Dave Finkelnburg on fri 23 mar 07
Dear Ivor,
Regarding your question, I do not know whether
anything has been added to this view.
There is much which I wish I did understand better
about clay bodies. I am much more comfortable with
the little I know about glazes!
As I'm sure you are aware plasticity of clay bodies
is influenced to some extent by at least these 5
factors and there must be others besides these:
1. particle packing
2. moisture content
3. adsorbed organics
4. pH
5. ionic concentration and strength
The question I keep asking myself is how to rank
these in order of how much effect they have.
Certainly 4 and 5, at the wrong end of the spectrum,
can deflocculate a clay body and utterly destroy
plasticity, so simply having clay and water present is
not enough.
I suspect 3 is the major player in ball clays, more
than fine particle size, and I recall some indication
from the literature that this is so. At least one
clay company here in the US added acrylic polymer
plasticizers to its bagged clay for a time, but then
abandoned them saying the effect was not great enough
to measure.
The effect of 2 is over a relatively narrow range.
Matt Katz cited 19-24% in his NCECA presentation.
Katz seemed to say 1 is the dominant factor. I am
sure that would require pH be neutral to below and
salts (5) not be present in the body in a high enough
concentration to cause deflocculation, and moisture be
in the range he cited.
I have a suspicion the importance of particle
packing on plasticity may not be anything new at all,
but rather something artisans figured out a long time
ago. It may just be news to ignorant folks like me!
:-)
I hope the changing of the seasons is bringing you
cooler days.
Dave Finkelnburg
Date: Fri, 23 Mar 2007 16:57:39 +1030
From: Ivor and Olive Lewis
Our main sources of information about the plastic
nature of clay seem to be the Hamer Dictionary and
Lawrence and West's book about ceramic science for
potters.
These give us the idea that clay is plastic because
clay crystals slide past each other lubricated by
water in the mixture.
Do you know if anything has been added to this view?
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Michael Wendt on sat 24 mar 07
Dave and others...
No one has mentioned the role of air
content in clay plasticity.
Entrained air is a major player in
seemingly short clay behavior.
I say this because when clay is mixed
from dry powder in a standard clay
mixer, much air in the form of variously
sized bubbles winds up dispersed
throughout the clay body.
Pugging in a non deairing pug mill does
not remove the air although it often
seems to disperse it even finer.
My first clue that air is a major player
came when I began producing clay
using the deairing clay mixer I built
in the late 70s . That clay could be
cut into a long strip and tied in a knot.
The same clay out of the non deairing
pug mill simply broke when bent, no
matter how long we aged it or added
flocculants.
I suggest that mode of production belongs
on the list of players in the plasticity
of a clay body,
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
USA
208-746-3724
http://www.wendtpottery.com
wendtpot@lewiston.com
Dave wrote:
Dear Ivor,
Regarding your question, I do not know whether
anything has been added to this view.
There is much which I wish I did understand better
about clay bodies. I am much more comfortable with
the little I know about glazes!
As I'm sure you are aware plasticity of clay bodies
is influenced to some extent by at least these 5
factors and there must be others besides these:
1. particle packing
2. moisture content
3. adsorbed organics
4. pH
5. ionic concentration and strength
Dave Finkelnburg on sat 24 mar 07
Michael,
You make an excellent point...and also add one more
item to my list of things to learn more about! :-)
Thanks for pointing out the importance of deairing on
plasticity of clay bodies.
Dave Finkelnburg, who lives in the same state as
Michael...which means we are only separated by about
550 miles, much of it on windy, 2-lane mountain
roads...
From: Michael Wendt :
> Dave and others...
> No one has mentioned the role of air
> content in clay plasticity.
> Entrained air is a major player in
> seemingly short clay behavior.
> I say this because when clay is mixed
> from dry powder in a standard clay
> mixer, much air in the form of variously
> sized bubbles winds up dispersed
> throughout the clay body.
> Pugging in a non deairing pug mill does
> not remove the air although it often
> seems to disperse it even finer.
> My first clue that air is a major player
> came when I began producing clay
> using the deairing clay mixer I built
> in the late 70s . That clay could be
> cut into a long strip and tied in a knot.
> The same clay out of the non deairing
> pug mill simply broke when bent, no
> matter how long we aged it or added
> flocculants.
> I suggest that mode of production belongs
> on the list of players in the plasticity
> of a clay body,
> Regards,
> Michael Wendt
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