iandol on thu 19 apr 01
Dear Vince,
I am asking questions about the Nature of Plastic Clay. "S" cracks are =
just one phenomenon which seem to be incompletely inderstood which =
depend on the nature of plastic clay flocs. I agree you that the =
empirical evidence you bring to the discussion, which is derived from =
your practical experience, provides a way of alleviating this nuisence. =
I know from my own practical experience about the flow patterns in wheel =
thrown and pugged clay.
Like you, I can expain the stress. But I cannot explain why there should =
be a rupture if the clay is uniform except by differential thickness or =
uneven drying. Something always initiates a crack. That is the beast I =
seek.
I am please you are making a valuable input into this discussion.
Best regards,
Ivor
Ross Hartman on fri 27 apr 01
FWIW;
iandol wrote:
> Like you, I can expain the stress. But I cannot explain why there shoul=
d be a rupture if the clay is uniform except by differential thickness or=
uneven drying. Something always initiates a crack.
I remember reading a paper in the ACerS digest (this was 25 or 30 years a=
go) that discussed this problem. As I remember the crack always occurs at=
the largest particle in the matrix. This was dealing with very small par=
ticle size as I remember and the distribution of the particle size. So wh=
en you say 'if the clay is uniform' I am assuming you mean the clay has a=
very even particle size distribution (not a trivial task). But the inclu=
sion
of a single larger particle would create a candidate position for where =
a crack might begin. Also it seems that this would be something that you =
would be able to control only under the most stringent manufacturing stan=
dards (perhaps in a lab setting). As I understand the problem... (if anyo=
ne would like to correct me I'm listening because I'm not sure I remember=
this 100 o/o correctly )
Hope this helps..
L8R/Ross
> That is the beast I seek.
> I am please you are making a valuable input into this discussion.
> Best regards,
> Ivor
>
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iandol on tue 1 may 01
Dear Ross Hartman,
Thank you for that information.
Rather than uniform particle size, I was thinking of uniform =
distribution of all of the materials which contribute to a prepared clay =
body. The differences between the smallest clay crystal and grog grains =
in an ordinary smooth throwing clay which may be designated as say, =
being 60# is considerable. But I believe that during its progress from =
the bag, through the pug mill, being pounded and kneaded on the bench =
before it gets to the wheel may change the distribution. Air inclusions, =
segregation, lamination, stray bits of grog or crumbs of clay can all be =
collected and be worked into soft clay as potential defects. Any of =
these could act as a stress raiser and act as a point from which a crack =
might initiate if the stress gets high enough.
I would like to get hold of that article. Do you have access to the =
AcerS archives?=20
Thank you for contributing to this discussion.
Best regards,
Ivor Lewis. Redhill, South Australia.
JOYCE LEE on fri 28 oct 05
Rick Bonomo asked:
Would someone be kind enough to post a link to a picture of an " =3D
S-crack" for those of us who haven't seen one?
-------------------------------------------------------------------------=
---
I'm sure someone will, Rick, but this might be a good time to
mention "The Potter's Dictionary" by Hamer and Hamer. They
post pictures of various types of cracks and give detailed
explanations of their cause as well as how to avoid them. I've
found it wonderfully helpful.
Joyce
In the Mojave Desert of California USA
Ivor and Olive Lewis on fri 3 nov 06
Dear Karen Nakakihara,
Sorry to learn you have had such a knock back with your work.
Well, you have had a lot of advice which seems to cover the main points. =
What does not seem to be fully understood is the way clay behaves when =
you apply pressure to the interior surface of the base of a pot. Done in =
the best manner this action can cause considerable motion within the =
clay.
Laying a flat blade across the floor of the pot and pressing this into =
the clay as it rotates does nothing to change existing flow structures =
that remain after pugging, wedging or kneading. So as it dries, =
especially if tension develops as outlined by Vince Pitelka and others, =
these structures are places of weakness and you get a typical S crack.
But if you commence your pressing run at the wall of the pot and push =
your fingers down and towards the centre several times you cause the =
clay to flow disrupting the weak places and refining the structure. This =
can be demonstrated by using markers of a differing colour (Think Agate =
patterns). When weak structures are removed the clay stands a better =
chance of surviving.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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