Wade Blocker on tue 13 nov 01
Gretchen,
Not only might you have a problem with your glaze fit, but possibly with
the claybody that yu are using. If the clay has a wide firing range, it is
the wrong clay to use. Mia in ABQ
Gretchen Woodman on tue 13 nov 01
Dear Clayarters,
I am making some large thin bowls and glazing
them only on the inside. I have found a beautiful
glaze that I would like to keep using, but I am having
a problem with the glaze cracking right across the
bottoms of the bowls as it cools. Is this a glaze fit
problem? They are on my counter right now, completely
cool and still making cracking noises. I fire ^6 ox.
I tried to fire a couple pieces again, adding more
glaze over the cracks, but I ended up with new cracks
to replace the old ones, and little pinhole bubbles
all over the bottom.
If this is a glaze fit problem, what is it that I
have to change in the recipe to fix it? If you need
to see the recipe, I will supply it in my next
post-It's over in my studio.
I appreciate any thoughts you may have.
Thank you,
Gretchen Woodman
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Cindy Strnad on tue 13 nov 01
Hi, Gretchen.
Your glaze does not fit. Maybe it's too tight,
maybe too loose. People will need a better
description of the cracking (large pattern? small?
does it flake off? are the bowls actually broken
in this process?) and the particulars on your
glaze to help you.
Best wishes,
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
Imzadi D. on tue 13 nov 01
gretchenwoodman@YAHOO.COM writes:
<< I am making some large thin bowls and glazing them only on the inside. I
have found a beautiful glaze that I would like to keep using, but I am
having a problem with the glaze cracking right across the bottoms of the
bowls as it cools. >>
It is the combination of glazing only one side of your pot and the piece
being so thin. The lopsided shrinkage of the glaze on only one side is
litterally tearing your too thin bowl apart. The thin wall is too fragile to
keep the bowl's structural integity intact.
You might try throwing the whole piece a bit thicker, or the bottom a bit
thicker and glaze inside the footring.
Imzadi
Jocelyn McAuley on tue 13 nov 01
Hi Gretchen,
You mentioned having only glazed the inside of your bowl, right?
Very often, dunting will occur in such an instance due to the loopsided
forces working on your clay. Having glaze just on the inside causes your
bowl to get stressed out from having the glaze expand and contract on that
side only. Kind of like a loopsided tug-of-war game. Usually if you have
that same, or a similar glaze on the outside as well, the sides will be
matched with similar stress from the glaze and not crack.
I would recommend reading up on dunting to come up with some engineering
solutions that can help you out whie keeping your aesthetic. (I've seen
some double walled bowls using this glaze inside only style, with glass as
well! The contrast was really nice.)
Good luck!
--
Jocelyn McAuley ><<'> jocie@worlddomination.net
Eugene, Oregon
http://www.ceramicism.com
Paul Lewing on tue 13 nov 01
on 11/13/01 6:46 AM, Gretchen Woodman at gretchenwoodman@YAHOO.COM wrote:
I am having
> a problem with the glaze cracking right across the
> bottoms of the bowls as it cools. Is this a glaze fit
> problem?
Sure is.
> If this is a glaze fit problem, what is it that I
> have to change in the recipe to fix it? If you need
> to see the recipe, I will supply it in my next
> post
Yes, we need to see the recipe. Chances are the coefficient of expansion is
too low (although it's possible that it's too high) and additions of
alumina, silica or some other oxide could fix it. Given the recipe (and
more importantly, the molecular formula) we could probably make a good stab
at telling you not only what to add or subtract, but how much.
Paul Lewing, Seattle
Wade Blocker on wed 14 nov 01
Bob,
Clays with a wide firing range are far from perfect. eg clay that can be
fired from cone 5 to 10,will not be vitrified at cone 5. Possibly at cone
6, and by cone 10 might be overfired.
I had that bad experience using a cone 5 to 10 white stoneware body from
Laguna.I now fire in the cone 5 to 6 range. Some of my glazes would crack.
A break of the glaze in the center of the ware which was visible in
bright light or was felt by touch.
There were some glazes I used which were not affected, but the basic
problem was still there. Brant from New Mexico Clay, a potter himself, had
alerted me that a cone 5 firing did not vitrify this clay. He suggested I
use a clay body made specifically for the cone 4 to cone 6 range.Since
then I have had no further glaze fit problems.Mia in damp and chilly ABQ
Ron Roy on wed 14 nov 01
Gretchen,
It's probably the glaze having too low an expansion for the clay - send me
the recipe and I may be able to confirm that - may even be able to fix it.
RR
> I am making some large thin bowls and glazing
>them only on the inside. I have found a beautiful
>glaze that I would like to keep using, but I am having
>a problem with the glaze cracking right across the
>bottoms of the bowls as it cools. Is this a glaze fit
>problem? They are on my counter right now, completely
>cool and still making cracking noises. I fire ^6 ox.
>
>I tried to fire a couple pieces again, adding more
>glaze over the cracks, but I ended up with new cracks
>to replace the old ones, and little pinhole bubbles
>all over the bottom.
>
> If this is a glaze fit problem, what is it that I
>have to change in the recipe to fix it? If you need
>to see the recipe, I will supply it in my next
>post-It's over in my studio.
>
>I appreciate any thoughts you may have.
>
> Thank you,
>
> Gretchen Woodman
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Robert Santerre on wed 14 nov 01
Wade, can you enlighten us a bit? Why would a clay with a wide firing range give
more problems with glaze fit and crazing then one with a narrower firing range?
Thanks, Bob
/////////////////////////////////////////////////////////////////////////////////////////////////////////
Wade Blocker wrote:
> Gretchen,
> Not only might you have a problem with your glaze fit, but possibly with
> the claybody that yu are using. If the clay has a wide firing range, it is
> the wrong clay to use. Mia in ABQ
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
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>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy on thu 15 nov 01
I manage the clays for Tuckers and most of Axners clays - together it's
millions of pounds a year. All the cone 6 bodies are to be fired at cone 6
all the cone 8 bodies at 8 and all the cone 10 bodies - well you get the
picture.
This has been going on since 1971 in some cases - yes it is true we have to
instruct some potters on how to fire, and cool and what to expect when the
rules are bent. Some potters get to have their bodies adjusted for the way
they work. Some potters need their minds adjusted and some their bodies but
for the most part the system is working very well.
Bodies are adjusted to take into account raw material variations - In the
case of Tuckers - all clays are tested as they come in and all the
vitrified clays are tested for shrinkage and absorbency.
I can't remember when we have had to take clay back - one advantage of
testing - if an error is made you can catch it.
Anyway this is just my rant to let everyone know - we have such a system
and we have very few customer problems - the key is - if you are going to
make clay close to the proper parameters you better be testing cause
otherwise - well get used to eating clay I guess.
RR
>I think that the commercial clay makers hedge their bets. If they made
>a cone 10 clay to vitrify at cone 10, they would have a lot of customer
>relations problems (more than they have now). There are just too may
>variables out there in the kilns and firings of the people they sell
>clay to. Heck, not all kilns fire evenly to the same cone, top to
>bottom, front to back. I can get the top and bottom even, in mine, but
>the middle is one cone hotter. People would be damaging kilns shelve etc.
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Nanci Bishof on thu 15 nov 01
This may be a dumb question, but why don't I then get cracking of the glaze
on carved tiles that have a big difference in their thickness within the tile
with some areas less than an eighth inch in diameter (relief carving free
from the base tile) and going from approximately 1/4" thick to over 5/8"
thick right next to each other? The tiles are press molded at a mid-carved
state using a rolling pin to push the clay into the latex mold. They are then
allowed to attain leatherhard state and carved again. Tiles are usually 6 to
7 inches in direction on both height and width.
nanci
Earl Brunner on thu 15 nov 01
I think that the commercial clay makers hedge their bets. If they made
a cone 10 clay to vitrify at cone 10, they would have a lot of customer
relations problems (more than they have now). There are just too may
variables out there in the kilns and firings of the people they sell
clay to. Heck, not all kilns fire evenly to the same cone, top to
bottom, front to back. I can get the top and bottom even, in mine, but
the middle is one cone hotter. People would be damaging kilns shelve etc.
Wade Blocker wrote:
> Bob,
> Clays with a wide firing range are far from perfect. eg clay that can be
> fired from cone 5 to 10,will not be vitrified at cone 5. Possibly at cone
> 6, and by cone 10 might be overfired.
> I had that bad experience using a cone 5 to 10 white stoneware body from
> Laguna.I now fire in the cone 5 to 6 range. Some of my glazes would crack.
> A break of the glaze in the center of the ware which was visible in
> bright light or was felt by touch.
> There were some glazes I used which were not affected, but the basic
> problem was still there. Brant from New Mexico Clay, a potter himself, had
> alerted me that a cone 5 firing did not vitrify this clay. He suggested I
> use a clay body made specifically for the cone 4 to cone 6 range.Since
> then I have had no further glaze fit problems.Mia in damp and chilly ABQ
>
> ______________________________________________________________________________
--
Earl Brunner
http://coyote.accessnv.com/bruec/
bruec@anv.net
vince pitelka on fri 16 nov 01
> Anyway this is just my rant to let everyone know - we have such a system
> and we have very few customer problems - the key is - if you are going to
> make clay close to the proper parameters you better be testing cause
> otherwise - well get used to eating clay I guess.
Ron Roy's message gives a good indication of the difference between clay
suppliers. Some, like Tucker and Axner, sell clays for specific
temperatures, and as Ron points out, these suppliers constantly test the
product to make sure it meets very rigid specifications. These clay bodies
are designed to mature at a specific temperature. As I said before, no one
should be suckered into buying a clay which is suppose to serve a broad
temperature range, because that is a lie.
I have no connection to any of these clay suppliers, but I want to see
credit given where credit is due. We all have so much to gain from quality
products and rigid product testing, and we need to suppport the suppliers
like Tucker and Axner who practice such integrity in product manufacture and
sale.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
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