karen lovenguth on mon 25 mar 02
Dear Clayarters,
It is a pleasure. This is my first
letter and I hope there is someone who can help me!
I build large coil pots and usually
smooth them to be able to burnish and apply terra
sigs. This past year I completed about twelve pieces
only to discover that my clay was not responding as it
had before .I have either lost them to cracking in my
gas kiln or in the second firing which are pit fired.
I live in Mexico and so all those great clay companies
that sell premixed clay here do not exist. I have
asked many ceramists locally for help and I am
currently mixing up a clay that consists of the
following
50 % OM 4 ball clay
30 % fire clay called zacatecas
10 % feldspar
10 % talco
to this I am adding paper about 1 toilet paper roll to
every 10 kilos.
I still have not fired one of these new large pots
with the new mix. I guess I should not despair yet. Is
there anyone who works in the same direction out there
that can help? About 60% of my work this year has
cracked. I am on the way to cracking myself.
Ok lets take it for granted that My new clay mix will
come out of the gas kiln intact. The pit firings are
tremendously violent. Let me put my questions into
form
1. I find that pit firings with flame seem to break
the ware almost always. Am I looking for more of a
smoldering fire?
2. I have used both dung and sawdust. The sawdust
fires quickly (one day) and the dung is amazingly slow
(3 weeks) to burn a meter deep. My pit kilns are built
with red brick and accomodated to the piece I am
firing. Perhaps to much air enters through the spaces
between the bricks. At night it also gets cool and
perhaps the temperature differences crack the clay?
3. Like i said I have not tried the new mix yet but
is there anything else I could add to the body to give
it strength with these types of firings? Is mullite a
possibility to open up the body?
4. Should I be looking at raku type bodies?
5. I would like to keep using the ball clay because I
feel it more reliable and I also use the om4 ball clay
for my sigillatas.Does anyone use a similar recipe and
fire in the way I do that could share their
experiences?
6. I am ordering a book by karin hessenburg about
sawdust firings. Are alot of my questions answered
there already? It is going to take at least a couple
of weeks for it to arrive and that is about the time I
have to fire before the rains start.
7. I thought that maybe sagger firing could also be a
solution but my pieces are so large and so that will
require many changes.
Does anyone have any ideas? Little tricks? A small
miracle or two. I fired a big pot today and it came
out in pieces. I will not give up. This is the work
that I want to do. I have one large beautiful
burnished piece with a spectacular cloud flower (unka)
effect as the japanese call the technique that worked.
Proof that it can be done. After that nothing has
worked. I thank you all for listening. Karen in
Sto.Domingo, Ocotitlan
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vince pitelka on mon 25 mar 02
> 50 % OM 4 ball clay
> 30 % fire clay called zacatecas
> 10 % feldspar
> 10 % talco
> to this I am adding paper about 1 toilet paper roll to
> every 10 kilos.
Karen -
I am surprised that you do not have 100% breakage with that recipe, with so
much ball clay and no grit at all. My heart goes out to you, but I wonder
where this recipe came from? There is a lowfire handbuilding body which is
50% ball clay and 50% talc, but it works only because of the high percentage
of talc and the peculiar behavior of talc. Ball clay is composed of
extremely fine particles, which means high drying shrinkage, which
drastically increases the likelihood of cracking in drying or firing. For a
handbuilding body intended for pitfiring I would reduce the ball clay to
20%, increase the fireclay to 35%, keep everything else as it is, and then
add 15% fine grog or sand. That should eliminate your problems, although
you will still want to fire cautiously to avoid abrupt thermal shock. Large
pieces are hard to pitfire because of uneven heating/cooling.
Good luck -
- 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/
Roger Korn on tue 26 mar 02
Or go even further, depending on how little plasticity you can work with:
40% Lincoln 60 Fireclay
20% OM4 ball clay
20% 60- sand or bisque grog
10% Potash spar
10% Talc
Just a thought. I use a gritty terra cotta straight out of the ground here at
Arkysanto - real open, but still sticky enough to work with.
Vince, you know this stuff way better than me, what do you think of the recipe
above?
Roger
vince pitelka wrote:
> > 50 % OM 4 ball clay
> > 30 % fire clay called zacatecas
> > 10 % feldspar
> > 10 % talco
> > to this I am adding paper about 1 toilet paper roll to
> > every 10 kilos.
>
> Karen -
> I am surprised that you do not have 100% breakage with that recipe, with so
> much ball clay and no grit at all. My heart goes out to you, but I wonder
> where this recipe came from? There is a lowfire handbuilding body which is
> 50% ball clay and 50% talc, but it works only because of the high percentage
> of talc and the peculiar behavior of talc. Ball clay is composed of
> extremely fine particles, which means high drying shrinkage, which
> drastically increases the likelihood of cracking in drying or firing. For a
> handbuilding body intended for pitfiring I would reduce the ball clay to
> 20%, increase the fireclay to 35%, keep everything else as it is, and then
> add 15% fine grog or sand. That should eliminate your problems, although
> you will still want to fire cautiously to avoid abrupt thermal shock. Large
> pieces are hard to pitfire because of uneven heating/cooling.
> Good luck -
> - 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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>
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>
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--
Roger Korn
McKay Creek Ceramics
In AZ: PO Box 463
4215 Culpepper Ranch Rd
Rimrock, AZ 86335
928-567-5699 <-
In OR: PO Box 436
31330 NW Pacific Ave.
North Plains, OR 97133
503-647-5464
Mondloch on tue 26 mar 02
vince,
Since it sounds like she has access to mullite, don't you think that would
also be a good addition to the body to improve thermal shock resistance?
Sylvia
---
Mark & Sylvia Mondloch
Silver Creek Pottery & Forge
W6725 Hwy 144
Random Lake ,Wi 53075
HotArt@silvercreekpottery.com
http://www.silvercreekpottery.com
----- Original Message -----
From: "vince pitelka"
To:
Sent: Monday, March 25, 2002 8:15 PM
Subject: Re: large coiled pots and breakage in pit firings
> > 50 % OM 4 ball clay
> > 30 % fire clay called zacatecas
> > 10 % feldspar
> > 10 % talco
> > to this I am adding paper about 1 toilet paper roll to
> > every 10 kilos.
>
> Karen -
> I am surprised that you do not have 100% breakage with that recipe, with
so
> much ball clay and no grit at all. My heart goes out to you, but I wonder
> where this recipe came from? There is a lowfire handbuilding body which
is
> 50% ball clay and 50% talc, but it works only because of the high
percentage
> of talc and the peculiar behavior of talc. Ball clay is composed of
> extremely fine particles, which means high drying shrinkage, which
> drastically increases the likelihood of cracking in drying or firing. For
a
> handbuilding body intended for pitfiring I would reduce the ball clay to
> 20%, increase the fireclay to 35%, keep everything else as it is, and then
> add 15% fine grog or sand. That should eliminate your problems, although
> you will still want to fire cautiously to avoid abrupt thermal shock.
Large
> pieces are hard to pitfire because of uneven heating/cooling.
> Good luck -
> - 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/
vince pitelka on tue 26 mar 02
> 40% Lincoln 60 Fireclay
> 20% OM4 ball clay
> 20% 60- sand or bisque grog
> 10% Potash spar
> 10% Talc
> Vince, you know this stuff way better than me, what do you think of the
recipe
> above?
> Roger
Roger -
I realized that my suggested recipe did not add up to 100 any more. I
should have said 45 fire clay, 30 talc, 15 fine sand or grog, etc. But
yours would work as well. The recipe I use for bonfiring is equal parts
goldart, ball clay, fire clay, and fine grog. Needless to say, at lowfire
temperatures it has extremely high thermal shock resistance.
- 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/
karen lovenguth on sat 30 mar 02
Hi everyone,
Its Karen and I want to give a big thanks
to you all. So many letters!!!! I have learned so
much. I will write a summary of all your collective
ideas for those ceramists that might one day have the
same questions and also for myself because I can be a
bit forgetful at times. Give me a few days to put it
all together. I felt a warm welcome to this wonderful
group!!!I will also keep you all posted on my
progress. I return to my studio once again with
energy!
Best wishes K
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