Craig Clark on tue 18 oct 05
With all of this discussion about S-cracks I keep seeing the word
compressed being used, as in compression of the clay. Is this in fact
what occurs? I was under the impression, perhaps mistaken, that clay is
like sand and it does not compress. I understand that a compressive
force may be applied to the clay but does an actual compression occur?
Thankyou for any and all info
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
Vince Pitelka on wed 19 oct 05
> With all of this discussion about S-cracks I keep seeing the word
> compressed being used, as in compression of the clay. Is this in fact
> what occurs? I was under the impression, perhaps mistaken, that clay is
> like sand and it does not compress. I understand that a compressive
> force may be applied to the clay but does an actual compression occur?
Craig -
Years ago, someone here on Clayart got quite upset about the use of the word
"compression" in this case, and actually wanted us to use a different word.
I think that it is enough that we apply a compressive force. I agree that
the clay and water are not compressible, but the word "compression" still
seems appropriate in this sense.
- 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 thu 20 oct 05
Dear Craig Clark,
In my opinion you are not in error in your thinking. Clay pastes, to the =
degree that we can apply pressure by hand and contain them are =
incompressible.
As I understand things, in geological terms, clay minerals responds to =
pressure by moving, then reorienting then undergoing metamorphism to =
become, first a shale, then slate. I have a sample of Shale that I am =
weathering. It is interesting to see that some of the fissures that are =
generating are at right angles to the bedding plane. Must take a picture =
of that sample before it finally disintegrates. Some shales can be a =
useful source of plastic clay.
People who follow the instruction to compress the inner base of a pot =
after removing water or slurry are applying pressure. There is no =
volumetric change ! The outer wall has to be contained to prevent the =
clay moving outwards if they move their fingers (or tool) from the =
centre of rotation to the inner wall. When the finger (or tool) is moved =
inwards applying pressure towards the centre of rotation, a button of =
clay builds up. The trick is to cause this to seemingly excess clay to =
submerge back into the centre. Never remove it ! This action forces any =
potential weak spots, voids or irregularities down to the wheelhead. =
Several passes inwards drive the submerged clay across and outwards =
where it sits supporting the cylindrical wall. A cyclic pattern can be =
exposed by using markers, pellets of contrasting clay.
Pulling the clay towards the cylinder thins the base, not a good thing =
to do if you pot design requires clay be left for a deep footring
I think part of the problem is that those authors who have written =
instructions for throwing have not thought to investigate or explain the =
techniques they promote.
Thanks for asking your question and putting forward a valid point of =
view.
Best regards,
Ivor
Gordon Ward on thu 20 oct 05
First, I don't know if clay is "compressible", but my experience tells me that something like it is
going on when using a throwing arm. Sometimes the bottoms of plates will actually bow up - and
never an s-crack.. This theoretically could be caused also by the relatively small amount of water
required to move the clay, so it's a little hard to say exactly what's going on.
When throwing off the hump, it is a good idea to minimize the water in the bottom as has been
noted. But the other little trick is before trimming, take your knuckle and tap on the center
portion of the foot area as the wheel slowly turns. You can make a small depression without
affecting the inside contour. I never seem to get an s-crack if I do this little step before trimming.
Gordon
Randall Moody on fri 21 oct 05
Of course clay is compressible. Otherwise we wouldn't use ram presses. Afte=
r
all of the discussion back and forth I am thinking that when we "Compress
the bottom" we are actually just aligning the clay particles. When I first
used B-mix all of my cups s-cracked even though I was doing the exact same
thing as I was when I was using Soldate 60 or porcelain. I stopped using
b-mix and had no more problems with s-cracks. I know that many people swear
by b-mix but if I want a finicky clay body I will stick with porcelain.
On 10/20/05, Gordon Ward wrote:
>
> First, I don't know if clay is "compressible", but my experience tells me
> that something like it is
> going on when using a throwing arm. Sometimes the bottoms of plates will
> actually bow up - and
> never an s-crack.. This theoretically could be caused also by the
> relatively small amount of water
> required to move the clay, so it's a little hard to say exactly what's
> going on.
>
> When throwing off the hump, it is a good idea to minimize the water in th=
e
> bottom as has been
> noted. But the other little trick is before trimming, take your knuckle
> and tap on the center
> portion of the foot area as the wheel slowly turns. You can make a small
> depression without
> affecting the inside contour. I never seem to get an s-crack if I do this
> little step before trimming.
>
> Gordon
>
>
> _________________________________________________________________________=
_____
> 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.
>
skiasonaranthropos@FSMAIL.NET on fri 21 oct 05
Hello Ivor,
How does your statement that =93Clay pastes ... are incompressible=94
correlate with clay bodies being able to exist at different bulk
densities? This can be illustrated with two examples: 1) a slip cast
article will have a higher dry bulk density than if plastic made 2) dry
bulk densities can vary across a cross section of a deaired pug column
Any thoughts?
Antony
Ivor and Olive Lewis on sat 22 oct 05
Dear Anthony,
I'm not sure if I comprehend your message. Can you give me a definition =
of Bulk Density?
As an example you say "..dry bulk densities can vary across a cross =
section of a de-aired pug column.." Surely a de-aired pug of clay is =
moist so it cannot have a dry bulk density.
I think the point I was making was that a deflocculated casting slip has =
a greater mass of solids per unit volume than a plastic clay body (Say a =
cubic foot of each). But the volume of water in the former is much =
smaller than the latter. The one with the least water is a liquid but =
the one with most water is a solid. This is incongruous. Both, under =
normal circumstances, are incompressible.
If you think about it, when an "S" crack forms the fracture is through =
water, not through clay ! ! !
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
skiasonaranthropos@FSMAIL.NET on mon 24 oct 05
Hello Ivor,
Thanks for your comments, and in reply:
1) Bulk density: is quite a common measurement in ceramics, and can be
defined as the ratio of the mass of the material to its bulk volume. A
typically unit is g per cm3.
2) Compressibility: To expand upon my earlier message; one check that can
be made on the performance of a pug is to take a cross sectional slice,
lay it flat on a bench and draw a standard grid across its surface. Each
drawn block is numbered for identification. The scored lines are cut
through to produce a series of blocks. The bulk densities of these are
determined using water displacement, Archimedes Principle, before drying
to 0% moisture. After drying the mositure content of the blocks are
determined and the bulk density remeasured using mercury rather than water
These results give the gradient of wet bulk density and moisture content
at extrusion, and the bulk density after drying. Such a procedure can be
useful to gauge the performance and particular set up of the pug. What is
evident though is that bulk density is not consistent across the slice,
and does not always parallel moisture content
My previous message also noted that a slip cast article would have a
higher bulk density than if made from a plastic body of the formulation
So what is the explanation for these phenomena?
3) Plastic body / casting slip. Im puzzled by your statement =93I think the
point I was making was that a deflocculated casting slip has a greater
mass of solids per unit volume than a plastic clay body (Say a cubic foot
of each). But the volume of water in the former is much smaller than the
latter. The one with the least water is a liquid but the one with most
water is a solid.=94
From my experience I suggest that:
i. A typically commercial plastic body could be expected to have a
moisture content of 18 to 22%. Assuming an average SG of 2.6 this equates
to 78 to 82% solids by mass
ii. Commercial casting slips are found slip densities of 1,700 to 1850 g
litre. Assuming an average SG of 2.6 this equates to 66.9 to 74.7% solids
by mass
Of course these values would be greatly influenced by a whole range of
factors, including properties of individual clays, ratio of non-plastics,
type and dosage of deflocculant
These suggest that a plastic body often, but this is not always so, has a
higher mass of solids than a casting slip. Were you just trying to
highlight the difference in SG between the slip raw materials and water?
Could you elaborate?
Its also probably a good idea not to think of a slip as being either
deflocculated or flocculated as this suggests just two conditions rather
than the actual case of something more akin to a continuum
Best regards, Antony
| |
|
