Paul Borian on thu 15 jul 10
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
i did mix a few batches using 100 mesh mulcoa grog and i think that
contributed to the problem because it increased the amount of fine particle=
s
in the body. It also caused some de-lamination of the pugged clay which may
also have played a factor in the cracking. Then i started using the 35 mesh
mulcoa and it helped but the cracks have not totally gone away.
in my next batch i plan to increase the kyanite and the mulcoa until the
total percentage of plastics is about 65%, rather than the 70% it is now. M=
y
old clay body was 65% plastics and never cracked - but it was not refractor=
y
enough for the temp i am firing to now so i had to change it. Basically, it
had too much fireclay and not enough kaolin.
i may go as high as 15% on the grog, at least for one batch, to see if it
solves the problem. I have an intuitive sense that it will (based not so
much on intuition but rather my experience of mixing clay for about 6
years). I have varied the percentage of grog quite a bit over the years and
it almost always seems that if i don't have enough of it (and enough other
non-plastics) then i get cracks.
any other comments?
thanks,
Paul
---------- Forwarded message ----------
From: Paul Herman
Date: Wed, Jul 14, 2010 at 9:30 PM
Subject: Re: Problems associated with too much grog?
To: Paul Borian
Cc: Clayart@lsv.ceramics.org
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 i=
s
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 f=
or
> 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 mo=
re
> grog is the best solution i can think of to cut back on that shrinkage ju=
st
> 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
>
Eric Hansen on thu 15 jul 10
Paul: that recipe is a lot of large particles and not much "glue" to hold i=
t
all together. Both the g200 and the Foundry Hill creme have alkaline fluxes=
,
now that is good.
I'd say the recipe is a bit too "short" however if you feel that it fluxes
to the right level then you can add more foundry hill creme to increase
plasticity and take away some of the g200 to keep the fluxes about the same=
.
Another way, and this might be better, it to add a good 200 mesh or whateve=
r
white-burning ball clay. The kyanite and mulcoa look like plenty of grog, I
wouldn't think increasing grog will help. You might try experimenting with
finer mesh grog as it will mean more grog particles throughout the mixture.
Any way, I'm no expert but normally have very good "horse sense"
h a n s e n
p.s. you can calcine your hawthorn clay to cone 9, the dry clay on a shelf,
and use it for grog, that also might keep the particle size smaller too.
On Thu, Jul 15, 2010 at 12:21 PM, Paul Borian wrote=
:
> 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
>
>
> i did mix a few batches using 100 mesh mulcoa grog and i think that
> contributed to the problem because it increased the amount of fine
> particles
> in the body. It also caused some de-lamination of the pugged clay which m=
ay
> also have played a factor in the cracking. Then i started using the 35 me=
sh
> mulcoa and it helped but the cracks have not totally gone away.
>
> in my next batch i plan to increase the kyanite and the mulcoa until the
> total percentage of plastics is about 65%, rather than the 70% it is now.
> My
> old clay body was 65% plastics and never cracked - but it was not
> refractory
> enough for the temp i am firing to now so i had to change it. Basically, =
it
> had too much fireclay and not enough kaolin.
>
> i may go as high as 15% on the grog, at least for one batch, to see if it
> solves the problem. I have an intuitive sense that it will (based not so
> much on intuition but rather my experience of mixing clay for about 6
> years). I have varied the percentage of grog quite a bit over the years a=
nd
> it almost always seems that if i don't have enough of it (and enough othe=
r
> non-plastics) then i get cracks.
>
> any other comments?
>
> thanks,
> Paul
>
>
>
> ---------- Forwarded message ----------
> From: Paul Herman
> Date: Wed, Jul 14, 2010 at 9:30 PM
> Subject: Re: Problems associated with too much grog?
> To: Paul Borian
> Cc: Clayart@lsv.ceramics.org
>
>
> 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 the=
re
> 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 material=
s
> 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 o=
n
> > 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 th=
e
> > 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 abo=
ve
> > mentioned cracks; so last week i stepped it up to 12% but have not fire=
d
> > 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
> >
>
--
Eric Alan Hansen
Stonehouse Studio Pottery
Alexandria, Virginia
americanpotter.blogspot.com
thesuddenschool.blogspot.com
hansencookbook.blogspot.com
"To me, human life in all its forms, individual and aggregate, is a
perpetual wonder: the flora of the earth and sea is full of beauty and of
mystery which seeks science to understand; the fauna of land and ocean is
not less wonderful; the world which holds them both, and the great universe
that folds it in on everyside, are still more wonderful, complex, and
attractive to the contemplating mind." - Theodore Parker, minister,
transcendentalist, abolitionist (1810-1860)
Paul Borian on thu 15 jul 10
the question was asked about how i mix the clay - so i will try to explain
that part briefly.
first of all, i have used the same mixing process for years and these types
of cracks did not occur. All aspects of the mixing, pugging, throwing and
bisque firing processes are exactly the same - the only thing that changed
was the clay body.
anyway, i mix my clay by dumping two 5 gallon buckets of wet reclaim into m=
y
mixer and at least 5 gallons of water, then let it mix for a while; then i
add all the clays, followed by the feldspar and kyanite, then the grog goes
in last. The total amount of dry materials is 300 lbs. I usually add more
water as needed.
the delamination was only happening when i used the 100 mesh grog. I would
cut a cross section of pugged clay (even after sitting for days/weeks) and
it would come apart in the pug spiral. This did not interfere all all with
the throwing process - the clay was excellent on the wheel - but it may hav=
e
played a part in the cracks.
Once i went back to the 35 mesh grog the delamination went away.
thanks,
paul
---------- Forwarded message ----------
From: L TURNER
Date: Thu, Jul 15, 2010 at 1:56 PM
Subject: Re: Problems associated with too much grog?
To: Paul Borian
Cc: Clayart@lsv.ceramics.org
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
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