Ann Brink on wed 14 jul 10
I hesitate to comment, because I don't often mix clay bodies, but I do
remember that I had cracking problems with some I made last year, and
thought it was from too much grog, instead of not enough. I believe it was
about 10% as well. I will watch for other responses.
Ann Brink in Lompoc Ca
(mostly about pottery)
----- Original Message -----
From: "Paul Borian"
To:
Sent: Wednesday, July 14, 2010 10:48 AM
Subject: Problems associated with too much grog?
>i recently made some major changes to my clay body which resulted in a
> considerable improvement to my product line. i have been fine tuning it
> for
> a while and i have one last problem to try and fix, which is a tendency t=
o
> crack on certain forms. It almost never is the common s-cracks, but rathe=
r
> cracks on the walls that start in the direction of the throwing lines.
> Mostly on thicker, straight walled forms - in fact it does not happen on
> bowls at all. I did thousands of such forms with my old clay body and thi=
s
> never happened, and all other aspects of the production system are the
> same.
>
> to try and make a long story short, i have a feeling that increasing the
> grog is the best way to deal with this kind of thing. The new body has a
> somewhat finer texture and a bit more shrinkage than the old one - and
> more
> grog is the best solution i can think of to cut back on that shrinkage
> just
> a bit and make the body a little more course.
>
> The last few batches i mixed up had 10% mulcoa grog and i got those above
> mentioned cracks; so last week i stepped it up to 12% but have not fired
> any
> of the pots yet. If that does not work, i am thinking to go as high as
> 15%.
>
> can anyone think of potential problems with using that much grog in a
> stoneware body? Anyone else using that much?
>
> thanks!
> Paul
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23:36:00
Paul Herman on wed 14 jul 10
Paul,
You might try using "non plastics" instead of grog. Materials that
don't shrink in drying are much more varied and available than just
grog.
Think about various mesh sizes of sand, silica and feldspar. I'm sure
there are a lot more potential ingredients that don't have a wet to
dry shrinkage.
Would you post the recipe? I'm a promoter of sharing clay recipes, if
you haven't noticed. Normally a body needs 20% or 30% of non plastic
materials to work right, IMHO. Of course some porcelain bodies are
less than half clay, but that is the exception, not the rule.
For my clay bodies, the highest percentage of sand and/or grog that I
use is 12%.
Good luck! Clay making is a whole nother deal.
Paul Herman
Great Basin Pottery
Doyle, California US
www.greatbasinpottery.com/
PS I think nother should be in the dictionary too, don't all you
calyartors?
On Jul 14, 2010, at 10:48 AM, Paul Borian wrote:
> i recently made some major changes to my clay body which resulted in a
> considerable improvement to my product line. i have been fine tuning
> it for
> a while and i have one last problem to try and fix, which is a
> tendency to
> crack on certain forms. It almost never is the common s-cracks, but
> rather
> cracks on the walls that start in the direction of the throwing lines.
> Mostly on thicker, straight walled forms - in fact it does not
> happen on
> bowls at all. I did thousands of such forms with my old clay body
> and this
> never happened, and all other aspects of the production system are
> the same.
>
> to try and make a long story short, i have a feeling that increasing
> the
> grog is the best way to deal with this kind of thing. The new body
> has a
> somewhat finer texture and a bit more shrinkage than the old one -
> and more
> grog is the best solution i can think of to cut back on that
> shrinkage just
> a bit and make the body a little more course.
>
> The last few batches i mixed up had 10% mulcoa grog and i got those
> above
> mentioned cracks; so last week i stepped it up to 12% but have not
> fired any
> of the pots yet. If that does not work, i am thinking to go as high
> as 15%.
>
> can anyone think of potential problems with using that much grog in a
> stoneware body? Anyone else using that much?
>
> thanks!
> Paul
Paul Borian on wed 14 jul 10
i recently made some major changes to my clay body which resulted in a
considerable improvement to my product line. i have been fine tuning it for
a while and i have one last problem to try and fix, which is a tendency to
crack on certain forms. It almost never is the common s-cracks, but rather
cracks on the walls that start in the direction of the throwing lines.
Mostly on thicker, straight walled forms - in fact it does not happen on
bowls at all. I did thousands of such forms with my old clay body and this
never happened, and all other aspects of the production system are the same=
.
to try and make a long story short, i have a feeling that increasing the
grog is the best way to deal with this kind of thing. The new body has a
somewhat finer texture and a bit more shrinkage than the old one - and more
grog is the best solution i can think of to cut back on that shrinkage just
a bit and make the body a little more course.
The last few batches i mixed up had 10% mulcoa grog and i got those above
mentioned cracks; so last week i stepped it up to 12% but have not fired an=
y
of the pots yet. If that does not work, i am thinking to go as high as 15%.
can anyone think of potential problems with using that much grog in a
stoneware body? Anyone else using that much?
thanks!
Paul
Lee Love on wed 14 jul 10
Grog makes the fired body weaker. See Pete Pinnell's posting on the subj=
=3D
ect.
--
Lee, a Mashiko potter in Minneapolis
http://mingeisota.blogspot.com/
=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi
paul gerhold on thu 15 jul 10
Paul,
It is not just the amount of grog but equally important is particle size
distribution. A wider range is better in almost all cases is better. One
of the keys to your problem may be your statement that your body is now
finer in texture and has more shrinkage. If you go too high in grog you ma=
y
have to make other changes to keep the body vitreous. If your body has
silica a larger particle size could help.
Paul
>
L TURNER on thu 15 jul 10
Paul,
Questions for clarification.
How do you mix your clay? How much water do you use and do you slake
the clay components before adding the non plastics?
Your comment "de-lamination of the pugged clay " makes me wonder if
the clay components (especially the 50 mesh fireclay) have been
uniformly moistened.
Regards,
L Turner
The Woodlands, TX
David Finkelnburg on thu 15 jul 10
Paul,
Grog should prevent, rather than cause, the problem you describe.
Are these cracks from throwing? In my own work I have seen cracks that
follow the throwing lines. I have hypothesized that the cause is forming a
layer of slip during the throwing process and then having greater shrinkage
in that slip. Thicker walls mean less mixing of the clay occurs during
throwing, so any slip worked up doesn't get mixed back into the rest of the
body as effectively. I have never seen cracks like this in thin pieces,
only in throwing thick forms. The grog may be coincidental rather than th=
e
cause.
Good potting!
Dave Finkelnburg
http://www.mattanddavesclays.com
Date: Wed, 14 Jul 2010 13:48:51 -0400
From: Paul Borian
Subject: Problems associated with too much grog?
i recently made some major changes to my clay body which resulted in a
considerable improvement to my product line. i have been fine tuning it for
a while and i have one last problem to try and fix, which is a tendency to
crack on certain forms. It almost never is the common s-cracks, but rather
cracks on the walls that start in the direction of the throwing lines.
Mostly on thicker, straight walled forms - in fact it does not happen on
bowls at all. I did thousands of such forms with my old clay body and this
never happened, and all other aspects of the production system are the same=
.
to try and make a long story short, i have a feeling that increasing the
grog is the best way to deal with this kind of thing. The new body has a
somewhat finer texture and a bit more shrinkage than the old one - and more
grog is the best solution i can think of to cut back on that shrinkage just
a bit and make the body a little more course.
The last few batches i mixed up had 10% mulcoa grog and i got those above
mentioned cracks; so last week i stepped it up to 12% but have not fired an=
y
of the pots yet. If that does not work, i am thinking to go as high as 15%.
can anyone think of potential problems with using that much grog in a
stoneware body? Anyone else using that much?
Michael Wendt on fri 16 jul 10
Paul,
Kyanite undergoes a physical size change during firing:
http://www.axner.com/kyanite-100-mesh.aspx
Axner claims 1-9% increase.
Brick makers add Kyanite to offset thermally induced
shrinkage in high temperature bricks.
Depending upon several factors including grain size , firing
temperature and dwell, the expansion can sometimes result in
body cracking according to Joe Brecha at the Clay Art Center
in Tacoma WA. He says he can't put that much Kyanite without
cracking problems.
On a separate note:
If you want to increase the refractory properties of your
body, have you considered trying Helmer Kaolin?
It is good to cone 34.
It is very plastic.
I can give you the protocol for body development I used for
body development and testing when I worked for the Idaho
Research Foundation in the early 70s.
The body I make is worth trying as a starting point since it
is ready mixed and can be tested right away and I can send
you a pure Helmer sample to try as a body addition.
Regards,
Michael Wendt
Paul listed his ingredients for his clay body as follows:
thanks for the replies so far.
here is the clay body i refer to:
epk 36
foundry hill creme 12
hawthorne bond (50 mesh) 22
g200 11
kyanite (100 mesh) 10
Mulcoa grog (35 mesh) 10
ivor and olive lewis on fri 16 jul 10
The major issues caused by introducing Grog depend in part on the origin of
the Grog and its Physical and Chemical characteristics.
The first thing to consider is the proportion of grog to compounded clay
body. If a body is designed to mature at a specific Cone Value then it will
have sufficient fluxing agents to accomplish that propose. Adding grog may
reduce or increase those necessary compounds. So Grog ( or a substance used
for the same purpose) may diminish or increase the volume of vitreous phase
contributed to the total new mixture. The overall effect may be to enhance
or diminish the strength and permeability of the fired product.
Regards,
Ivor Lewis,
REDHILL,
South Australia
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