Earl Brunner on mon 16 jul 01
I have used Tom Coleman Porcelain (made by Aardvark) for several years
now. This porcelain can be very stiff until you get it moving, slamming
the bag around would definitely make a difference in softening it up.
However, I have never noticed any thixotropic tendencies with this
porcelain while throwing. It is one of the best porcelains that I have
ever thrown. I'm wondering if there isn't some other mechanism at work here.
--
Earl Brunner
http://coyote.accessnv.com/bruec/
bruec@anv.net
Marjorie Beynon on tue 17 jul 01
Could it be that the very large pugmills used by clay body producers, create
some heat and the outside of the pug has the most compression - therefore
the clay pug feels stiff, but as you "bang it around" it moves and feels as
if you have softened it, but you have just evened out the moisture. MarjB
iandol on thu 19 jul 01
Dear Earl Brunner,
You say here.>
Perhaps it is a question of degree, rather than an all or nothing case. =
If you have slammed your clay and kneaded it to its optimum degree of =
plasticity then it will change no further, having used all the free =
charge which causes the breakdown of structure.
But to answer your question. Perhaps there is. It is superbly simple and =
explains just about everything you ever wanted to know about clay =
plasticity. But if I were to make the suggestion, most knowledgable =
members of the group would howl me down as an idiot.
Anyway, without knowing the contents the truth will not be known.
Best regards,
Ivor Lewis. Redhill, South Australia
Tom Wirt/Betsy Price on thu 19 jul 01
>>snip)Your summary of what may be happening in a bag of clay by
classing it as a "Bingham Fluid" seems to be at odds with the
information I have which suggests that "thixotropic" materials are
non-Newtonian fluids whose viscosity varies with applied shear force
(Enc.Brit.). Would you confirm the these terms are not equivalent.
I tend to agree Ivor. I had not remembered having heard of a Bingham
fluid, but on researching it a little, it seems the properties of a
Bingham fluid are more influenced by the mechanical properties of the
solution, where it seems clay and thixotropic actions are electron
charge influenced. The yield force causes shearing of the positive
and negative charges and thus movement of the particles.
We may be confusing, I don't know, thixotropy with what happens when
you slam a pug of clay down. One is a sharp force, the other a more
smoothly applied force which, as soon as it is relaxed, the clay
should stiffen up....although in throwing with larger weights of clay,
these bodies tend to keep flowing once they get started...like a
Bingham fluid. Maybe when you get that higher power microscope you
can look at this and see something. If you need a sample of a clay
that definitely does this, will be glad to send it.
Ahhh mystery.
Tom Wirt
iandol on fri 20 jul 01
Dear Rob Muylaert (Netherlands)
Thanks for making a contribution from the Low Countries. No problems =
understanding your message except that where you say "They can look very =
dry" I feel you must mean that they have become solid but they are still =
wet to the touch and not powdery..=20
Your summary of what may be happening in a bag of clay by classing it as =
a "Bingham Fluid" seems to be at odds with the information I have which =
suggests that "thixotropic" materials are non-Newtonian fluids whose =
viscosity varies with applied shear force (Enc.Brit.). Would you confirm =
the these terms are not equivalent.
I would also be pleased if you would elaborate on your description of =
the relationship between clay and water. Are you saying that clay =
particles and water combine to become individual solid entities which =
form a uniform interlocking crystalline like structure? If so, I would =
like to read the documents relating to the research which established =
this fact. Perhaps you would be kind enough to provide your references =
since such a concept would be contrary to that currently used by potters =
who believe plastic clay consists of plate like crystals of clay which =
slide past each other, lubricated with liquid water.
Best regards,
Ivor Lewis. Redhill, South Australia
Rob Muylaert on mon 23 jul 01
=20
Dear Ivor Lewis,
You are making it a bit difficult for me ( technical stuff translating =
in English), but I will try.
First the short answers, then the explanation.
----- Original Message -----=20
From: iandol=20
To: rmk@tref.nl=20
Thanks for making a contribution from the Low Countries. No problems =
understanding your message except that where you say "They can look very =
dry" I feel you must mean that they have become solid but they are still =
wet to the touch and not powdery..=20
Your description is correct
Your summary of what may be happening in a bag of clay by classing it as =
a "Bingham Fluid" seems to be at odds with the information I have which =
suggests that "thixotropic" materials are non-Newtonian fluids whose =
viscosity varies with applied shear force (Enc.Brit.). Would you confirm =
the these terms are not equivalent.
This is the pitch of the matter. I think it is more complex then you =
suggest, see explanation below.
I would also be pleased if you would elaborate on your description of =
the relationship between clay and water. Are you saying that clay =
particles and water combine to become individual solid entities which =
form a uniform interlocking crystalline like structure?
No, I do not. I used a 'model' to explain the phenomenon.=20
If so, I would like to read the documents relating to the research which =
established this fact. Perhaps you would be kind enough to provide your =
references since such a concept would be contrary to that currently used =
by potters who believe plastic clay consists of plate like crystals of =
clay which slide past each other, lubricated with liquid water.
Best regards,
Ivor Lewis. Redhill, South Australia
=20
A more comprehensive explanation.
First of all, I am not an expert. I am just more interested in glaze ( =
and the chemistry of it) then most of my colleagues.
Some years ago, one of our members (Dutch Ceramist Association) =
organized a workshop Rheologie (streaming and distortion of fluids). The =
practical part was changing glaze- and clay suspensions (viscosity, =
thixotropy, yield value etc) with commercial available deflocculants =
etc.
Now I only refer to the theoretical part.
If you make a graph from the relation between force and velocity (horz. =
- vert. Baseline), then you can define several different kind of fluids. =
For a Newton fluid it is a strait line from point 0,0.
There are several non Newton fluids, all with a different graph, and at =
least three or four we are dealing with when working with clay and =
glazes.
1. Bingham or Plastic fluids, like Arabic gum, gelatine, clay with a lot =
off bentonite or montmorillonite . There is an initial force necessary =
before anything happens, but then the relation force/velocity is a =
straight line.
2. Pseudo plastic fluid, fluids with long chains of molecules. A curved =
line, initially the viscosity is staying behind the applied force.
3. Dilatant fluid, the opposite of pseudo plastic. Think of quicksand =
and wet sand along the seashore. You can run over it ( a great force and =
it is almost solid, but just walking and you get stuck in it). Some =
kaolins can cause this unwanted property to castingslips.=20
4. Thixotropic fluids, the ascending and descending line of the curve is =
not the same. And the process is also not identical when repeated. =
Examples are some glues, bentonite clays and many ceramic suspensions.=20
=20
This is theory, in nature you mostly see a more complex process, a =
combination of several symptoms.
Back to our 'slam in a bag' clay. I think the phenomenon has properties =
from a Bingham and a Thixotropic fluid.
In clay I recognize more the properties from Bingham and Pseudo plastic =
fluids. In suspensions ( glaze and slips) more from thixotropic fluids.
But as we say in Holland 'I give my opinion (away) for a better one'
Rob
http://www.rmkeramiek.nl
info@rmkeramiek.nl
---------------------------------
Nederlandse Vakgroep Keramisten (Dutch Ceramist Association)
http://www.nvk-keramiek.nl
info@nvk-keramiek.nl
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