as a vessel=3D0Afor frozen leftovers, and will uncer=
em=3D
oniously dump it into a=3DA0freezer for a week, and thence into a=3DA0=3D0A=
hot ov=3D
en with two quarts of frozen cioppino in it?=3DA0 Physical stresses from th=
e =3D
ice while freezing =3D0Acould be an issue, the=3DA0melting ice and so on ar=
e=3DA0=3D
yet another=3DA0threat, etc.=3DA0 A host of other uses, =3D0Aand potential =
hazard=3D
s, exist.=3D0A=3DA0=3D0AI have one pot that I have repeated used for oven-b=
aking,=3D
and not started in a cold oven.=3DA0 It =3D0Ahad no visible cracks in 1981=
whe=3D
n I made it.=3DA0 It was shipped overseas and back to the US=3D0A4 times, I=
bel=3D
ieve, survived several moves within the US, and after being used for a pot =
=3D
pie=3D0Ain the oven I discovered a visible crack on the bottom.=3DA0 I have=
no =3D
idea if it was that use or=3D0Aa previous use, or a previous abuse (it's be=
en=3D
dropped several times) that led to the crack.=3D0AAlthoug there is no visi=
bl=3D
e sign of the crack on the top, glazed surface, I no longer use it.=3DA0 Bu=
t=3D
=3D0Ais the crack from thermal shock?=3DA0 I can't possibly say.=3DA0 It co=
uld be=3D
from physical stress.=3DA0 It=3D0Acould be a minute crack eventually revea=
led =3D
through repeated cycles in the dishwasher and=3D0Asuch.=3DA0 It could be th=
at w=3D
hen my younger son was loading silverware in the dishwasher -- =3D0Awhich h=
e =3D
did from across the kitchen, cheerfully throwing the spinning cutlery into =
=3D
the silverware=3D0Abaskets at top speed -- a mis-guided =3DA0whirling knife=
may=3D
have thumped into the pot.=3DA0 Or,=3D0Athere may have been an invisible s=
urfa=3D
ce flaw that allowed water to permeate the bottom=3D0Aof the pot and cause =
st=3D
ress with heating=3DA0and cooling.=3D0A=3DA0=3D0AWhen I'm selling things an=
d people=3D
ask me the question, I tell them that the safe thing is=3D0Ato start with =
ro=3D
om-temp pot, contents, and oven, and then raise the entirety to the target=
=3D
=3D0Atemperature.=3DA0 I also point out that commercial ovenware typically =
advi=3D
ses avoiding thermal=3D0Ashock.=3D0A=3D0ABest wishes --=3D0ASteve Slatin=3D=
A0 =3D0A=3D0A=3D
=3D0AN48.0886450=3D0AW123.1420482=3D0A=3D0A=3D0A___________________________=
_____=3D0ALo=3D
ng ago, in the 50's, When I was learning clay, the instructor, a knowledgea=
=3D
ble, respected clay artist, when asked whether fired (^4) pots were oven-sa=
=3D
fe, replied, "of course, they were fired at over 2000=3DB0, they certainly =
ca=3D
n survive home oven temperatures."=3D0A=3D0ARecent Clayart discussion concl=
uded=3D
, yes, but start in a cold oven. My 50's teacher never said anything about =
=3D
a cold oven.=3D0A=3D0AMy friend, a Clayarter (hi, Marcey!) who makes beauti=
ful =3D
pie dishes, tells her customers to start cold.=3D0A=3D0AIn the latest Clay =
Time=3D
s (pre-holiday 2011) there is an article by Bill van Gilder on how to make =
=3D
a ceramic muffin pan followed by a recipe for muffins to be baked at 375=3D=
B0=3D
. No mention in the article or the recipe about a cool oven.=3D0A=3D0AI use=
com=3D
mercial glass or metal ware for my own baking and I don't make pots for ove=
=3D
n use so I have never test, test, tested this. =3D0A=3D0AI can understand t=
hat =3D
freezer-to-oven might suffer a shock, but has anyone ever had a cool room-t=
=3D
emperature pie or casserole explode or crack when put into a hot oven? =3D0=
A=3D
=3D0AI fire ^6 electric oxidation.=3D0A=3D0AI am confused.=3D0A=3D0AEleanor=
Kohler =3D
=3D0ACenterport, NY=3D0A=3D0AMerry Christmas to all to whom it applies and =
Happy =3D
Whatever to everyone else.=3D0AI don't much go for PC=3D0A=3D0A=3D0A=3D0A=
=3D0A=3D0A"reali=3D
ty is a great teacher.=3DA0 fantasy is the enemy"=3D0A=3D0Amel jacobson, po=
tter, =3D
Clayart moderator
Pottery by John on wed 14 dec 11
From: Steve Slatin "I also point out that commercial ovenware typically
advises avoiding thermal shock."
Steve, et al,
This past weekend I made a chicken pot pie. While looking for a vessel to
put into the oven, I considered several; glass, an imported French ceramic
casserole, and a Corningware fluted casserole, I even for a moment
considered using a "casserole" I made in the fashion of an example in a
Ceramic Arts Daily posting about throwing round and then squaring up on the
wheel. My final selection was based (probably quite erroneously) on the
notation fired into the bottom of the Corningware piece saying "Oven to
Table", after all Corning is commercial, and surely Oven to Table must
include going into a hot oven as well. Despite the permutations I could
have made from their fired in invitation, I did put the pot pie into a cold
oven and went to temperature and added a little hold at the end. Delicious
and safe. I do wonder how my little casserole would have handled it though;
alas the recipe was larger than my work would handle.
In curiosity I looked up the Corningware site and found their warnings for
their French White line (which is stoneware)
http://corningware.com/index.asp?pageId=3D80
Sounds very familiar to a Clayart reader, but for the next round we need to
add:
"Do not put under broiler/griller, under a microwave browning element, in a
toaster oven, or on or near any other direct heating source such as range
vents, open flames, etc". Sound advice.
John Lowes
Sandy Springs, Georgia
http://wynhillpottery.weebly.com/
Gayle Bair on wed 14 dec 11
Just in case anyone questions the viability of a potters ceramics as
opposed to commercial ware I printed Corning use and care info and plan to
keep it with me when I'm selling my work.
I've not had that happen but you never know!
I have experienced Pyrex shattering... it took a very long time to make
sure I got all those tiny shards of glass off the floor and counter
surfaces!
Gayle
Gayle Bair
Bainbridge Island WA
gayle@claybair.com
www.claybair.com
On Wed, Dec 14, 2011 at 11:54 AM, Pottery by John <
potterybyjohn@bellsouth.net> wrote:
> From: Steve Slatin "I also point out that commercial ovenware typically
> advises avoiding thermal shock."
>
> Steve, et al,
>
> This past weekend I made a chicken pot pie. While looking for a vessel t=
o
> put into the oven, I considered several; glass, an imported French cerami=
c
> casserole, and a Corningware fluted casserole, I even for a moment
> considered using a "casserole" I made in the fashion of an example in a
> Ceramic Arts Daily posting about throwing round and then squaring up on t=
he
> wheel. My final selection was based (probably quite erroneously) on the
> notation fired into the bottom of the Corningware piece saying "Oven to
> Table", after all Corning is commercial, and surely Oven to Table must
> include going into a hot oven as well. Despite the permutations I could
> have made from their fired in invitation, I did put the pot pie into a co=
ld
> oven and went to temperature and added a little hold at the end. Delicio=
us
> and safe. I do wonder how my little casserole would have handled it thoug=
h;
> alas the recipe was larger than my work would handle.
>
> In curiosity I looked up the Corningware site and found their warnings fo=
r
> their French White line (which is stoneware)
> http://corningware.com/index.**asp?pageId=3D80ex.asp?pageId=3D80>
>
> Sounds very familiar to a Clayart reader, but for the next round we need =
to
> add:
>
> "Do not put under broiler/griller, under a microwave browning element, in=
a
> toaster oven, or on or near any other direct heating source such as range
> vents, open flames, etc". Sound advice.
>
> John Lowes
> Sandy Springs, Georgia
> http://wynhillpottery.weebly.**com/
>
--
ronroy@CA.INTER.NET on wed 14 dec 11
Hi Eleanor,
The most important thing is to get the clay/glaze fit right - get that =3D2=
0
right and you have made a better pot.
The second part is to get some granular material like grog into the =3D20
clay body - that helps in a few ways to mediate heat shock.
Remember - a crazed glaze never broke ovenware - it makes pots easier =3D20
to break however so a glaze that is just below crazing in terms of =3D20
expansion is your best bet.
Unglazed would be the easiest to do as long as there was not =3D20
cristobalite present.
If I made ovenware one of the instructions I would include would be - =3D20
keep the pot in the middle so it's not closer to one of the elements - =3D2=
0
makes for a more even heating.
RR
Quoting Eleanor :
> Long ago, in the 50's, When I was learning clay, the instructor, a =3D20
> knowledgeable, respected clay artist, when asked whether fired (^4) =3D20
> pots were oven-safe, replied, "of course, they were fired at over =3D20
> 2000=3DB0, they certainly can survive home oven temperatures."
>
> Recent Clayart discussion concluded, yes, but start in a cold oven. =3D20
> My 50's teacher never said anything about a cold oven.
>
> My friend, a Clayarter (hi, Marcey!) who makes beautiful pie dishes, =3D2=
0
> tells her customers to start cold.
>
> In the latest Clay Times (pre-holiday 2011) there is an article by =3D20
> Bill van Gilder on how to make a ceramic muffin pan followed by a =3D20
> recipe for muffins to be baked at 375=3DB0. No mention in the article or =
=3D
=3D20
> the recipe about a cool oven.
>
> I use commercial glass or metal ware for my own baking and I don't =3D20
> make pots for oven use so I have never test, test, tested this.
>
> I can understand that freezer-to-oven might suffer a shock, but has =3D20
> anyone ever had a cool room-temperature pie or casserole explode or =3D20
> crack when put into a hot oven?
>
> I fire ^6 electric oxidation.
>
> I am confused.
>
> Eleanor Kohler
> Centerport, NY
>
> Merry Christmas to all to whom it applies and Happy Whatever to =3D20
> everyone else.
> I don't much go for PC
>
>
>
>
>
> "reality is a great teacher. fantasy is the enemy"
>
> mel jacobson, potter, Clayart moderator
>
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