Dave Finkelnburg on mon 7 jul 03
Earl, Eric,
Earl asks, "So if what we are describing isn't particle alignment, what
is it?"
There are too many variables involved for the answer to this to be
simple. First, you can get cracking along layers due to a variation in
moisture within the clay before it is allowed to dry. That's the most
common culprit. I think you are just seeing inadequate mixing -- the clay
isn't totally homogeneous.
Also, as you knead the clay, just as happens in a clay mixer or pugmill,
or opening clay on the wheel, there is a tendency to form microscopic tears
or small openings in the clay. If these are within the clay, it will want
to shrink more there during drying, and later during firing. This is the
"un-twist" you observe.
Finally, what Eric asked about originally is the tendency of some clay
bodies to behave in a thixotropic manner. Our clay bodies are not truly
thixotropic. A true thixotropic material is like a solid until pressure is
applied, at which point the material flows like a fluid. Clay bodies have
some tendency to behave this way. With little stress applied to them they
are quite stiff, especially if they have been left undisturbed for a "long"
time. When stress is applied, the clay appears to be softer and more
workable.
I don't understand the weak attractive forces between particles of a
thixotropic clay, but I think they are mostly electrostatic. Clay platelets
are notoriously unbalanced electrically, being positive on some surfaces and
negative on others. In a wet environment (a wet clay body) these charges
cause the particles to cling to each other.
To answer your question Eric (or not answer it, sorry), I don't think it
is possible to say absolutely how long it takes for the disrupted
electrostatic bonds of a thixotropic clay to reestablish themselves because
that depends on the clay body (bentonite content, magnesium sulfate content,
soluble alkali content, etc), how it's been handled, its moisture content,
how soft a clay is soft enough for you -- too many variables.
All the best to both of you!
Dave Finkelnburg
Earl Brunner on mon 7 jul 03
Yeah, I wasn't trying to get argumentative, just discussing. Actually I
can think of a couple of possible answers for why the lump of wedged
clay might break along the spiral lines of the wedging. One would be
the possible "skinning" of the clay from having some moisture taken out
on the wedging table and the subsequent layering of the slightly dryer
surfaces in the wedging process.
I also thought about the ion charge or electrostatic possibility. Maybe
if we lined our wet box with magnets or an electromagnetic coil, they
would stay soft indefinitely. :) I mean magnets have been touted for
all kinds of other things including healing broken bones........
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Dave
Finkelnburg
Sent: Monday, July 07, 2003 3:38 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: longevity of wedging benefits -- thixotropic clay
Earl, Eric,
Earl asks, "So if what we are describing isn't particle alignment,
what
is it?"
There are too many variables involved for the answer to this to be
simple. First, you can get cracking along layers due to a variation in
moisture within the clay before it is allowed to dry. That's the most
common culprit. I think you are just seeing inadequate mixing -- the
clay
isn't totally homogeneous.
Also, as you knead the clay, just as happens in a clay mixer or
pugmill,
or opening clay on the wheel, there is a tendency to form microscopic
tears
or small openings in the clay. If these are within the clay, it will
want
to shrink more there during drying, and later during firing. This is
the
"un-twist" you observe.
Finally, what Eric asked about originally is the tendency of some
clay
bodies to behave in a thixotropic manner. Our clay bodies are not truly
thixotropic. A true thixotropic material is like a solid until pressure
is
applied, at which point the material flows like a fluid. Clay bodies
have
some tendency to behave this way. With little stress applied to them
they
are quite stiff, especially if they have been left undisturbed for a
"long"
time. When stress is applied, the clay appears to be softer and more
workable.
I don't understand the weak attractive forces between particles of a
thixotropic clay, but I think they are mostly electrostatic. Clay
platelets
are notoriously unbalanced electrically, being positive on some surfaces
and
negative on others. In a wet environment (a wet clay body) these
charges
cause the particles to cling to each other.
To answer your question Eric (or not answer it, sorry), I don't
think it
is possible to say absolutely how long it takes for the disrupted
electrostatic bonds of a thixotropic clay to reestablish themselves
because
that depends on the clay body (bentonite content, magnesium sulfate
content,
soluble alkali content, etc), how it's been handled, its moisture
content,
how soft a clay is soft enough for you -- too many variables.
All the best to both of you!
Dave Finkelnburg
________________________________________________________________________
______
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.
Eric B on mon 7 jul 03
thank you everyone for your input. i certainly learned a few things. as far
as how long wedged clay can sit before losing that "quality" it has, it
sounds like it might be as subjective a question as it is scientific. so i think i
must follow the advice one of you gave, which is just wedge up a series of
similar balls over a few days, one day after the next, then throw them at the
same sitting and see how they suit me.
thanks again!
| |
|
