Ivor and Olive Lewis on thu 8 apr 04
Dear Michael Wendt,
The amazing Power of Two.
Given that many clays contain particles much thicker than the average
0.1mu of clay crystals, does the disruption of the layers by such
stuff as Grog have any noticeable effects on a clay fabric.
Have you observed segregation in layers which contact the table or can
we assume that thinner, purer clay layers deform around the larger
fragments.
Your proposition that the layered structure, being placed horizontally
to the wheelhead is interesting. Do you know if there is a rippling or
folding effect in the basal layers as wheel torque twists the plastic
mass against the hands of a thrower?
Best regards,
Ivor Lewis. Redhill, South Australia
Michael informed us << The best way to use the clay is to leave the =
piece oriented so the sheets are parallel to the wheel head. This gives =
the bottom no crack starting points.
You reorient the walls when you throw. Cutting the block into multiples =
is easy too... just be sure the part placed on the wheel head is one of =
the slammed faces.
Michael Wendt on fri 9 apr 04
Ivor,
I have noted only that my clay becomes stiffer around 2^30
layers which is contrary to the usual wisdom that wedging
softens clay.
I would welcome conjecture as to why.
My own theory is that packing begins around then with the clay
acting like mortar to neat stacks of bricks which increase the load
bearing character of the clay. My tests with the softness tester
confirm what I can feel with my hands as I wedge. The clay suddenly
becomes much stiffer around 28 - 30 doublings. The device measures
nearly twice the load bearing value of the clay before the effect occurs.
I also do not detect any torsional effects when I throw ("S"- cracks) nor
do I need to pin tool the lip when done as the uniformity of the body is
unmatched by any other wedging method I have tried.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
Ivor wrote:
Given that many clays contain particles much thicker than the average
0.1mu of clay crystals, does the disruption of the layers by such
stuff as Grog have any noticeable effects on a clay fabric.
Have you observed segregation in layers which contact the table or can
we assume that thinner, purer clay layers deform around the larger
fragments.
Your proposition that the layered structure, being placed horizontally
to the wheelhead is interesting. Do you know if there is a rippling or
folding effect in the basal layers as wheel torque twists the plastic
mass against the hands of a thrower?
Best regards,
Ivor Lewis. Redhill, South Australia
Ivor and Olive Lewis on sun 11 apr 04
Dear Michael,
I will get some clay out and take it up to 2^30 to see if I record the
same effect rather than just speculating. It will also give me a
chance to try out an idea I have for looking at thin sections of
unfired clay under the (X 500) microscope. Although this will not
resolve clay particles it will show anything larger that 1mu and if
there is any segregation it may be revealed.
I had better get my act together and construct the remainder of my
plasticity tester. The active part is to be a quartz crystal ground
square and given a 45=BA pyramid point. Just need a frame and an
actuator.
Interesting proposition, I like the way you are thinking.
Best regards,
Ivor Lewis. Redhill, South Australia
Michael Wendt on mon 12 apr 04
Ivor and others,
To really see the power of doubling, use two different clay
bodies that are different colors. 50:50 red and white for
example. By the time you reach 2^30 power, the stria are
no longer visible to the naked eye!
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
John Kudlacek on mon 12 apr 04
Hi, Haven't thoroughly studied this discussion but appears to have
been generated by spiral or "s" cracking. I discoverd long ago, contrary
to Leache's advise, that if the wedged clay is placed on the wheel with
the "roll" on its side rather than on the end, I virtually eliminated "S"
cracks. Now I am plagued with the concentric cracks near the base of
bowls, on the exterior, which appear to be a result of a lack of
compression of the clay which is left idle in the throwing process.
John in Topeka
Ivor and Olive Lewis on tue 13 apr 04
Dear John Kudlecek,
This thread started in relationship to the wedging process called "Cut
and Slam" which doubles and redoubles the layers in the block of clay.
By now you will have been told that such arrays of cracks are due to
dunting. They are illustrated in The Potter's Dictionary by Hamer
under Cracks and under Dunt.
If you are containing your clay on the outside at the wheelhead as you
apply pressure to the base from within your clay should maintain its
internal integrity.
There is an abundance of information in the archives about "S" cracks
and their cure.
Best regards,
Ivor Lewis. Redhill, South Australia
Ivor and Olive Lewis on tue 13 apr 04
Dear Michael,
See my message dated today. "Optimum Wedging Part 1"
No, I agree, my eyes are not as good as that, even with bi-focal
glasses
I would add to what you have said.
Take equal quantities of differing coloured clay and knead the same
using the Chrysanthemum method. After twenty pushes slice the clay in
tow, end to end. Do the same with Rams head.
Very interesting knowledge.
Best regards,
Ivor Lewis. Redhill, South Australia
.
mailtoandrew@FSMAIL.NET on wed 14 apr 04
Hi,
I m coming into this a little bit late, and with the renaming of the topic
tracking back has been a little difficult.
Anyway as I read the posts it seems to have been suggested that after
prolonged wedging a clay body loses some plasticity. Correct?
If this is what has been observed could it not simply be that mositure is
being lost? Certainly handing damp clay does take water from the clay
body, and with reducing mositure the workability falls.
The increase in plasticity after mechanical input, be it from wedging or
pug, has been well established.
Wondering if Ive misuderstood these last few posts ....
Andrew
Ivor and Olive Lewis on thu 15 apr 04
Dear Andrew,
The proposition was that extended periods of wedging using the cut and
double method, creating a myriad parallel layers, eventually seemed to
give firmer clay which had superior plastic and mechanical qualities.
So far we have all, possibly deliberately, ignored the obvious notion
that evaporation will cause hardening.
As you say <from wedging or
pug, has been well established.>> As an observation, yes. But how do
you explain this improvement in quality and what proof is available to
justify these claims.
Best regards,
Ivor Lewis. Redhill, South Australia
mailtoandrew@FSMAIL.NET on sat 17 apr 04
Dear Ivor,
In response to your ...as an observation, yes. But how do you explain this
improvement in quality and what proof is available to justify these
claims ...
Proof of the effect of mechanical input can, given access to suitable test
equipment, easily be had by simple experimentation
Take a lump of freshly dug kaolin and divide into two.
Deair one half. Extrude set of pencil sized rods. Leave to dry.
Heavily wedge other half. Deair. Extrude another set of rods and dry.
Determine breaking load of all using suitable equipment.
The highest load will be from wedged kaolin. Although simplistic for many
kaolins an increase in unfired strength will be paralleled by plasticity.
And as noted in my earlier post this change against input is subject to
the law of diminishing returns with a plateau eventually reached.
Although I dont know what material you use if you buy a commercial,
processed grade there is a good chance it was pugged by the processor to
benefit for this phenomenon.
As far as a mechanism ... it easy to simply think that as energy is being
put into the clay some form of change must result. A more detailed view is
that the size distribution of the particles is being changed as
agglomerates are separated and even bits are broken off individuals.
Hope that something to mull on ....
Andrew
Ivor and Olive Lewis on sun 18 apr 04
Dear Andrew,
Until it is possible to make images of clay materials at a
magnification where the form of smallest of the particles can be
detailed and its identity determined one guess is as good as another
as to the nature and properties of any plastic clay body.
I have yet to read one satisfactory explanation of the phenomenon of
plasticity in books published for potters.
You say << The highest load will be from wedged kaolin. Although
simplistic for many
kaolins an increase in unfired strength will be paralleled by
plasticity.>>
Can you please explain how you arrive at a direct correlation between
the deformability of a plastic substance and the yield point of a
rigid substance obtained via a notched bend test?.
As far as I understand things, these improvements are to some degree
subjective since they relate to tactile sensation noticed while we are
working the clay. I accept the evidence Michael Wendt has given from
his measurements using his indentometer. But solutions to this problem
are bedevilled by the need to solve a puzzle with many variables, few
of which can be controlled. So I doubt if "Mulling" will provide an
answer.
Ivor
mailtoandrew@FSMAIL.NET on fri 30 apr 04
Hello Ivor,
Sorry for the late response to your post, Ive been away for over a week.
Anyway in reply ...
Imaging of individual clay particles has long been possible.
You statement that ... one guess is as good as another as to the nature
and properties of any plastic clay body ... is quite sweeping. Whilst
being facetious is the suggestion that plasticity is due to the action of
fairies as good? There is broad agreement about much concerning clay and
plasticity. If however you choose to ignore it ....
As far as plasticity and green strength ... Although seemingly obvious it
is important to note that these two properties are not the same. Both can
be assessed by a choice of apparatus that have no recourse the
subjectiveness of a human.
You mention a notched bend test? The test methods for determining the
mechanical strength of ceramics use specimens of regular geometry with
smooth surfaces. If a notch was used a failure inducing crack would
propagate there rather than the material breaking at its own weakest
point. ie simply put the flaw would be tested and not the ceramic.
There is a general trend that for many ball and china clays those which
are mechanically stronger are also more plastic. The increase in
plasticity and unfired strength of clays resulting from pugging or wedging
has long been established, both experimental and experientially. The cause
is that factors that lead to high strength also lead to high plasticity,
with particle size being particularly significant. No parallel with other
substances or materials is suggested.
I was not disputing Michael Wendt=92s observations, just suggesting knowns
that may have been worth considering.
Regards,
Andrew
Bruce Girrell on fri 30 apr 04
> Imaging of individual clay particles has long been possible.
Imaging of platelets is no problem for an SEM. Imaging of clay platelets
unaltered by drying... a little tougher. To investigate the properties that
are being discussed, imaging of the clay in its plastic state is essential.
> As far as plasticity and green strength ... Both can
> be assessed by a choice of apparatus that have no recourse the
> subjectiveness of a human.
How does a plasticimeter work? Where does one obtain one of these devices?
What are the units of plasticity?
Bruce "inquiring minds want to know" Girrell
mailtoandrew@FSMAIL.NET on fri 30 apr 04
Hello Bruce,
SEM imaging. I certainly agree with your comment. Amongst other things
specimens are sputtered with gold before held in a vacuum. Perhaps I
should have added to my brief sentence something like ... but not in a
state that most potters will use. Still SEMS are vaulable tools,
facsinating to use and do provide very useful information.
And plastometers ... I described them as assessing rather than measuring
plastcity; an important distinction as this property is more easily
described than defined. There have been a range of designs over the years
including Attenberg, Pfefferkorn, BCRA compression, MFI.
Plasticty itself does not have units but these analysers use arbitary
values or even just a Index. (Plasticity Index, or PI)
As far as where to buy? Certainly Wal Mart don't stock them but if you
have deep pockets try Ceram Research.
Hope thats at bit of use,
Regards,
Andrew
Bruce Girrell on fri 30 apr 04
Andrew wrote:
> Plasticty itself does not have units but these analysers use arbitary
> values or even just a Index. (Plasticity Index, or PI)
>
> As far as where to buy? Certainly Wal Mart don't stock them but if you
> have deep pockets try Ceram Research.
And this is exactly what Michael was trying to address. He was trying to
make an instrument that
1) produced a meaningful measurement of clay body plasticity
2) produced a measurement that would be reproducible by potters and clay
suppliers anywhere, not an arbitrary index
3) was inexpensive
4) could be built by anyone with basic construction skills
(hopefully I'm not putting words in your mouth Michael - please correct me
if I have misstated your goals)
In checking out plastometers on the web I see that there are several methods
used: compression, extrusion, torsion, shearing, and perhaps others. What
means would do you feel would produce the most meaningful measurement of
plasticity for potters? Can a semi-static method such as slow compression
actually yield a useful value or do you feel that a dynamic method would be
required?
Perhaps even more fundamentally, what do we actually need to measure, i.e.,
what is there about this nebulous term "plasticity" that is of value to
potters? To me, plasticity entails three essential properties:
1) the ability to deform with applied pressure
2) cohesion, i.e., no rupturing upon deformation
3) retention of the new shape after removal of the deforming pressure
Feel free to modify or expand upon that definition.
If those are the properties that are important to potters, what is the best
way to measure them such that the goals outlined above can be met?
Bruce "how about we change the subject line?" Girrell
Ivor and Olive Lewis on sat 1 may 04
Dear Bruce,
You are right on the money with:
<> 1) the ability to deform with applied pressure
> 2) cohesion, i.e., no rupturing upon deformation
> 3) retention of the new shape after removal of the deforming
pressure>>
When consideration is given to the notion of Plasticity (as the
converse of "Elastic") as a general condition applicable to all solid
materials, facts emerge which seem to be at odds with the models of
plastic clay that have been proposed in recent and current literature
written for the potter and ceramic artyst.
Enjoy your weekend.
Best regards,
Ivor Lewis. Redhill, South Australia
----- Original Message -----
From: "Bruce Girrell"
To:
Sent: Saturday, 1 May 2004 5:56
Subject: Re: : Stack & Slam wedging was StaRe: Wedging Table Height
> Andrew wrote:
>
> > Plasticty itself does not have units but these analysers use
arbitary
> > values or even just a Index. (Plasticity Index, or PI)
> >
> > As far as where to buy? Certainly Wal Mart don't stock them but if
you
> > have deep pockets try Ceram Research.
>
> And this is exactly what Michael was trying to address. He was
trying to
> make an instrument that
> 1) produced a meaningful measurement of clay body plasticity
> 2) produced a measurement that would be reproducible by potters and
clay
> suppliers anywhere, not an arbitrary index
> 3) was inexpensive
> 4) could be built by anyone with basic construction skills
> (hopefully I'm not putting words in your mouth Michael - please
correct me
> if I have misstated your goals)
>
> In checking out plastometers on the web I see that there are several
methods
> used: compression, extrusion, torsion, shearing, and perhaps others.
What
> means would do you feel would produce the most meaningful
measurement of
> plasticity for potters? Can a semi-static method such as slow
compression
> actually yield a useful value or do you feel that a dynamic method
would be
> required?
>
> Perhaps even more fundamentally, what do we actually need to
measure, i.e.,
> what is there about this nebulous term "plasticity" that is of value
to
> potters? To me, plasticity entails three essential properties:
> 1) the ability to deform with applied pressure
> 2) cohesion, i.e., no rupturing upon deformation
> 3) retention of the new shape after removal of the deforming
pressure
>
> Feel free to modify or expand upon that definition.
>
> If those are the properties that are important to potters, what is
the best
> way to measure them such that the goals outlined above can be met?
>
>
> Bruce "how about we change the subject line?" Girrell
>
>
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Ivor and Olive Lewis on sat 1 may 04
Dear Andrew,
I repeat, and I apply this to plastic clay and not to leather hard or
green clay, "one guess is as good as another as to the nature and
properties of any plastic clay body ". If you wish to invoke
"Faieries" then perhaps they do help.
I know full well that clay minerals have been imaged using SEM
techniques. But plastic clay has not been so imaged. I do not dispute
your assertions about the relationship between plasticity and green
mechanical strength
Regarding the testing of green clay specimens, I think there are two
test which use notched specimens. One is the single edge notched beam
specimen and the other is the chevron notched specimen. Yes, these
conformations do not rely on a weakness to determine the measured
strength of the material. If you rely on an random indiscriminate flaw
for a result then the measure is not calculable since the area under
test is undetermined.
Best regards,
Ivor Lewis. Redhill, South Australia
mailtoandrew@FSMAIL.NET on tue 4 may 04
Dear Ivor,
Of course it is entirely a personal decision to consider that "one guess
is as good as another as to the nature and properties of any plastic clay
body ". However researchers and other professionals are largely satisfied
with the explanations concerning plasticity.
Regarding the testing of green clay specimens. Perhaps you are confused
with impact testing? These, such as Charpy, are very occasionally
undertaken on clay based ceramics, and then on fired specimens. Whilst I
would not be as categorical as to say unfired notched specimens are never
tested the acknowledged method, detailed in various international
standards and used by all the kaolin producers, is Modulus of Rupture.
With methods widely published much elaboration here is perhaps unnecessary
beyond that specimens are rods of either circular or rectangular cross
section with smooth external faces.
Your comment about relying on a random indiscriminate flaw? The strength
of ceramics is strongly dependent on flaws. Their presence, magnitude and
their effect are major limiting factors on mechanical strength, and why
actual values are often just 1% of the theoretical. Flaws disrupt the
distribution of an applied load, and therefore the ability of a ceramic to
withstand a load is dependent on the probability of a failure-initiating
flaw occurring in a suitably stressed area. This statistical nature is the
cause of the wider spread of mechanical test results for ceramics than for
non-brittle materials.
I would certainly agree with the fundamentals of plasticity that have been
listed earliwer in this thread. These have long been noted, with the
earliest reference I have by E.O Wilson in 1936, which was slightly
modified by Ryan & Radford in 1986 to .... =93that property which enables a
material to be changes in shape without rupturing by the application of an
external force, and to retain that shape when the force is removed or
reduced below a certain level.=94
Regards,
Andrew
Dear Bruce,
I agree with your suggestion about changing the subject title. Any
suggestions?
Further to your bulleted points
1) Meaningful measurement of clay body plasticity
2) Measurement that would be reproducible by potters and clay suppliers
anywhere, not an arbitrary index
3) was inexpensive
4) could be built by anyone with basic construction skills
Point 1 ... This will be at best very, very difficult. Only when something
is precisly and quantifiable defined can it be measured. After many years
of research there is much that is understood about the mechanisms and the
factors influencing plasticity but an absolute definition appears
unobtainable.
I can see an analogy with the concept of heatwork that is used in
connection with kilns. This has no units, is not a precise measure of
anything but is easily appreciated in production.
G. W. Scott Blair, an early researcher into plasticity, summarised it
neatly in 1930s by stating ... plasticity is like honesty, something
indefinable but which has certain characteristics associated with it.
Point 2 ... Many methods and instruments to assess plasticity have been
proposed, built and sold. Their greatest success has been for comparative
assessments. Values quoted have been either an Index or simply the
parameter measured which is then related to plasticity.
Point 3 and 4 ... Although probably quite challenging these two should be
possible if points 1 and 2 are solved.
I wish Michael great success with his instrument. However it will most
likely be a version of existing comparison plastometers, the descriptions
given being very similar. Whilst not wishing to sound negative, and I
apologies if I do, I wonder about its likely uptake. None of the clay body
suppliers I have known have ever used, or even considered, the current
plastometers. Some use the hand held penetrometers, though these only
really gauge the mositure content of indentical formulations.
Also whilst a repeatable assessment of plasticity could be useful to
industrial manufacture where consistency is paramount is there much demand
from the craft market?
With the scientific study of plasticity dating back to at least the 1920s
when a series of Symposiums were held on the subject much has been
published. For anyone interested the following are a good start:
Whitewares, production testing and quality control
Ryan W. and Radford C.
Pergamon Press. 1986
A very good start to the technology of ceramic whitewares. It succinctly
explains a numbers of the influencing factors and the operation of a
number of plastometers.
Plasticity, a critical survey
Bloor E.C.
Trans. Brit. Ceram. Soc. 56, 423, 1957.
A very comprehensive, and highly readable, discussion on the subject in
relation to clay bodies.
Theoretical Aspects of Paste Extrusion
Horrobin D.J.
Phd thesis. Queens=92 College, Cambridge University. 1999
A detailed appreciation of the underlying principles of plasticity.
Regards,
Andrew
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