Gay Judson on mon 11 sep 06
I decided that I wanted to work with clay that would be vitrified at cone 6.
So I tested the clay I was using. It falied the test. So did then next 3
or 4 clays I tested. I finally hit on a winner--tested at 1.23% porosity.
Great. Now I make a bunch of work and start glaze firing. !@#$#$!@$!@$
Every peice is terribly pitted. Well, not every piece. The test tiles did
OK with a double dip. And the small test bowls came out with very few
pinholes. But all the larger pieces--just bowls and pitchers even my
mugs--are all impossibly pinholed. In some, the pinholes are like small
volcanos--rough and glassy to the touch. I am using glazes that did fine on
the clay that did not vitrify at cone 6.
Here is my firing schedule: fire up to 2185 F, soak for 20 minutes, down to
1900 F and hold for 30 minutes and then off. I fire in an electronic Skutt
wiht a vent. I place cone packs on the bottom, top, and one of the middle
shelves. My cones come out very consistant, and very evenly fired top to
bottom. Cone 5 is flat, cone 6 is touching and cone 7 is barely moving
forward. I bisque to cone 04.
What do I do now? Give up on the vitrified clay? I am using Armadillo's
Cinco Blanco. I called Armadillo today--they suggested firing a little
higher assuming that the glaze is under fired and that is causing the pin
holes. (In spite of the fact that all their literature cautions against
firing above cone 5.) That is the same schedule I was using before on other
clays without getting the pinholes.
Help, anyone?
Gay Judson, San Antonio, TX
Lynn Goodman Porcelain Pottery on mon 11 sep 06
It sounds like you are doing everything right. If you mixed the glaze
yourself, you could tinker with the recipe to stop the problem; since
this is a commercial glaze, that is out of the question.
Consider the possibility that the glaze and clay may be incompatible.
You may need to switch either the clay or the glaze. Maybe other
clayarters know what vitreous claybody will work with this glaze--what
claybody is this, and which others have you tried?
Lynn
On Sep 11, 2006, at 7:12 PM, Gay Judson wrote:
> I decided that I wanted to work with clay that would be vitrified at
> cone 6.
> So I tested the clay I was using. It falied the test. So did then
> next 3
> or 4 clays I tested. I finally hit on a winner--tested at 1.23%
> porosity.
> Great. Now I make a bunch of work and start glaze firing.
> !@#$#$!@$!@$
> Every peice is terribly pitted. Well, not every piece. The test
> tiles did
> OK with a double dip. And the small test bowls came out with very few
> pinholes. But all the larger pieces--just bowls and pitchers even my
> mugs--are all impossibly pinholed. In some, the pinholes are like
> small
> volcanos--rough and glassy to the touch. I am using glazes that did
> fine on
> the clay that did not vitrify at cone 6...
Lynn Goodman
Fine Porcelain Pottery
Cell 347-526-9805
www.lynngoodmanporcelain.com
Alisa Liskin Clausen on tue 12 sep 06
> Hi Gay,
Argg. I know that you are working very hard mixing glazes and learning a
lot about chemistry.
Regarding pinholing, you said your test work did show some pinholing.
This is a sign that you could have bigger problems with bigger batches. I
would work out more tests that show no pinholing before you glaze a kiln
load. Pinholes usually show up more frequently on horizontal surfaces
than verticle (because the glaze does not movce as much to heal
pinholing), so your test tiles should be ideally both flat and vertical.
But with that said, thanks a lot because now you have glazed, I would also
ask if your bisque ware was clean?
Debris and dust can be a problem, making making pinholes in the glaze
surface.
I would sample your glazes on test pieces and repeat the test in different
places in your kiln, even if your kiln does fire evenly. I would
personally, find others clays to sample. Many clay bodies are listed in a
wide range, like mid fire stoneware, from cone 6 to 10. Absorbency tests
on your clay bodies will tell you much more than the manufacturer's label.
Best regards from Alisa in Denmark
Veena Raghavan on tue 12 sep 06
Hi Gay,
I did not see your original post, but when I was in New York, we had a lot of
pinholing. When we raised the bisque fire (Ron Roy's advice) to cone 04 from
06, the pinholing stopped. Hope this helps along with all the advice you have
received.
Veena
VeenaRaghavan@cs.com
dwichman@frontiernet.net on tue 12 sep 06
I am curious about this too. I had a glaze which I used to consider
bullet proof because it seemed to work well on everything. Then making
some tall round pots, all of a sudden I would get bad pinholing/small
crater blemishes - mainly on the bottom half before the widest part of
the pot. These would not appear on tall cylinders, just the very round
or flat disk shaped pots. Can the shape of a pot make a difference as
to these blemishes? Or the fact the clay may be slightly thicker there?
Debi Wichman
http:/www.elementterra.com
Dave Finkelnburg on tue 12 sep 06
Debi,
I think you are on the right track in suspecting
clay thickness may be at fault. My research indicated
glaze thickness is a factor in formation of
bubbles/pinholes/blisters. There seemed to be a
minimum glaze thickness below which no glaze faults
were observed.
Thicker parts of ware invite a thicker glaze coat.
I think the ware acts as a sponge and the thicker the
"sponge" the thicker the resulting glaze coat.
Trimming groggy clay, however, also causes glaze
faults in my observation, so don't overlook that as a
possibility. The bottom portion of a pot is more
likely to be trimmed.
Good glazing!
Dave Finkelnburg
--- wrote:
> I am curious about this too. I had a glaze which I
> used to consider
> bullet proof because it seemed to work well on
> everything. Then making
> some tall round pots, all of a sudden I would get
> bad pinholing/small
> crater blemishes - mainly on the bottom half before
> the widest part of
> the pot. These would not appear on tall cylinders,
> just the very round
> or flat disk shaped pots. Can the shape of a pot
> make a difference as
> to these blemishes? Or the fact the clay may be
> slightly thicker there?
> Debi Wichman
> http:/www.elementterra.com
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Nancy Braches on tue 12 sep 06
The other day my supplier suggested I "hold" the kiln at the end for 30 minutes to let any remaining toxins to escape at cone 6 to eliminate the pinholing and also the problem I am having with John's Clear over underglazes (Johns clear is a laguna cone 5-10 clear glaze). I have a manual kiln sitter so I have to wait for the kiln to go off , then I turn it back on Medium for 30 minutes to hold the temp. I'm going to try it on my next glaze firing to see if it works.
Nancy
Hilltop Pottery
Veena Raghavan wrote: Hi Gay,
I did not see your original post, but when I was in New York, we had a lot of
pinholing. When we raised the bisque fire (Ron Roy's advice) to cone 04 from
06, the pinholing stopped. Hope this helps along with all the advice you have
received.
Veena
VeenaRaghavan@cs.com
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Lee Love on tue 12 sep 06
On 9/12/06, Dave Finkelnburg wrote:
>
> Debi,
> I think you are on the right track in suspecting
> clay thickness may be at fault.
>
Sponging the bisque with water adjusts for wall thickness.
--
Lee in Mashiko, Japan
http://potters.blogspot.com/
"Let the beauty we love be what we do." - Rumi
Gay Judson on wed 13 sep 06
Thanks to the many who have contributed their thoughts on my dilemma. The
consensus seems to be that the clay is still releasing gasses from burning
impurities. I am about to refire the pieces that came out such a mess to a
little higher temperature and see if the glazes smooth out. And then I am
going to fire the next bisque firing longer, slower. See if I can catch the
villan! What an incredible resource this group is. I am AGAIN most
greateful for the generously shared wisdom here at Clayart!
Gay Judson, San Antonio, TX
Snail Scott on wed 13 sep 06
On Sep 12, 2006, at 3:24 PM, Nancy Braches wrote:
> The other day my supplier suggested I "hold" the kiln at the end for
> 30 minutes...I have a manual kiln sitter so I have to wait for the
> kiln to go off , then I turn it back on Medium for 30 minutes to hold
> the temp...
>
Why let it turn off? Use a witness cone to determine
the proper temperature, and use a higher cone in
the sitter as a failsafe. Turning to 'medium' tends to
allow more cooling than is best for a soak (I use that
method to do give a slower cooling for my matte
glazes), but try turning just the middle to 'medium' -
for a mid-range stoneware firing, that often seems
sufficient to stall it out without actually allowing the
temperature to drop much. It can be a little uneven,
but not as much as you might think. Turning all the
switches to 'medium' for a short time, then back to
'high' and alternating for a while, will give a better
soak, but be more labor-intensive.
-Snail
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