Cindy Strnad on tue 24 oct 00
Norman,
I don't consider myself a guru of any shape or size, but no, there's nothing
wrong with firing below vitrification if that gives you the results you
want. It's a good idea for things like the tahine because, yes, it does make
them more likely to be able to withstand thermal shock.
That's not to say they are thermal shock proof, however. Not to be a wet
blanket, but I would be afraid to make a tahine meant for using over a
direct heat source for anyone but myself. I don't know what the situation is
overseas, but here in the good old litigation happy U.S.A., I'd think
selling or giving away such a piece would constitute a huge financial risk
to the maker and/or seller. It's a shame, but there you are. You have to
think about these things.
I don't do oven ware, and I no longer write "oven safe" on my pottery for
this reason. Personally, I use my pottery in the oven all the time, but I
can't imprint complete common-sense instructions into the base of each piece
to protect myself if someone does something stupid, can I?
Oven ware might be all right, but no way would I even think about doing
flame-ware. If I did, I certainly wouldn't advertise it as such.
But, back to the vitrification question. Work fired under vitrification is
good for certain uses because of its porosity. And it's not good for other
uses, again, because of its porosity. If allowed to remain damp, it will
grow mold, and it's likely to seep if unglazed or glazed with an imperfectly
fitting glaze. As long as you realize that, fire away.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
Norman van der Sluys on tue 24 oct 00
One more example of "primitive" or "unsophisticated" (meaning
non-western or
pre-industrial) potters achieving what cannot be done with our current
technology.
There seems to be definite disadvantages to firing clay bodies to
vitrification.
Sure, it is desireable for wind chimes and may make the ware more impact
resistant,
but it makes the ware much less tolerant of thermal shock. Those of us
who do smoke
firing and raku know this well. As far as porosity is concerned, isn't
that what
glazes are for?
I'd like to hear from the Gurus just what the case is for considering
firing below
vitrification undesireable. I regularly use a stoneware clay rated at
^8-10 for
functional ware fired to ^6-7, and find it very practical - use it
everyday,
including oven dishes.
Richard Jeffery wrote:
> never made one, but cooked with them, and have friends to do.
> Moroccan/Algerian/etc in origin. Ones I have seen have been of
various
> sizes - from a sort of single portion size - say 7" across, to about
12"
> across. Base is deep - say 1 to 1 1/2". All parts containing food,
> including conical lid, are glazed. Lid does not have a steam vent in
the
> top. Only seen earthenware, and design intends use on a direct heat
source
> as well as in an oven - in fact as I understand it, primary use is
over a
> heat source - so take note of recent posts on other heat proof clay
bodies.
>
--
Norman van der Sluys
by the shore of Lake Michigan
amy parker on wed 25 oct 00
Norm - not to put myself into "guru" status, but rather from the
perspective of a pottery "consumer", my experience has been good with
"underfired" ovenware, and most other forms, as long as the glaze fits well
enough to seal the piece for its intended use. The biggest problem I have
had is with a cone 10 body fired to cone 6 used as plates. After removing
them from the dishwasher and stacking them in a closed cupboard, they tend
to mildew on the back sides. If I hand wash them, they don't seem to soak
up as much water, and I don't have a problem.
Amy
At 04:32 PM 10/24/00 -0400, you wrote:
....There seems to be definite disadvantages to firing clay bodies to
vitrification...it makes the ware much less tolerant of thermal shock. ...
I'd like to hear from the Gurus just what the case is for considering
firing below vitrification undesireable.
Norman van der Sluys
Amy Parker
Lithonia, GA
Jon Singer on wed 25 oct 00
If I may quote Norman van der Sluys --
>One more example of "primitive" or "unsophisticated" (meaning
>non-western or
>pre-industrial) potters achieving what cannot be done with our current
>technology.
Well, no. That simply isn't true.
While it is true that "primitive" or "unsophisticated" peoples have
achieved many things that would (or do) surprise us, and while it is
true that this is noteworthy and worthy of careful study, it's a bad
idea to claim that they achieve something we can't, unless you do
your homework and make sure that your claim will hold water.
Anybody on this list who cares to do a little research can make a body
that will fire to full vitrification at cone 10 or 11 and is thoroughly
flameproof. There are recipes for flameproof stonewares (and glazes
to fit them) in books that some of us already have on our shelves.
I thought I'd come up with something fairly new, a flameproof
porcelain, but when I did some reading I found out that an entire
family of flameproof porcelains has been known since 1968.
In addition, the compositions we've been using as kiln furniture for
many decades are flameproof. That is, after all, a good part of what
kiln furniture is about...
....And who says we can't make porous earthenware pots with our
current technology (e.g., electric kilns and wheels) that are every bit
as flameproof as the "primitive" ones?
Best --
jon
Craig Martell on wed 25 oct 00
Norman commented:
>There seems to be definite disadvantages to firing clay bodies to
>vitrification. Sure, it is desireable for wind chimes and may make the
>ware more impact resistant, but it makes the ware much less tolerant of
>thermal shock.
Hi:
There are advantages and disadvantages to everything. Sometimes it's
called gain and loss. Resistance to thermal shock is not the prime
function of many clays although it's a very good quality to have for ware
that is going to be subjected to rapid temp changes.
FYI, I make high fire porcelain and it is vitrified. What I prefer about
vitreous ware is the development of a very strong interface between clay
and glaze. This does not happen with ware that isn't fired to maturity, or
the point of vitrification. Non vitreous ware that is sealed by glazing is
the defintion of earthenware. I like earthenware mugs but have seen many
of these pieces with "shelling" glazes due to the lesser quality of the
fusion bond between clay and glaze.
Thermal shock can be controlled in vitreous ware by limiting the amount of
free silica and supplying the body with sufficient flux to attach and
nullify cristobalite. I have very few problems with thermal shock in my
porcelain mugs, teabowls, teapots because of the degree of fusion attained
by a high amount of feldspar in the body. Porcelain is pretty much
composed of fused silica after firing to cone 10 and above. Fused silica
has a lower coefficient of expansion (COE) than crystalline silica so by
adding enough spar to the body to fuse things, one can eliminate the
thermal shock effect to a large extent. I don't, however, make ovenware
with porcelain.
My view is that a properly compounded high fire body will not be of lesser
quality than a non vitreous earthenware in regard to thermal shock
properties. These high fire bodies have a stonger clay/glaze boundary
layer and are superior in mechanical strength to unvitrified ware. A
potter needs to control the point of vitrification though and this is done
by adjusting the amount of flux at the desired temperature. Stonewares
that are near zero in absorbtion are too vitrified and brittle. China clay
bodies, or porcelains can be quite near zero absorbtion and not be brittle
and easily shattered.
There's a lot more I could say but I think I should get back to making pots.
later, Craig Martell in Oregon
John Hesselberth on wed 25 oct 00
Norman van der Sluys wrote:
>I'd like to hear from the Gurus just what the case is for considering
>firing below vitrification undesireable.
Well for one thing it makes it totally unsatisfactory for use in today's
microwave world. A pot that absorbs moisture is a dangerous thing in a
microwave. Of course you are also totally dependent, then, on the glaze
to hold liquids and that means a single craze or glaze defect and you
have a leaky pot.
I don't totally agree withyour statement that a vitrified pot is unable
to withstand thermal shock. I guess it is a matter of degree, but a well
vitrified pot can withstand enough thermal shock for many uses. A well
matched glaze/vitrified clay combination should certainly be able to
withstand freezer to boiling water level of shock repeatedly. Even 300
deg F oven to immersion in room temperature water shouldn't bother it
much. I guess you might not be able to put it right in the coals of an
open fire; if that is important to you, you might want to use unvitrified
clay and warn people not to put it in the microwave or use it for
liquids. It is really of matter of understanding your material and
designing for the intended use (and of course in today's world, worrying
about what the lawyers will say when someone does something stupid with
it). One size doesn't fit all. John
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is, perhaps, still necessary to say that the very best glazes cannot
conceal badly shaped pots..." David Green, Pottery Glazes
Des Howard on thu 26 oct 00
Norman
Our standard body is a virtually 0% porosity/absorption porcelaneous
stoneware fired to Cone 12 red., if the opportunity presents during
a kiln cooling & visitors are present we delight in removing pots
from the kiln with a towel, carrying the smoking towel to a glaze wash
tub & dropping the pot in cold water, without harm to the pot.
Glazes are for enhancing a pot &/or producing a smoother surface
for dinnerware, not for rendering the ware non-porous.
Any crazing that may develop does not make the pot leak.
Des
Norman van der Sluys wrote (snipped):
> There seems to be definite disadvantages to firing clay bodies to
> vitrification.
> Sure, it is desireable for wind chimes and may make the ware more impact
> resistant,
> but it makes the ware much less tolerant of thermal shock. Those of us
> who do smoke
> firing and raku know this well. As far as porosity is concerned, isn't
> that what
> glazes are for?
--
Des & Jan Howard
Lue Pottery
LUE NSW 2850
Australia
Ph/Fax 02 6373 6419
http://www.luepottery.lisp.com.au
Norman van der Sluys on thu 26 oct 00
John,
What you say may be true for grossly underfired pieces (of course my smoke-fired
things are not functional in the sense of use for holding food or liquids,) but my
daily experience contradicts all the caveats this thread has generated thus far. I
have never seen evidence of mildew or leaking, even with oil lamps, after several
years of everyday use. I use my ware in the microwave extensively and it seems to
outlast the glass plates that come with the microwaves! My coffee mug gets nuked
several times a day, and food stored in the refrigerator is reheated in
serving-storage bowls I make. Even a coffee mug with noticeable crazing passes
David Hendley's overnight soak then boil test. This despite the fact that the clay
I use (Minnesota Clay Northern Light w/iron) is rated as ^8 - 10 and I typically
fire to ^6 well down with ^7 horizontal. Yes, my glazes seem to fit the body well
at this temperature, and I consider a pot with a glaze miss on the interior to be a
second at best, but is this not part of craftsmanship? If a piece is to be
considered functional shouldn't the glaze coat be functional as well as the body
and the design?
There seems to be a certain high-heat mystique that says "hotter is better." Most
of the advocates of this philosophy seem to be heavily influenced by traditional
Japanese and Sung Dynasty Chinese ware. From my recent reading (Tichane, and
Richard L. Wilson's excellent Inside Japanese Ceramics, among others,) I gather
that many of these inspiring pieces were not fired higher than about 1230 oC. I
have no objection to people firing their stoneware so high that they need wadding
or alumina powder to keep unglazed surfaces from sticking to the shelves, if that
is the way they want to spend their money, but pracical experience tells me this is
neither beneficial nor desireable from a functional or environmental standpoint.
John Hesselberth wrote:
> Well for one thing it makes it totally unsatisfactory for use in today's
> microwave world. A pot that absorbs moisture is a dangerous thing in a
> microwave. Of course you are also totally dependent, then, on the glaze
> to hold liquids and that means a single craze or glaze defect and you
> have a leaky pot.
>
--
Norman van der Sluys
by the shore of Lake Michigan
John Hesselberth on thu 26 oct 00
Hi Norman,
I suspect you are more vitrified than you think. Just because a clay
supplier rates a clay at a certain cone range, doesn't mean
(unfortunately) that is an accurate maturity cone range (well except for
those clays watched over by our own Ron Roy). I'll bet the actual water
absorption on your clay fired to a very strong cone 6 is certainly less
than 3% and probably less than 2%. At a full cone 10 in may really be
overfired. Greater than 2-3% absorption seems to be where problems like
I described begin to occur.
Absorption is pretty easy to measure; although you have to do it
carefully. Make a fairly thin test bar. Fire it. Weigh it carefully
immediately on removing it from your kiln. Boil it for a couple hours
completely submerged in water and then let it cool to room temperature
leaving it submerged. When cool, quickly dry off the surface water and
immediately reweigh it. Then calculate % absorption:
[(Final weight - initial weight) / initial weight] x 100
I have found the same thing on the clay I use (Standard 306). It is one
of those rated with an extremely wide range of 4-10. I fire it to cone 6
tip touching and get between 1.5 and 2% absorption on the tests I have
run. I'm guessing it is pretty brittle at cone 10, but I have no data on
that.
Regards, John
Norman van der Sluys wrote:
>I
>have never seen evidence of mildew or leaking, even with oil lamps, after
>several
>years of everyday use. I use my ware in the microwave extensively and it
>seems to
>outlast the glass plates that come with the microwaves! My coffee mug gets
>nuked
>several times a day, and food stored in the refrigerator is reheated in
>serving-storage bowls I make. Even a coffee mug with noticeable crazing
>passes
>David Hendley's overnight soak then boil test. This despite the fact that
>the clay
>I use (Minnesota Clay Northern Light w/iron) is rated as ^8 - 10 and I
>typically
>fire to ^6 well down with ^7 horizontal. Yes, my glazes seem to fit the
>body well
>at this temperature, and I consider a pot with a glaze miss on the
>interior to be a
>second at best, but is this not part of craftsmanship? If a piece is to be
>considered functional shouldn't the glaze coat be functional as well as
>the body
>and the design?
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is, perhaps, still necessary to say that the very best glazes cannot
conceal badly shaped pots..." David Green, Pottery Glazes
Janet Kaiser on fri 27 oct 00
> I suspect you are more vitrified than you
think.
We ALL probably are! Not just vitrified, but
totally cracked too!!!
Ha! Ha! Hahaha! Ha! Sorry, but it tickled my
funny bone...
Janet Kaiser
The Chapel of Art . Capel Celfyddyd
HOME OF THE INTERNATIONAL POTTERS' PATH
Criccieth LL52 0EA, GB-Wales Tel: (01766) 523570
E-mail: postbox@the-coa.org.uk
WEBSITE: http://www.the-coa.org.uk
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