Cobus Potgieter on sun 10 dec 00
The following appear in a little book edited by Barbara Tipton "Great =
Ideas for Potters", published by CERAMICS MONTHLY:
"Models for casting molds of thrown ware may be formed directly on the =
wheel with the following mixture:
Plaster Throwing Body
Kentucky Ball Clay (OM4)...................14.00%
Pottery Plaster (1) ............................78.50%
Bentonite .......................................... 6.00%
Sodium Citrate ................................... 1.50%
Make up only as much of the throwing plaster as will be used in making a =
single object on the wheel. Mix the dry ingredients and add them to =
water equivalent to 25% of the weight of the dry ingredients. A little =
more water may be added as necessary. Wedge the materials thoroughly. =
The throwing plaster will remain workable for about 20 minutes. This =
period of time may be extended by increasing the sodium citrate =
slightly."
=20
I dry mixed the materials by putting them all in a Tupperware container =
and shake and turn the container a while. At first I felt frantic when =
I mixed the water and started wedging: the stuff would not get plastic, =
and I was afraid that it was going to set. I added more water, kept on =
wedging, and it started to respond like clay, and it didn't set. I =
added enough water until it felt like the clay that I throw with. From =
then I took my time. I centered, threw and worked on the piece until I =
was satisfied with it. Still it didn't set. Then I left it. After =
quite a while, it started setting. During the process of drying, it =
developed a white hairy growth on the surface (I suspect that these are =
salts that migrated to the surface together with the water). I left it =
until the piece was complete dry, and I sanded it lightly. The dry =
piece was like plaster - hard. I sealed the surface and continued to =
make a mold for slipcasting as if this was a piece that was turn on the =
plaster wheel.
I didn't try to fire it. I don't think that this will pose such a big =
problem. Some of the brick- and tile clays from Europe have lots of CaO =
in it (some red clays here in Namibia as well). The CaO however must be =
finer than 100# and it cannot be fired over 1100=B0C. The higher the =
CaO content, the weaker will the fired body be (depending on temperature =
and other fluxes) , but for wall tiles, and architectural mouldings, it =
would not matter that much.
Cobus Potgieter.
NAMIBIA CERAMICS Tel: +264-61-264526
PO Box 6742 Fax: +264-61-261590
Ausspannplatz Mobile: 081 124 7094
Windhoek
NAMIBIA
http://www.orusovo.com/namceram
namceram@orusovo.com
Ababi Sharon on fri 15 dec 00
Hello Cobus!
Did you try to prepare theClay/plaster by soaking the bentonite with water
overnight?
Ababi Sharon
ababisharon@hotmail.com
http://www.milkywayceramics.com/cgallery/asharon.htm
http://www.israelceramics.org/index.html
* * * * * * * *
www.photoisland.com
ID: sharon@shoval.org.il
Password:clay
----- Original Message -----
From: "Cobus Potgieter"
To:
Sent: Sunday, December 10, 2000 10:17 AM
Subject: Clay/Plaster
The following appear in a little book edited by Barbara Tipton "Great Ideas
for Potters", published by CERAMICS MONTHLY:
"Models for casting molds of thrown ware may be formed directly on the wheel
with the following mixture:
Plaster Throwing Body
Kentucky Ball Clay (OM4)...................14.00%
Pottery Plaster (1) ............................78.50%
Bentonite .......................................... 6.00%
Sodium Citrate ................................... 1.50%
Make up only as much of the throwing plaster as will be used in making a
single object on the wheel. Mix the dry ingredients and add them to water
equivalent to 25% of the weight of the dry ingredients. A little more water
may be added as necessary. Wedge the materials thoroughly. The throwing
plaster will remain workable for about 20 minutes. This period of time may
be extended by increasing the sodium citrate slightly."
I dry mixed the materials by putting them all in a Tupperware container and
shake and turn the container a while. At first I felt frantic when I mixed
the water and started wedging: the stuff would not get plastic, and I was
afraid that it was going to set. I added more water, kept on wedging, and
it started to respond like clay, and it didn't set. I added enough water
until it felt like the clay that I throw with. From then I took my time. I
centered, threw and worked on the piece until I was satisfied with it.
Still it didn't set. Then I left it. After quite a while, it started
setting. During the process of drying, it developed a white hairy growth on
the surface (I suspect that these are salts that migrated to the surface
together with the water). I left it until the piece was complete dry, and I
sanded it lightly. The dry piece was like plaster - hard. I sealed the
surface and continued to make a mold for slipcasting as if this was a piece
that was turn on the plaster wheel.
I didn't try to fire it. I don't think that this will pose such a big
problem. Some of the brick- and tile clays from Europe have lots of CaO in
it (some red clays here in Namibia as well). The CaO however must be finer
than 100# and it cannot be fired over 1100°C. The higher the CaO content,
the weaker will the fired body be (depending on temperature and other
fluxes) , but for wall tiles, and architectural mouldings, it would not
matter that much.
Cobus Potgieter.
NAMIBIA CERAMICS Tel: +264-61-264526
PO Box 6742 Fax: +264-61-261590
Ausspannplatz Mobile: 081 124 7094
Windhoek
NAMIBIA
http://www.orusovo.com/namceram
namceram@orusovo.com
____________________________________________________________________________
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
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Cobus Potgieter on sat 16 dec 00
Hi Ababi,
No, I just dry mixed all the ingredients (which were all over 200#)
thoroughly, and started adding water. If you soak this mixture overnight,
you will have problems with the plaster setting. You can't leave the
plaster out, soak, and mix the next day. The plaster has to be mixed with
the rest of the ingredients, whilst all are still dry.
In the Middle East, people use a technique called "gach-e-koshteh" (killed
plaster). According to Nader Khalili: CERAMIC HOUSES & EARTH ARCHITECTURE,
p.141: "To kill the plaster we must make it into a doughy paste and knead it
for over half an hour. Such a mix will stay pliable for two to three days".
I am under the impression that Hassan Fathy also referred to this technique
in his books.
The sodium citrate is the retarding agent to keep the plaster from setting.
Salt, or previously set and ground plaster has the opposite effect.
I suggest that you mix about a kilo and play with it - all you can loose is
a bit of time and a small amount of raw materials. You will learn more from
this practical experience than from first trying to enquire all the methods
and scientific explanations (a lesson that took me a long time to learn).
In rethinking I am of the opinion that my earlier statement that the given
recipe (throwable plaster)might not pose problems in firing, I must add the
following:
Gault clays (UK) have a CaO content of +/- 20%. The bricks were fired to
1050°C, and the high CaO content caused the Fe2O3 to bleach to a nearly
white brick (in reduction atmosphere). These clays were formed during the
Cretaceous period. (HEAVY CLAY TECHNOLOGY, by FH Clewis)
Some Namibian clays also have a high CaOCO3 content, and sometimes Gypsum as
the source for CaO. By grinding these clays very fine, and adding BaCO3 to
the clays containing Gypsum, some problems can be solved.
In conclusion, the throwable plaster recipe has a CaO content of nearly 45%
(calculated on INSIGHT), which will result in a very porous and weak body.
I experimented with bodies by adding Portland Cement up to 20%, and even
higher percentages of Dolomite. These in general wielded soft fired bodies
(at earthenware temperatures).
----- Original Message -----
From: "Ababi Sharon"
To:
Sent: Friday, December 15, 2000 7:00 PM
Subject: Re: Clay/Plaster
Hello Cobus!
Did you try to prepare theClay/plaster by soaking the bentonite with water
overnight?
Ababi Sharon
ababisharon@hotmail.com
http://www.milkywayceramics.com/cgallery/asharon.htm
http://www.israelceramics.org/index.html
* * * * * * * *
www.photoisland.com
ID: sharon@shoval.org.il
Password:clay
----- Original Message -----
From: "Cobus Potgieter"
To:
Sent: Sunday, December 10, 2000 10:17 AM
Subject: Clay/Plaster
The following appear in a little book edited by Barbara Tipton "Great Ideas
for Potters", published by CERAMICS MONTHLY:
"Models for casting molds of thrown ware may be formed directly on the wheel
with the following mixture:
Plaster Throwing Body
Kentucky Ball Clay (OM4)...................14.00%
Pottery Plaster (1) ............................78.50%
Bentonite .......................................... 6.00%
Sodium Citrate ................................... 1.50%
Make up only as much of the throwing plaster as will be used in making a
single object on the wheel. Mix the dry ingredients and add them to water
equivalent to 25% of the weight of the dry ingredients. A little more water
may be added as necessary. Wedge the materials thoroughly. The throwing
plaster will remain workable for about 20 minutes. This period of time may
be extended by increasing the sodium citrate slightly."
I dry mixed the materials by putting them all in a Tupperware container and
shake and turn the container a while. At first I felt frantic when I mixed
the water and started wedging: the stuff would not get plastic, and I was
afraid that it was going to set. I added more water, kept on wedging, and
it started to respond like clay, and it didn't set. I added enough water
until it felt like the clay that I throw with. From then I took my time. I
centered, threw and worked on the piece until I was satisfied with it.
Still it didn't set. Then I left it. After quite a while, it started
setting. During the process of drying, it developed a white hairy growth on
the surface (I suspect that these are salts that migrated to the surface
together with the water). I left it until the piece was complete dry, and I
sanded it lightly. The dry piece was like plaster - hard. I sealed the
surface and continued to make a mold for slipcasting as if this was a piece
that was turn on the plaster wheel.
I didn't try to fire it. I don't think that this will pose such a big
problem. Some of the brick- and tile clays from Europe have lots of CaO in
it (some red clays here in Namibia as well). The CaO however must be finer
than 100# and it cannot be fired over 1100°C. The higher the CaO content,
the weaker will the fired body be (depending on temperature and other
fluxes) , but for wall tiles, and architectural mouldings, it would not
matter that much.
Cobus Potgieter.
NAMIBIA CERAMICS Tel: +264-61-264526
PO Box 6742 Fax: +264-61-261590
Ausspannplatz Mobile: 081 124 7094
Windhoek
NAMIBIA
http://www.orusovo.com/namceram
namceram@orusovo.com
____________________________________________________________________________
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
____________________________________________________________________________
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
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