Gloria Davidson on tue 27 may 97
Hi Everyone,
I have recently begun making ovenware from a white porcelineous
stoneware and have had many returns from cracking after the piece leaves the
oven. I live in British Columbia and have been using a clay bought in N.
Oregon. Unfortunately, I have had little success with Plainsman's Clay which
is more accesible geographically to me.
I wondered if anyone in this region is using a cone 10-11 white
claybody that it doing well with ovenware. Also, I am going to Vancouver in
a couple of weeks and would like to find the most reasonably priced High
Temp soft brick for some Kiln repairs on my Minnesota Flat top. (Love this
Kiln!)
Sure would appreciate any info that you might be able to send my way.
Thanks Gang!.
Gloria
Craig Martell on wed 28 may 97
At 09:01 AM 5/27/97 EDT, Gloria Davidson wrote:
>----------------------------Original message----------------------------
>Hi Everyone,
> I have recently begun making ovenware from a white porcelineous
>stoneware and have had many returns from cracking after the piece leaves the
>oven.
Hi Gloria:
Ovenware that fails during use is generally the fault of Cristoballite in
the claybody. Cristoballite will invert very rapidly and produce a larger
volume change than the silica inversion that occurs at about 1060 degrees F.
The cristoballite inversion happens at about 479 F., but the exact temp is
VERY unpredictable.
The remedy for this defect is to increase the feldspar in the body formula
and incorporate the offending Cristoballite into the body glass. Fused
silica has a lower coefficient of expansion than crystalline silica and this
will stop the failure of the ovenware pieces. Since you are using a
commercial body, you will have to advise the manufacturer of the problem and
hope they will be responsive and adjust the formula.
Another possible remedy is to fire the body to maturation and cool quickly
to below cone 5, which will reduce the amount of Cristoballite that is being
formed. The formation of Cristoballite begins at about cone 5 or 6 (thanks
to Ron Roy and his dilatometry experiments) and accelerates with temperature
increase to cone 10. If you hold at cone 9 and 10 to soak, and cool slowly,
you will develop a lot of Cristoballite. Fast cooling is not my first
choice of remedies for this problem, but it has been proven to work in some
instances.
To respond to your other question, one good source of reasonably priced
refractories in the US is Hi-Temp NW in Portland Oregon. They also have
Kaowool pipe liners for kiln stacks priced at $7.50 per unit.
Kind Regards, Craig Martell-Oregon
Tony Hansen on wed 28 may 97
Gloria Davidson wrote:
> I have recently begun making ovenware from a white porcelineous
> stoneware and have had many returns from cracking after the piece leaves the
> oven. I wondered if anyone in this region is using a cone 10-11 white
> claybody that it doing well with ovenware.
For greatest success make sure your ware has even cross section, no
residual stresses from the process, and that the glaze really fits
(Plainsman
can send you test bars to glaze, fire, and return for strength testing).
Use a body that is not too vitreous (it terminates micro cracks better).
Use a body and firing technique that minimizes the presense of
cristobalite.
Plainsman is working on a low-expansion pyrax ovenware body but
unfortunately
it will be much tougher to fit glazes to. It really is a wonder that we
potters
can make ovenware that actually works when Corningware spends millions
to get
theirs working.
--
=================================================================
Tony Hansen, Plainsman thansen@mlc.awinc.com
Ron Roy on wed 28 may 97
>----------------------------Original message----------------------------
>Hi Everyone,
> I have recently begun making ovenware from a white porcelineous
>stoneware and have had many returns from cracking after the piece leaves the
>oven. I live in British Columbia and have been using a clay bought in N.
>Oregon. Unfortunately, I have had little success with Plainsman's Clay which
>is more accesible geographically to me.
> I wondered if anyone in this region is using a cone 10-11 white
>claybody that it doing well with ovenware.
>Gloria.
Hi Gloria,
Remember - the problem is the combination of clay and glaze. Send me the
recipe and I will tell you how to alter the glaze to get rid of the
problem. Include as much info about the way the glaze looks and as much as
you know about the body.
Some clay bodies develop enough cristobalite to be a big part of this
problem. Call your supplier and see if you can get them to tell you how
much feldspar is in the body.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
ronroy@astral.magic.ca
Emily Henderson on sun 1 jun 97
At 09:01 AM 5/27/97 EDT, you wrote:
>----------------------------Original message----------------------------
> I have recently begun making ovenware from a white porcelineous
>stoneware and have had many returns from cracking after the piece leaves the
>oven. I live in British Columbia and have been using a clay bought in N.
>Oregon. Unfortunately, I have had little success with Plainsman's Clay which
>is more accesible geographically to me.
>_____________________________________________________________________
>Hi There Gloria. I live in Astoria OR. I have had very bad luck in
obtaining good clays in my area. I do buy from George Wright who operates a
small clay company in Manning OR (His phone is the same as his private line;
this is a postage stamp sized town) The body is buff open and short. They
say it fires ^6-11, but I don't thibk it vitrifies 'til a good ^7 I have
had nightmaeres trying to get a glaze to fit at ^6-8 over this stuff, but
some high magnesia glazes with thermal expansions of 58 59 seem to work
quite well at ^10. I have had GREAT success with clays (like White Rose or
MAC) from ClayArt (yes, the same name as the list) in Tacoma. They have
been a little less great lately, but these folks are fast effecient
shippers. I can't handle their shipping costs though and need to either get
into a buyers consortium or keep trying to fit George Wright's clay. At
least I can drive there. I have had horrible experiences with Georgie's in
Portland but have tried only one ^10 clay from Seattle Pottery Supply. It
was nice but I could get just as good, and cheaper, from Tacoma. Can you
let me know who your vendor was for the stuff you weren't thrilled with and
cracked so? Emily, in Astoria, and not at the Saturday Astoria Farmer's
Market cuz it's raining just TOO hard
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