Gaye Sekula on fri 4 feb 05
A friend and I are having a discussion of which clay(s) make the best
ovenware and welcome your input. Is earthenware preferable, since it is such a good
insulator, or stoneware, due to its strength? Then, I suppose, fireclay
should also be considered, but should it be a consideration if the pot would not
be placed on an open flame?
Looking forward to your responses,
Gaye Sekula
San Antonio
David Beumee on sun 6 feb 05
Dear Gaye,
I would think that fired earthenware would be less of an insulator than a vitrified stoneware body because of the more open structure of a fired earthenware body. The advantage of earthenware as good ovenware to my mind would be in its resistance to thermal shock, by virtue of its very open structure as fired clay. Its disadvantage might be that earthenware may be more suseptable to chipping and cracking in comparison to vitrified stoneware. So one question might be: how to have the strength of fired stoneware and the thermal shock resistance of earthenware?
There are some unglazed stoneware bakers on the market that are wonderful in their ability to brown a crust evenly because of stoneware's ability to retain heat. To say it another way, fired stoneware is a poorer conductor of heat than fired earthenware. These stoneware bakers are fired fairly high, I'm guessing around cone 8, so the ware has plenty of strength, and the body has been intentionally left "open" for thermal shock resistance. But, the directions for use say don't wash with soap. In other words, the manufacturer has intentionally cut down on the relative amount of flux, meaning a more open body, meaning better thermal shock resistance, but not as open as a fired earthenware body would be. The disadvantage of lowering the flux content in a high fire stoneware body is that cristobalite problems may occur, so the manufacturer has to be careful in choosing relatively low silica clays, and has to be careful about the choice of silica used in the body. A fired and glazed earthe
nware might well have such an open structure that water would be drawn in where the piece was dry footed, and get under the glaze to cause eventual crazing problems. So what about leaving the earthenwa
re intentionally unglazed? This has been done as well, the Romertoph covered bakers, where you soak the piece in water prior to baking your chicken, and the moisture poaches your roast or chicken as it cooks.
Fire clay would undoubtably be part of any high fire ovenware clay body, as it is an extremely useful part of most high fire stoneware bodies, but would not be useful as a single ingredient to make up any stoneware body, if I understand your question.
In conclusion, both earthenware and stoneware have advantages and disadvantages, but not many eathenware ovenware pieces appear commercially that I'm aware of, whereas stoneware bakers and pizza bakers are sold by the millions.
David Beumee
davidbeumee.com
Lafayette, CO
-------------- Original message ----------------------
From: Gaye Sekula
> A friend and I are having a discussion of which clay(s) make the best
> ovenware and welcome your input. Is earthenware preferable, since it is such a
> good
> insulator, or stoneware, due to its strength? Then, I suppose, fireclay
> should also be considered, but should it be a consideration if the pot would
> not
> be placed on an open flame?
>
> Looking forward to your responses,
> Gaye Sekula
> San Antonio
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
gjudson on tue 8 feb 05
Thanks, Lee. Yes, that is encouraging. I had hoped to find a =
ready-made
clay that others have used successfully for ovenware. I have not yet =
tried
making my own clay--not sure I'm ready to do that--but maybe I am. In
reading through the literature (Hamer & Hamer, for example) there are SO
many variables that affect thermal shock vulnerability perhaps I should =
just
stay away from ovenware. I don't sell any work (yet) so this is just =
for my
own use--which will provide lots of testing if I do make some ovenware. =
I
will also do the freezer/boiling water test--can't imagine anything
surviving that! Enough!! Thanks for taking the time to respond. Gay
Judson
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Lee Love
Sent: Monday, February 07, 2005 7:49 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Ovenware-earthenware or stoneware?
There are several recipes in Chappel, but he stesses the importance of
proper construction and slow firing. I recommend his section on oven
and rangetop ware.
--
Lee in Mashiko, Japan http://mashiko.org
http://potters.blogspot.com/ WEB LOG
http://claycraft.blogspot.com/ Photos!
_________________________________________________________________________=
___
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Lee Love on tue 8 feb 05
There are several recipes in Chappel, but he stesses the importance of
proper construction and slow firing. I recommend his section on oven
and rangetop ware.
--
Lee in Mashiko, Japan http://mashiko.org
http://potters.blogspot.com/ WEB LOG
http://claycraft.blogspot.com/ Photos!
Janet Kaiser on tue 8 feb 05
That is a very interesting answer, David. I am
going to go off the original question a little,
but I am personally most interested in:
> In conclusion, both earthenware and stoneware
have advantages and
>disadvantages, but not many eathenware ovenware
pieces appear commercially
>that I'm aware of, whereas stoneware bakers and
pizza bakers are sold by
>the millions.
This is quite a different perspective to mine,
not least, because I am not too sure that the
studio potter can compare his/her products with
manufactured ware. There is very little
commercial manufacturing being undertaken in the
UK these days, but the last time I spoke to
anyone in the industry, they said, "No plastic
clay is used any more". The last turners were
being made redundant in the 1990s... That is the
"throwers" who used horizontal lathes to form
pots. In other words it is all slip moulded or
dry press moulded.
I actually have mugs which claim to be "stone
made" but I very much doubt that they are what
any potter would understand to be stoneware and
in the absence of a universally accepted
"definition" of any of the terms craftsmen use --
porcelain, stoneware, earthenware with equivalent
ISOs, DINs, BSs or whatever -- it is not
"illegal" to claim something is "stoneware" when
the studio potter would probably call it low-fire
or earthenware! Hence I personally have great
difficulty distinguishing between manufactured
"earthenware" and "stoneware" products, usually
classing them in the low-fire regions most of the
time. It is pure guestimation, but it is the
reverse of your experience.
Seems to me that a studio or production potter
depends on their market in choosing between
earthenware and stoneware. The traditional
regional dishes of anywhere from Provence to
Yorkshire have fairly strict "expectations". Of
course everyone here will be familiar with the
dark brown exterior with lighter buff interior of
the traditional ovenware used for lidded
casseroles, Lancashire hotpot and the likes, but
the more you travel, the more it becomes apparent
how vast a range of traditional ware is still
being commercially produced -- even if some is
imported from low-cost Asian and South American
countries.
A couple of examples regional ware that I have
been delighted to discover in the past include:
+ the glazed fluted Kugelhopf shapes of Alsace,
often with jaunty slip trailed or combed designs
as much "peasant pottery" throughout Europe --
earthenware;
+ the cobalt blue on grey salt glazed domestic
kitchen, dairy and public house ware of Germany
-- stoneware;
+ the lightweight, red clay skillets and
terraines of Provence and South of France, some
of which are tin glazed but many of which just
have a clear glaze liner -- earthenware.
They are what I can bring to mind off the top of
my head... Mostly because they are in regular use
or are "decorations" around the home. They are
all earthenware, except for the German salt
glaze, and are still in local production, or at
least were up to 10 years ago although they were
struggling to remain mainstream "functional" and
not simply "tourist" ware. A lot of the original
loose, naive artistry has also been lost where
designs doggedly copy what grandpa did 100+ years
ago. Not one had contemporary themes, such as
someone watching tv or wearing clothes post 1920!
Which brings me an awful long way for the
original earthenware or stoneware! Sorry!
Sincerely
Janet Kaiser
**************************************************
**********
THE CHAPEL OF ART - or - CAPEL CELFYDDYD
8 Marine Crescent : Criccieth : GB-Wales LL52 0EA
Tel: ++44 (01766) 523122
http://www.the-coa.org.uk
Contact: Janet Kaiser: The International Potters
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David Beumee on tue 8 feb 05
Hi Janet,Lee, Gay and Gaye,
It probably isn't fair to compare our handmade work with commercially pressed bakers, but I think concerning the particular question it is interesting to note what is sold in the department store as "ovenware", in light of of past suits filed against potters making ovenware that is used and abused by the customer. For instance, I'm looking at the back of the cardboard box that we use to store our "superstone" pie and pizza baker (superb for foccacia bread and quiche). No warning whatsoever is given concerning what might happen if the baker were taken hot from the oven and placed on a wet counter, or what might happen if the baker were placed frozen in a preheated oven with frozen food in it. Yet I have (wrongly) printed detailed cards of all the restrictions on using my casseroles. I liked Mel's answer. He doesn't say anything, and if people ask he says "I use pots in the oven, but I am careful. Cold to hot is dangerous, as is hot to cold." "Good pots should not be abused." In other
words, Mel doesn't make statements in print so he isn't caught having said something he can't back up. Maybe this is the philosophy (not) expressed on the back of the "superstone" carton. Don't say to
o much. "Heat and reheat foods on your stone and use it as a portable food warmer!" "Our baker is simply a piece of clay pressed into a pan shape and fired at temperatures of over 2200 degrees F."
Yes,it would be of tremendous help to find out what goes into such a clay body and what kinds of thermal shock tests were done to give the manufacturer such confidence in the product. On the other hand, I know how hot they are fired, and I now know from Lee that Chappell gives ovenware recipes in his book. All I need is an accepted thermal shock test and I'm in business, as long as I don't say too much to my customers and don't put anything in writing.
Can anybody guide me to an "accepted" thermal shock test?
David Beumee
davidbeumee.com
Lafayette, CO
-------------- Original message ----------------------
From: Janet Kaiser
> That is a very interesting answer, David. I am
> going to go off the original question a little,
> but I am personally most interested in:
>
> > In conclusion, both earthenware and stoneware
> have advantages and
> >disadvantages, but not many eathenware ovenware
> pieces appear commercially
> >that I'm aware of, whereas stoneware bakers and
> pizza bakers are sold by
> >the millions.
>
> This is quite a different perspective to mine,
> not least, because I am not too sure that the
> studio potter can compare his/her products with
> manufactured ware. There is very little
> commercial manufacturing being undertaken in the
> UK these days, but the last time I spoke to
> anyone in the industry, they said, "No plastic
> clay is used any more". The last turners were
> being made redundant in the 1990s... That is the
> "throwers" who used horizontal lathes to form
> pots. In other words it is all slip moulded or
> dry press moulded.
>
> I actually have mugs which claim to be "stone
> made" but I very much doubt that they are what
> any potter would understand to be stoneware and
> in the absence of a universally accepted
> "definition" of any of the terms craftsmen use --
> porcelain, stoneware, earthenware with equivalent
> ISOs, DINs, BSs or whatever -- it is not
> "illegal" to claim something is "stoneware" when
> the studio potter would probably call it low-fire
> or earthenware! Hence I personally have great
> difficulty distinguishing between manufactured
> "earthenware" and "stoneware" products, usually
> classing them in the low-fire regions most of the
> time. It is pure guestimation, but it is the
> reverse of your experience.
>
> Seems to me that a studio or production potter
> depends on their market in choosing between
> earthenware and stoneware. The traditional
> regional dishes of anywhere from Provence to
> Yorkshire have fairly strict "expectations". Of
> course everyone here will be familiar with the
> dark brown exterior with lighter buff interior of
> the traditional ovenware used for lidded
> casseroles, Lancashire hotpot and the likes, but
> the more you travel, the more it becomes apparent
> how vast a range of traditional ware is still
> being commercially produced -- even if some is
> imported from low-cost Asian and South American
> countries.
>
> A couple of examples regional ware that I have
> been delighted to discover in the past include:
> + the glazed fluted Kugelhopf shapes of Alsace,
> often with jaunty slip trailed or combed designs
> as much "peasant pottery" throughout Europe --
> earthenware;
> + the cobalt blue on grey salt glazed domestic
> kitchen, dairy and public house ware of Germany
> -- stoneware;
> + the lightweight, red clay skillets and
> terraines of Provence and South of France, some
> of which are tin glazed but many of which just
> have a clear glaze liner -- earthenware.
>
> They are what I can bring to mind off the top of
> my head... Mostly because they are in regular use
> or are "decorations" around the home. They are
> all earthenware, except for the German salt
> glaze, and are still in local production, or at
> least were up to 10 years ago although they were
> struggling to remain mainstream "functional" and
> not simply "tourist" ware. A lot of the original
> loose, naive artistry has also been lost where
> designs doggedly copy what grandpa did 100+ years
> ago. Not one had contemporary themes, such as
> someone watching tv or wearing clothes post 1920!
>
> Which brings me an awful long way for the
> original earthenware or stoneware! Sorry!
>
> Sincerely
>
> Janet Kaiser
> **************************************************
> **********
> THE CHAPEL OF ART - or - CAPEL CELFYDDYD
> 8 Marine Crescent : Criccieth : GB-Wales LL52 0EA
> Tel: ++44 (01766) 523122
> http://www.the-coa.org.uk
> Contact: Janet Kaiser: The International Potters
> Path
>
>
>
> --
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> Checked by AVG Anti-Virus.
> Version: 7.0.300 / Virus Database: 265.8.5 - Release Date: 03/02/2005
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
John Hesselberth on wed 9 feb 05
On Tuesday, February 8, 2005, at 06:43 PM, David Beumee wrote:
> Can anybody guide me to an "accepted" thermal shock test?
Hi David,
You can check out ASTM C554-93. It is titled "Standard Test Method for
Crazing Resistance of Fired Glazed Ceramic Whitewares by Thermal Shock
Method". While the title indicates it is aimed at craze resistance, the
test it quite rigorous. Since ASTM charges about $25 for each
procedure, here is a brief synopsis.
You find an oven capable of 250-450 deg F. You take your test piece (a
whole piece of ware, not a test tile) and start at 250. Heat for 20
minutes and plunge into 68 deg F water. Examine for crazing (or
shattering I guess). Increase temperature 25 deg F. Repeat. Continue to
450 deg F. Pieces that pass 3 cycles at 300 deg F are considered craze
resistant. You also have to report body cracks but the procedure
doesn't say what you do about them or what they mean.
Lots of people will swear that pottery can't/won't pass this kind of
testing, but it will if the glaze and body are properly matched.
While ASTM doesn't say this test could be used for ovenware, it
certainly provides a standard thermal shock test which may help you
test your own ware. I don't know if they have a test more specific to
your question or not. Searching through their test methods is not the
easiest task in the world. Their abstracts are not very revealing and
after you spend $25 a few times and come up with nothing you get
discouraged. I have purchased a total of 8 of their test methods and
feel I got value from about 3 of them.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Janet Kaiser on thu 10 feb 05
Ah-ha! That is the crux of the matter, David:
"Accepted thermal shock test".
I have never heard of any "official" testing
guidelines. What would be the extremes to shock a
pot to? Dry or liquid content? Empty or full...
There are an awful lot of variables... Beyond
which it leads me to some parallel thoughts...
First of all, how can such a standardised test be
devised, when the objects to be tested vary so
much? Thickness, shape, clay, glaze, method...
Even something as unnoticed or little regarded as
the precise throwing method... A pot which
appears the same size and shape as its neighbour
may not be when compared in detail. Perhaps one
was more compressed than the other, or the base
was thicker, or any of the minor differences
which would affect any attempt at satisfactory
testing.
Secondly, is it even appropriate? Why should a
potter who makes perfectly functional pots, test
them for safety in the first place? All factors
are highly variable in a studio or small workshop
versus a production line churning out thousands
of identical pieces, yet we are all brain-washed
into being "health and safety" mad, as if we can
legislate for appropriate and misappropriate use
of the perfectly functional and practical ceramic
ware we produce.
In many ways, studio potters should thank those
nameless mug makers for ensuring the whole
industry remains viable, so it cannot be closed
down by officious government departments or the
courts. Without big business "benefitting" there
is a tendency to close down "unsafe" small
enterprise or put them under such burocratic
pressure they find it is not worth their while.
Legislation aimed at protecting the personal
healthy and safety of humans (individually and
collectively) often includes environmental
concerns, yet they can be dropped completely
under the lobby pressure of Big Business.
As a little sop to the underfunded and
unrepresented concerns of the population as a
whole, guess who bares the brunt of watered-down
legislation? Many have been manipulated into
classifying this as a left versus right issue,
but that is a dangerous over-simplification. What
was only a Third World problem is now affecting
our own small producers right across the board.
It is made an even more difficult issue in a
society which supports suing for personal gain
whilst opting out of personal responsibility.
Whether suing a corporation (which an increasing
number of people seem to feel is "fair game" no
matter how just or unjust) or an individual like
a potter, ethics seem to fly out of the window in
the face of "easy money"...
I have two new/unused industrially manufactured
mugs, both of which have what could be classed as
"dangerous" faults. The one has a fractured
handle (bottom only, approx. 2 mm away from the
mug) which would no doubt cause a very nasty
accident if it sheered off full of hot liquid.
The other is also 6 inches high and there is what
I suspect is an impact crack in the side.
Although not the normal star shaped or radiating
cracks, nor pinpoint hole caused by hard impact,
there is a hairline crack up the side of the mug,
which seams to peter out top and bottom. It
leaked near to a half pint of boiling water over
a couple of minutes, although Himself should have
realised that the ominous shishing sound was not
normal when he made tea for the first time in his
Christmas present.
The thing is that the only recourse for me, is to
return them to the seller and ask for a
replacement or a refund. I do not think I would
want more than that even though they are faulty
goods. The second could be difficult because I
could not prove it was already faulty when
bought, because the nature of the fault is
unclear. So they sit here holding pens and
pencils quite happily. Someone else would not
doubt make a huge song and dance about them...
And if the handle had fallen off and dumped
nearly a pint of hot liquid in my lap? I would
not dream of suing the mug manufacturer, nor
would any British court allow me to do so.
Sincerely
Janet Kaiser
P.S. I liked Craig's little outburst about OSHA
being castrated thanks to political pressure
from up high, not simply within the
Administration, putting lobbyists out of a job!
Ha! Ha! Yes, it is what comes of the richest
candidates being chosen to rule a nation,
especially from one of the dirtiest industries of
all. Why on earth you guys celebrate July 4th
when you have replaced a monarchy with a series
of oligarchs who are treated like royalty, I
guess you have to be American to completely
understand! LOL!
*** IN REPLY TO THE FOLLOWING MAIL:
*** From: David Baumee
>All I need is an accepted thermal shock test and
I'm
>in business, as long as I don't say too much to
my customers and don't put
>anything in writing.
> Can anybody guide me to an "accepted" thermal
shock test?
*** PREVIOUS MAIL ENDS HERE ***
**************************************************
**********
THE CHAPEL OF ART - or - CAPEL CELFYDDYD
8 Marine Crescent : Criccieth : GB-Wales LL52 0EA
Tel: ++44 (01766) 523122
http://www.the-coa.org.uk
Contact: Janet Kaiser: The International Potters
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Jon Pacini on thu 10 feb 05
Greetings All---Hi David
As John H has posted, multiple cycling of a piece of ware thru heat and cold
is probably the best test you can do. This will not only let you know if the
glaze will craze, as the test is designed to do, but also if the clay/glaze
combination will survive intact.
You can have the best ovenware body in the world, but if the glaze ends up
tearing the piece in half, then you re in trouble.
For the most part if you are careful with your intended ovenware, nearly any
clay will work. In the past we at Laguna have relied on customer feed back
and practical experience to lead us to our recommendations on which bodies
work best. Earthenware tends to be the best insulator, keeping food warm.
Stoneware is no doubt the most durable and Porcelains the most elegant.
However, as with flame ware and refractories, the role thermal expansion
plays in ovenware could be looked at as a criteria for success. The biggest
problem with heating something cold is not necessarily the speed with which
you heat it, but how evenly it heats. If one side of a pot heats up and the
other doesn t, one side of the pot is expanding and the other isn t. The
stress developed at the point between the two will cause failure of the pot.
Now, pots can handle a certain amount of stress, but if you keep the COE of
a clay low, you will find that the clay won t expand as much as a clay with
a high COE. Low expansion is a good thing; less stress develops with less
expansion.
As I said nearly any clay will work. And if you look at the COE s for
typical commercial clays they reside in the range of 3.6- to 6.6 x 10 -6,
with of course, exceptions on either end of the scale. If your glaze fits
well and your technique is consistent, then you will likely have success
with any of these clays.
However, if you want to better your odds, then go with the clays with the
lower COE s.
Of course I just finished testing all the Laguna/miller clays and I now have
these numbers. And as self-serving as this may seem, it IS good information
to have. God bless Ron R who poked and prodded me, kicking and screaming, to
the point of doing the tests.
I hope to get them charted and on the website soon. I may even have a chart
ready for NCECA for those who are interested in this type of info. Till
then, you can call or e-mail me for individual clays.
Best regards
Jon Pacini
Clay Manager
Laguna Clay Co.
Jon Pacini on sat 12 feb 05
Greetings All-Hi David---
With ceramics, the denser the material is, the less it insulates heat.
You don't need to look any farther than a hard brick and a soft brick to
confirm this. One is full of air and the other is not, but the material they
are made out of is the same.
It's the density of the earthenware that makes it a better insulator in
regards to heat, not the temperature it's fired to.
Best regards
Jon Pacini
Clay Manager
Laguna Clay Co.
-----Original Message-----
From: claybuds@att.net [mailto:claybuds@att.net]
Sent: Saturday, February 12, 2005 8:30 AM
To: Jon Pacini; CLAYART@LSV.CERAMICS.ORG
Cc: Jon Pacini
Subject: Re: Ovenware-earthenware or stoneware?
Hi everyone,
Grateful thanks to John H and Jon P.
"> You can have the best ovenware body in the world, but if the glaze ends
up
> tearing the piece in half, then you re in trouble."
One of the most reliable recipes I have used is the so-called Babu
porcelain recipe:
55 Grolleg
20 Custer
13 Silica 90
12 Pyrax
2 Bentone MA
This produces a low expansion body, so a good beginning toward an ovenware
porcelain, but the body is generally hard to fit for those of us not
inclined toward computer glaze calc programs. Then there's the question of
aesthetics. I've been working hard trying to find a base glaze that enhances
rather than deadens color. I need help lowering my best results to fit this
low expansion body. But I know that there's a certain point where a low
expansion glaze can put a clay body in just the right amount of compressive
fit to vastly strengthen the whole piece overall. Am I correct that only MOR
testing will reveal this?
" Earthenware tends to be the best insulator, keeping food warm.
> Stoneware is no doubt the most durable and Porcelains the most elegant."
WOW! Earthenware a better insulator than stoneware? Surprises never end!
"> As I said nearly any clay will work. And if you look at the COE s for
> typical commercial clays they reside in the range of 3.6- to 6.6 x 10 -6,
> with of course, exceptions on either end of the scale. If your glaze fits
> well and your technique is consistent, then you will likely have success
> with any of these clays."
Hurray to Jon for doing the COE tests on individual clays! Yes, I most
certainly am interested in starting with the lowest expansion clays.
Thankyou! I will be emailing you and looking for info at NCECA. Excellent!
David Beumee
davidbeumee.com
Lafayette, CO
-------------- Original message ----------------------
From: Jon Pacini
> Greetings All---Hi David
>
> As John H has posted, multiple cycling of a piece of ware thru heat and
cold
> is probably the best test you can do. This will not only let you know if
the
> glaze will craze, as the test is designed to do, but also if the
clay/glaze
> combination will survive intact.
>
> You can have the best ovenware body in the world, but if the glaze ends up
> tearing the piece in half, then you re in trouble.
>
> For the most part if you are careful with your intended ovenware, nearly
any
> clay will work. In the past we at Laguna have relied on customer feed back
> and practical experience to lead us to our recommendations on which bodies
> work best. Earthenware tends to be the best insulator, keeping food warm.
> Stoneware is no doubt the most durable and Porcelains the most elegant.
>
> However, as with flame ware and refractories, the role thermal expansion
> plays in ovenware could be looked at as a criteria for success. The
biggest
> problem with heating something cold is not necessarily the speed with
which
> you heat it, but how evenly it heats. If one side of a pot heats up and
the
> other doesn t, one side of the pot is expanding and the other isn t. The
> stress developed at the point between the two will cause failure of the
pot.
>
> Now, pots can handle a certain amount of stress, but if you keep the COE
of
> a clay low, you will find that the clay won t expand as much as a clay
with
> a high COE. Low expansion is a good thing; less stress develops with less
> expansion.
>
> As I said nearly any clay will work. And if you look at the COE s for
> typical commercial clays they reside in the range of 3.6- to 6.6 x 10 -6,
> with of course, exceptions on either end of the scale. If your glaze fits
> well and your technique is consistent, then you will likely have success
> with any of these clays.
>
> However, if you want to better your odds, then go with the clays with the
> lower COE s.
>
> Of course I just finished testing all the Laguna/miller clays and I now
have
> these numbers. And as self-serving as this may seem, it IS good
information
> to have. God bless Ron R who poked and prodded me, kicking and screaming,
to
> the point of doing the tests.
>
> I hope to get them charted and on the website soon. I may even have a
chart
> ready for NCECA for those who are interested in this type of info. Till
> then, you can call or e-mail me for individual clays.
>
> Best regards
> Jon Pacini
> Clay Manager
> Laguna Clay Co.
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
David Beumee on sat 12 feb 05
Hi everyone,
Grateful thanks to John H and Jon P.
"> You can have the best ovenware body in the world, but if the glaze ends up
> tearing the piece in half, then you re in trouble."
One of the most reliable recipes I have used is the so-called Babu porcelain recipe:
55 Grolleg
20 Custer
13 Silica 90
12 Pyrax
2 Bentone MA
This produces a low expansion body, so a good beginning toward an ovenware porcelain, but the body is generally hard to fit for those of us not inclined toward computer glaze calc programs. Then there's the question of aesthetics. I've been working hard trying to find a base glaze that enhances rather than deadens color. I need help lowering my best results to fit this low expansion body. But I know that there's a certain point where a low expansion glaze can put a clay body in just the right amount of compressive fit to vastly strengthen the whole piece overall. Am I correct that only MOR testing will reveal this?
" Earthenware tends to be the best insulator, keeping food warm.
> Stoneware is no doubt the most durable and Porcelains the most elegant."
WOW! Earthenware a better insulator than stoneware? Surprises never end!
"> As I said nearly any clay will work. And if you look at the COE s for
> typical commercial clays they reside in the range of 3.6- to 6.6 x 10 -6,
> with of course, exceptions on either end of the scale. If your glaze fits
> well and your technique is consistent, then you will likely have success
> with any of these clays."
Hurray to Jon for doing the COE tests on individual clays! Yes, I most certainly am interested in starting with the lowest expansion clays. Thankyou! I will be emailing you and looking for info at NCECA. Excellent!
David Beumee
davidbeumee.com
Lafayette, CO
-------------- Original message ----------------------
From: Jon Pacini
> Greetings All---Hi David
>
> As John H has posted, multiple cycling of a piece of ware thru heat and cold
> is probably the best test you can do. This will not only let you know if the
> glaze will craze, as the test is designed to do, but also if the clay/glaze
> combination will survive intact.
>
> You can have the best ovenware body in the world, but if the glaze ends up
> tearing the piece in half, then you re in trouble.
>
> For the most part if you are careful with your intended ovenware, nearly any
> clay will work. In the past we at Laguna have relied on customer feed back
> and practical experience to lead us to our recommendations on which bodies
> work best. Earthenware tends to be the best insulator, keeping food warm.
> Stoneware is no doubt the most durable and Porcelains the most elegant.
>
> However, as with flame ware and refractories, the role thermal expansion
> plays in ovenware could be looked at as a criteria for success. The biggest
> problem with heating something cold is not necessarily the speed with which
> you heat it, but how evenly it heats. If one side of a pot heats up and the
> other doesn t, one side of the pot is expanding and the other isn t. The
> stress developed at the point between the two will cause failure of the pot.
>
> Now, pots can handle a certain amount of stress, but if you keep the COE of
> a clay low, you will find that the clay won t expand as much as a clay with
> a high COE. Low expansion is a good thing; less stress develops with less
> expansion.
>
> As I said nearly any clay will work. And if you look at the COE s for
> typical commercial clays they reside in the range of 3.6- to 6.6 x 10 -6,
> with of course, exceptions on either end of the scale. If your glaze fits
> well and your technique is consistent, then you will likely have success
> with any of these clays.
>
> However, if you want to better your odds, then go with the clays with the
> lower COE s.
>
> Of course I just finished testing all the Laguna/miller clays and I now have
> these numbers. And as self-serving as this may seem, it IS good information
> to have. God bless Ron R who poked and prodded me, kicking and screaming, to
> the point of doing the tests.
>
> I hope to get them charted and on the website soon. I may even have a chart
> ready for NCECA for those who are interested in this type of info. Till
> then, you can call or e-mail me for individual clays.
>
> Best regards
> Jon Pacini
> Clay Manager
> Laguna Clay Co.
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Janet Kaiser on mon 14 feb 05
>> " Earthenware tends to be the best insulator,
keeping food warm.
>> Stoneware is no doubt the most durable and
Porcelains the most elegant."
>
>WOW! Earthenware a better insulator than
stoneware? Surprises never end!
Do you not cook, David? Slow, oven-cooked dishes
like Boston Baked Beans for example, call for
earthenware covered crocks or casseroles and some
cook books will even suggest earthenware as the
correct or preferred type for individual recipes.
I would personally question the concept that
stoneware is the most durable... It ain't
necessarily so!
Sincerely
Janet Kaiser
**************************************************
**********
THE CHAPEL OF ART - or - CAPEL CELFYDDYD
8 Marine Crescent : Criccieth : GB-Wales LL52 0EA
Tel: ++44 (01766) 523122
http://www.the-coa.org.uk
Contact: Janet Kaiser: The International Potters
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Lee Love on tue 15 feb 05
Janet Kaiser wrote:
>I was recently told the same thing in no
>uncertain manner. By someone on this list, who
>claimed that all pots are now primarily nothing
>more than "decorative" and never functional, so
>they had no need to take any notice of what could
>be considered "good practice" when designing and
>making them.
>
>
This is not a potter, but a clay artist or sculptor.
>
>I would personally question the concept that
>stoneware is the most durable... It ain't
>necessarily so!
>
This is only true when all your forms are some variation of a
brick. ;-)
The oldest pots I have made, that got everyday use in my house, were
raku latte/coffee bowls (I usually prefer using other people's work,
except to bring something in to "test."). The latte bowls have been in
storage in St. Paul for the last 5 years. I should make some more (got
frit for the glaze!)
--
Lee in Mashiko, Japan http://mashiko.org
http://potters.blogspot.com/ WEB LOG
http://claycraft.blogspot.com/ Photos!
Jon Pacini on tue 15 feb 05
Greetings All Hi Janet--- Yes there are exceptions to all the rules
in ceramics. And every stoneware clay body is not superior to every
earthenware body. Some earthenwares posses tremendous resilience. And you
can certainly misfire even a great stoneware body and make it fail
miserably. But from a handling standpoint, a well formulated, well produced
stoneware vessel will generally posses better chip and crack resistance
than an earthenware one.
But that does not diminish the advantages of using earthenware that you have
noted for cooking with. I much prefer earthenware for that purpose myself.
Best regards
Jon Pacini
Clay manager
Laguna Clay Co.
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