Ivor and Olive Lewis on sun 26 sep 04
Dear Ron Roy.
Some interesting knowledge coming from this thread.
I have been trying to visualise the effects on the structure of a slab
of clay as it is rolled and manipulated by beating, pushing against
guide rods and so on and I am getting some contradictory visions.
Your remark ...<< I make my slabs by throwing them onto the table -
which tends to even out the density. >>...has me puzzled, since it
seems to infer that clay which has been well wedged and kneaded has
variations of density (Mass per unit volume) within its fabric.
A second point is the idea of beating the sides inwards. If pressure
that has been applied to thin the clay aligns the clay plates so that
they are in parallel sheets (as is often said) then does pushing
against the edges of those layers cause them to fold and wrinkle,
giving a structure similar to that of a "folded gneiss" or do the
crystals remain flat but slide inwards over each other?
However, if it can be accepted that plastic clay is composed of flock
structures an alternative picture begins to form.
Relatively uniform flock structures of pristine wedged and kneaded
clay begin to reshape from their random polyhedron forms into a
flattened lenticular shapes, extending in area as they thin, becoming
almost leaf like in form in the extreme. However, if there are
inequalities in the application of pressure then these changes may not
be uniform and differences in flock shape will prevent uniform drying,
building stress into the structure. Stress relief during firing will
cause warping.
Just "Brainstorming"
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From: "Ron Roy"
To:
Sent: Saturday, 25 September 2004 4:27
Subject: Re: warping slabs
> Hi Kate,
>
> Tricky question to answer - I can be the clay and it can also be the
way
> you make slabs - or both - or as you say - how you use em.
>
> I don't have a roller - I make my slabs by throwing them onto the
table -
> which tends to even out the density. I finish them off with a few
rolls of
> a rolling pin.
>
> What is required is to try and imagine - where is the clay
compressed and
> where is it stretched.
>
> Using rails on the sides will help keep the stretch out of the
sides - if
> the clay is forced up against the rails - and you can - as someone
else
> suggested - paddle the sides and ends to recompress the clay. You
can also
> draw a tool around the slab to recompress the edges - maybe the
inside part
> of a piece of split bamboo - which would compress and round at the
same
> time. Same idea as recompressing the rim of a thrown pot.
>
> If a slab roller is used - recompress the sides and ends of the
slabs and
> then roll it again.
>
> Just keep in mind - if parts of the slab are stretched and other
parts not
> - the stretched parts are going to shrink more during drying and
firing.
> Anything you do to counter the uneven stretching will improve the
> situation.
>
> RR
>
>
> >> Stretched clay always shrinks more than compressed clay - usually
the out
> >> side part of a slab is stretched and the inside compressed - if
you can
> >> compress the outside clay between rolling it you will help solve
the
> >> problem.
> >
> >Hi Ron...I haven't had this problem much yet (knock wood), but how
should
> >one compress the outside clay without changing the thickness? I
don't have
> >a slab roller and do all my work with wooden spacers and a straight
rolling
> >pin. I turn the clay 90 degrees so I roll it in both directions,
and
> >usually flip it over once during the process, too. Before I start
rolling
> >at all, I flatten in by hand, pushing and patting it till it's near
the
> >final thickness needed. Is there something else I should be doing?
> >
> >The only real problem I had with warpage was in making a fairly
deep
> >bowl...may have been too much uneven compression needed to make the
slab fit
> >the depth of the press mold, or I may have removed it too
soon/handled it
> >too much. Anyway, I'd like to avoid the problem in the future,
because I'd
> >like to be able to offer more deep bowls!
> >
> >Regards,
> >Kate Johnson
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
>
______________________________________________________________________
________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your
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> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Ron Roy on mon 27 sep 04
Hi Ivor,
Can we agree that wedged clay is uniform - if we can't then there is no use
going any further.
When you make a slab by throwing it - you are stretching the clay evenly -
especially if you turn it each time.
My contention is - if you have compressed and streched clay in the same
slab you are asking for trouble.
RR
<< I make my slabs by throwing them onto the table -
>which tends to even out the density. >>...has me puzzled, since it
>seems to infer that clay which has been well wedged and kneaded has
>variations of density (Mass per unit volume) within its fabric.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ivor and Olive Lewis on tue 28 sep 04
Dear Ron,
I will most certainly agree with you on that point.<< Can we agree
that wedged clay is uniform - if we can't then there is no use going
any >>.
I will also agree with your implication, as an implication and not as
a fact, that the effects of stretching differ from the effects of
applying a squeeze to reduce the thickness of the clay.
But until you, or I or someone else publishes images obtained either
by reflected or transmitted light from specimens which represent the
three situations we speak of, thoroughly prepared plastic clay,
stretched clay and pressed clay, I prefer to keep an open mind on the
topic.
And on your final point I whole heartedly agree <compressed and stretched clay in the same slab you are asking for
trouble.>>
Dare I say Clay has a MEMORY. ( I shouldn't, it's anthropomorphic not
scientific)
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From: "Ron Roy"
To:
Sent: Monday, 27 September 2004 4:18
Subject: Re: warping slabs. Imagination may help understanding.
> Hi Ivor,
>
further.
>
> When you make a slab by throwing it - you are stretching the clay
evenly -
> especially if you turn it each time.
>
> My contention is - if you have compressed and streched clay in the
same
> slab you are asking for trouble.
>
> RR
>
> << I make my slabs by throwing them onto the table -
> >which tends to even out the density. >>...has me puzzled, since it
> >seems to infer that clay which has been well wedged and kneaded has
> >variations of density (Mass per unit volume) within its fabric.
>
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
>
______________________________________________________________________
________
> 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.
Joyce LEE on tue 28 sep 04
ok
----- Original Message -----=20
From: Jim Murphy=20
To: CLAYART@LSV.CERAMICS.ORG=20
Sent: Tuesday, September 28, 2004 12:37 PM
Subject: Re: warping slabs. Imagination may help understanding.
Ivor wrote:
<as
a fact, that the effects of stretching differ from the effects of =
applying a
squeeze to reduce the thickness of the clay.>>
<asking
for trouble.>>
Hello Ivor,
I find this discussion - compression & stretching effects with respect =
to
thickness - very interesting.
I've got an open mind and a bit of an imagination muyself, so ... what =
the
heck!
During compression or stretching, I'll bet actual discrete clay =
particle
"shape" is altered very little - if at all.
The voids - or capillaries - between clay particles are normally =
filled by
water.
Whether one believes capillary water-nature to be "ice-like", =
"slush-like",
or "fluid", I think what's most important is the size & shape of the
capillaries themselves are being changed during claybody compression =
or
stretching.
Modern plasticity-theory suggests there are "opposing forces" - =
capillary
water force is said to help pull the clay particles together while the
electric charge of clay surfaces keeps them apart.
Long small-diameter capillaries help create greater hydrostatic =
"suction
forces" [compared to shorter large-diameter capillaries], however, =
there is
a tradeoff in that "shearing force" of the claybody may be reduced. =
[Too
much shearing is "bad" for plasticity.]
In visualizing what's happening during compression (squeezing) or =
stretching
of clay to the same thickness, I'm inclined to "imagine" for ALL
uniform-density plastic bodies - when clay is stretched beyond a =
certain
"critical state" - the density of longer small-diameter capillaries =
becomes
too great and shearing force is decreased.
So, for two equivalent volumes of the same claybody - one compressed =
to a
certain critical thickness and the other stretched to the same =
critical
thickness - the claybody behavior "differences" [warpage, etc.] may be
explained by understanding the nature of changing capillary size & =
shape.
The "stretched" clay perhaps develops a greater proportion of the long
small-diameter capillaries.
Just my $0.02.
Best wishes,
Jim Murphy
=
_________________________________________________________________________=
_____
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.
Jim Murphy on tue 28 sep 04
Ivor wrote:
<a fact, that the effects of stretching differ from the effects of applying a
squeeze to reduce the thickness of the clay.>>
<for trouble.>>
Hello Ivor,
I find this discussion - compression & stretching effects with respect to
thickness - very interesting.
I've got an open mind and a bit of an imagination muyself, so ... what the
heck!
During compression or stretching, I'll bet actual discrete clay particle
"shape" is altered very little - if at all.
The voids - or capillaries - between clay particles are normally filled by
water.
Whether one believes capillary water-nature to be "ice-like", "slush-like",
or "fluid", I think what's most important is the size & shape of the
capillaries themselves are being changed during claybody compression or
stretching.
Modern plasticity-theory suggests there are "opposing forces" - capillary
water force is said to help pull the clay particles together while the
electric charge of clay surfaces keeps them apart.
Long small-diameter capillaries help create greater hydrostatic "suction
forces" [compared to shorter large-diameter capillaries], however, there is
a tradeoff in that "shearing force" of the claybody may be reduced. [Too
much shearing is "bad" for plasticity.]
In visualizing what's happening during compression (squeezing) or stretching
of clay to the same thickness, I'm inclined to "imagine" for ALL
uniform-density plastic bodies - when clay is stretched beyond a certain
"critical state" - the density of longer small-diameter capillaries becomes
too great and shearing force is decreased.
So, for two equivalent volumes of the same claybody - one compressed to a
certain critical thickness and the other stretched to the same critical
thickness - the claybody behavior "differences" [warpage, etc.] may be
explained by understanding the nature of changing capillary size & shape.
The "stretched" clay perhaps develops a greater proportion of the long
small-diameter capillaries.
Just my $0.02.
Best wishes,
Jim Murphy
Ivor and Olive Lewis on wed 29 sep 04
Dear Jim,
You make some very good points.
Agreed, clay minerals are not going to change their shape but force
causing movement of plastic clay will alter the conformation and
distribution of spaces that separate all particles. So your
proposition that variations in the geometry of the spaces will
determine the direction and magnitude of gross motion, that is,
visible distortion or warping as clay dries, seems plausible and
reasonable.
Two factors which have been ignored in this discussion so far are Time
and Temperature. Consider the face of the clay exposed to an adsorbent
surface, be it plyboard, canvas or a deep plaster mould. The higher
the temperature and the longer the time of contact the greater the
volume of water that is removed from the surface layer through
evaporation or capillary wicking. There may even be a difference in
the rate of water loss between the two exposed faces. This will cause
differences in mechanical stress with the interior of the clay. At
some point warping or fracture is inevitable if the stresses exceed
the structural strength of the clay.
These things can be reasoned out without recourse to esoteric
scientific theory.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From: "Jim Murphy"
To:
Sent: Wednesday, 29 September 2004 5:07
Subject: Re: warping slabs. Imagination may help understanding.
> Ivor wrote:
>
> <as
> a fact, that the effects of stretching differ from the effects of
applying a
> squeeze to reduce the thickness of the clay.>>
>
> <asking
> for trouble.>>
>
> Hello Ivor,
>
> I find this discussion - compression & stretching effects with
respect to
> thickness - very interesting.
>
> I've got an open mind and a bit of an imagination muyself, so ...
what the
> heck!
>
> During compression or stretching, I'll bet actual discrete clay
particle
> "shape" is altered very little - if at all.
>
> The voids - or capillaries - between clay particles are normally
filled by
> water.
>
> Whether one believes capillary water-nature to be "ice-like",
"slush-like",
> or "fluid", I think what's most important is the size & shape of the
> capillaries themselves are being changed during claybody compression
or
> stretching.
>
> Modern plasticity-theory suggests there are "opposing forces" -
capillary
> water force is said to help pull the clay particles together while
the
> electric charge of clay surfaces keeps them apart.
>
> Long small-diameter capillaries help create greater hydrostatic
"suction
> forces" [compared to shorter large-diameter capillaries], however,
there is
> a tradeoff in that "shearing force" of the claybody may be reduced.
[Too
> much shearing is "bad" for plasticity.]
>
> In visualizing what's happening during compression (squeezing) or
stretching
> of clay to the same thickness, I'm inclined to "imagine" for ALL
> uniform-density plastic bodies - when clay is stretched beyond a
certain
> "critical state" - the density of longer small-diameter capillaries
becomes
> too great and shearing force is decreased.
>
> So, for two equivalent volumes of the same claybody - one compressed
to a
> certain critical thickness and the other stretched to the same
critical
> thickness - the claybody behavior "differences" [warpage, etc.] may
be
> explained by understanding the nature of changing capillary size &
shape.
> The "stretched" clay perhaps develops a greater proportion of the
long
> small-diameter capillaries.
>
> Just my $0.02.
>
> Best wishes,
>
> Jim Murphy
>
>
______________________________________________________________________
________
> 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.
Jim Murphy on wed 29 sep 04
on 9/29/04 2:49 AM, Ivor and Olive Lewis at iandol@WESTNET.COM.AU wrote:
<and Temperature. ... The higher the temperature and the longer the time of
contact the greater the volume of water that is removed from the surface
layer through evaporation or capillary wicking.>>
Ivor,
Yes, perhaps one of the advantages of "compression" may also be that a
greater proportion of exposed [surface] capillary "openings" - perpendicular
to slab length - get "sealed-over" during rolling, etc., creating slab
surfaces which may dry more slowly and evenly [more in tune with internal
capillary structure "drying"] under given time/temp conditions.
Whereas in "stretched" clay areas, there may be a greater proportion of
short large-diameter capillary "openings" [per unit surface area] which may
speed drying through evaporation & capillary wicking as you've described.
Only conjecture on my part. Makes sense to me though.
Best wishes,
Jim Murphy
Ivor and Olive Lewis on thu 30 sep 04
Dear Jim Murphy,
Surface consolidation seems an acceptable thought and one which might
be amenable to experimental investigation.
Soaking dried specimens in a dyed fluid resin then preparing ground
and polished samples might reveal alterations in pore structure. Could
be a useful exercise for someone who is doing research at one of the
universities.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Ron Roy on fri 1 oct 04
Hi Ivor,
Shame on you - memory indeed!
Fishing line - when just off the spool remembers the spool - same thing -
compressed on one side and stretched on the other.
Seems that using an anthropomorphic term is better when talking to
fishermen and potters - they understand the concept better because they do
not have the scientific vocabulary.
Understanding is more important than being accurate in this case - because
it leads to solutions.
RR
>Dare I say Clay has a MEMORY. ( I shouldn't, it's anthropomorphic not
>scientific)
>Best regards,
>Ivor Lewis.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Vince Pitelka on fri 1 oct 04
> Shame on you - memory indeed!
> Fishing line - when just off the spool remembers the spool - same thing -
> compressed on one side and stretched on the other.
> Seems that using an anthropomorphic term is better when talking to
> fishermen and potters - they understand the concept better because they do
> not have the scientific vocabulary.
> Understanding is more important than being accurate in this case - because
> it leads to solutions.
Thanks for that Ron. And Ivor, you are such a productive contributor to
Clayart, but Ron is absolutely right here. Language gets adapted to the
particular circumstance, and that is as it should be. I do not like to see
inappropriate use of language, especially ceramics terminology, but this is
one of those cases where quibbling over "scientific versus anthromoporphic"
language isn't productive. Same thing is true with the term "compression"
in talking about what happens when we apply pressure against clay. Whether
or not clay or water is compressible is irrelevant, because we are applying
a compressing force, and therefore the language is appropriately
descriptive.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Ivor and Olive Lewis on sat 2 oct 04
Dear Vince Pitelka,
No argument with you there.
Language is a strange phenomenon. Gets begged, borrowed and stolen,
adopted, adapted and variously misused.
Divested of precision it leads to some strange situations.
But consider this. In my first teaching position I was told by Flo
Sergeant, who taught English and Human Biology. "Never knowingly
convey misinformation to your students". In other words, do you
homework and get it right.
In the light of your argument, she was wrong and I have been mislead.
Best regards and enjoy your weekend.
Ivor Lewis.
Redhill,
S. Australia.
Ivor and Olive Lewis on sat 2 oct 04
Dear Ron Roy,
You say <case - because it leads to solutions. >>
In the case of your example, how does your understanding lead to a
solution?
Best regards,
Ivor
Ron Roy on sat 2 oct 04
Hi Ivor,
If one part of a slab shrinks more because it is stretched - different
memories in different parts of the slab - and you understand that - would
it not stand to reason that you would perhaps find a way to make a slab
without conflicting memories - and would that not lead to a solution?
RR
>Dear Ron Roy,
>You say <>case - because it leads to solutions. >>
>In the case of your example, how does your understanding lead to a
>solution?
>Best regards,
>Ivor
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ivor and Olive Lewis on sun 3 oct 04
Dear Ron,
I can imagine a slab with anisotropic internal stress patterns imposed
by contrary strains acting in opposing directions will warp in
peculiar, unfamiliar and unexpected ways. That is knowledge I have in
my memory. For example, I remember that when a slab is rolled in one
direction, and only on one side, will curl as it dries. So, unless I
need a slab like sixteen foot wave on Ninety Mile Beach I use my
memory
Hence it is possible to devise techniques that minimise or avoid
stress development through promoting an isotropic structure and so
prevent movement during stress relief. That is the purpose of
flipping and rotating a slab as it is being worked, either when
throwing, free hand rolling or using the Clay Mangle.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Joyce LEE on sun 3 oct 04
ok
----- Original Message -----=20
From: Ron Roy=20
To: CLAYART@LSV.CERAMICS.ORG=20
Sent: Saturday, October 02, 2004 3:50 PM
Subject: Re: warping slabs. Imagination may help understanding.
Hi Ivor,
If one part of a slab shrinks more because it is stretched - =
different
memories in different parts of the slab - and you understand that - =
would
it not stand to reason that you would perhaps find a way to make a =
slab
without conflicting memories - and would that not lead to a solution?
RR
>Dear Ron Roy,
>You say <>case - because it leads to solutions. >>
>In the case of your example, how does your understanding lead to a
>solution?
>Best regards,
>Ivor
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
=
_________________________________________________________________________=
_____
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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