Vince Pitelka on fri 14 oct 05
Steve Graber wrote:
> no - i don't think so. the "S" crack indicates limited centering. better
> centering would be a smaller "s" followed by best centering with the basic
> "dot" or no crack. from a good foundation pulling anything away toward
> walls & such doesn't affect the center. the compression everyone talks
> about is really bridging the possible "S" crack undernieth. hiding it."
Steve,
If you really think about this, you will see that the "S" crack has nothing
to do with centering, other than the fact that the crack is a different
shape when the clay is poorly centered. It is all about compression. The
proof is in throwing off the hump. A student can cone the clay up and down
at the top of the hump, centering the clay thoroughly through to its core,
but then when they throw a tea bowl, if they do not carefully compress the
bottom, they will get an "S"-crack every time, and it will look identical to
the "S"-crack you get when throwing a bowl or cup right on the wheelhead
without properly compressing the bottom. This really doesn't have anything
to do with whether you center properly.
It is a well known phenomenon that many people widen the bottom of a pot by
pulling clay away from the center, and that the bottom remains uncompressed
in the process. If they do not go back and compress the bottom, then the
walls will be more compressed, and in drying the bottom will shrink more
than the walls. The clay will crack along the throwing spiral - thus the
"S" crack - this not conjecture - it has been proven by studies of the clay
structure.
You said:
> if you get down to the detail grains of clay they are connected by their
> little fish scale like platelets. a good centering doesn't disrupt that."
That's not quite accurate. The clay is not in grains. The "fish scales"
you refer to ARE the platelets -the particles of clay, and it is the water
layers between all those platelets that give clay its plasticity. Throwing
orients the platelets in a linear grain structure in a spiral opposite the
direction of the wheel, because your hands are holding the clay back
clockwise while the wheel is pulling it counterclockwise.
You wrote:
good centering yields no "S". that's why we seldom see it in small pots.
Where are you getting this information? Exactly the opposite is true. We
see "S" cracks FAR more often on small pots - tea bowls, sake cups, coffee
mugs - no matter how well-centered the clay.
Think about this some more and you will see that centering has little to do
with "S" cracks.
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/
Craig Martell on sat 15 oct 05
Vince was saying:
>Where are you getting this information? Exactly the opposite is true. We
>see "S" cracks FAR more often on small pots - tea bowls, sake cups, coffee
>mugs - no matter how well-centered the clay.
Hello Vince et al:
I've been following this thread and wondered when someone would bring up
the above. My experience has been that I see many more smaller pots,
thrown off the hump, develop "s" cracks. I honestly haven't seen as many
larger pots with this sort of crack in my own work or that of others. This
also accounts for the fact that there are more smaller pots made than larger.
The crux of the issue is particle alignment. If there are different kinds
of alignment in a pot this can cause unequal rates of shrinkage which can
produce all kinds of cracks. The most common is the "s" crack where the
walls of the pot are subject to compression from the inside and outside
during the throwing process, and sometimes compression of the base or
bottom is neglected which gives a different particle alignment and a
different rate of shrinkage. As a bonus you can get a nice "s"
crack. See: Hamer and Hamer, The Potter's Dictionary of Materials and
Techniques, in the "C" section under "Crack."
The idea that incomplete centering is the main cause of an "s" crack is not
borne out by years of inquiry and practice in the world of wheel thrown
pottery. Lots of things can be made "off center" and emerge from the kiln
in a-ok shape. Seen this and have done it myself many times. I've
discussed this many times with my friend Jerry at the Off Center Cafe on
Center St. That's not a joke the Cafe and St. are the real deal.
regards, Craig Martell Hopewell, Oregon
Steve Slatin on sat 15 oct 05
Vince --
This is one of the best summaries I've seen of our
understanding of the S crack penomenon, and I should
hesitate to ask this question ... especially as it is
on the order of an irregular observation and not based
on any theory of any kind, but --
My (wholly subjective, anecdotal) observation is that
in smaller pieces the S crack occurs on pieces that
had a good deal of slip at the bottom at the end of
the second pull or later. Is there a correlation
between the moisture and the S crack? I recognize
that there is an exogenous variable in the nature of
some possible compression applied in the process of
getting the moisture out ...
Maybe I need to get a microscope so I can get a better
look at the little stuff.
Best wishes -- Steve S.
--- Vince Pitelka wrote:
> Steve,
> If you really think about this, you will see that
> the "S" crack has nothing
> to do with centering, other than the fact that the
> crack is a different
> shape when the clay is poorly centered. It is all
> about compression. The
> proof is in throwing off the hump. A student can
> cone the clay up and down
> at the top of the hump, centering the clay
> thoroughly through to its core,
> but then when they throw a tea bowl, if they do not
> carefully compress the
> bottom, they will get an "S"-crack every time, and
> it will look identical to
> the "S"-crack you get when throwing a bowl or cup
> right on the wheelhead
> without properly compressing the bottom. This
> really doesn't have anything
> to do with whether you center properly.
>
> It is a well known phenomenon that many people widen
> the bottom of a pot by
> pulling clay away from the center, and that the
> bottom remains uncompressed
> in the process. If they do not go back and compress
> the bottom, then the
> walls will be more compressed, and in drying the
> bottom will shrink more
> than the walls. The clay will crack along the
> throwing spiral - thus the
> "S" crack - this not conjecture - it has been proven
> by studies of the clay
> structure.
>
> You said:
> > if you get down to the detail grains of clay they
> are connected by their
> > little fish scale like platelets. a good
> centering doesn't disrupt that."
>
> That's not quite accurate. The clay is not in
> grains. The "fish scales"
> you refer to ARE the platelets -the particles of
> clay, and it is the water
> layers between all those platelets that give clay
> its plasticity. Throwing
> orients the platelets in a linear grain structure in
> a spiral opposite the
> direction of the wheel, because your hands are
> holding the clay back
> clockwise while the wheel is pulling it
> counterclockwise.
>
> You wrote:
> good centering yields no "S". that's why we seldom
> see it in small pots.
>
> Where are you getting this information? Exactly the
> opposite is true. We
> see "S" cracks FAR more often on small pots - tea
> bowls, sake cups, coffee
> mugs - no matter how well-centered the clay.
>
> Think about this some more and you will see that
> centering has little to do
> with "S" cracks.
> Best wishes -
> - Vince
>
> Vince Pitelka
Steve Slatin --
Drove downtown in the rain
9:30 on a Tuesday night
Just to check out the
Late night record shop
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Vince Pitelka on sat 15 oct 05
Steve Slatin wrote:
> My (wholly subjective, anecdotal) observation is that
> in smaller pieces the S crack occurs on pieces that
> had a good deal of slip at the bottom at the end of
> the second pull or later. Is there a correlation
> between the moisture and the S crack? I recognize
> that there is an exogenous variable in the nature of
> some possible compression applied in the process of
> getting the moisture out ...
Steve -
In my experience, having water or slip in the teabowl or cup during throwing
is not an issue, as long as one doesn't take TOO long to create the vessel,
and as long as one removes all water or slip from the inside before removing
the pot from the wheel. And yes, to some extent I think the act of removing
the water can apply some compressive force. I guess that depends on
individual technique. Some people swab out the water with a pretty light
hand.
In my experience, the "S" crack is almost entirely an issue of compression.
Craig Martell provided great clarification. We can't "compress" clay and
water as an air compressor compresses air. When we compress the bottom of a
pot, we are simply applying compressive that increases alignment of
platelets in a flattened grain structure parallel to the bottom, which makes
the clay in the bottom shrink more in thickness than in diameter, and thus
"S" cracks are eliminated or reduced.
Regarding the problems caused by water or slip left in the bottom of a pot -
clay is plastic because platelets are flat and smooth and have an afinity
for water, maintaining a water layer between each platelet. Drying
shrinkage occurs as the water layers disappear and the platelets move closer
together. If water or slip is left in the bottom of a pot, it absorbs into
the clay, moving the platelets farther apart, disrupting the grain
structure. But the bottom of the pot is confined by the walls, and thus the
increase in volume occurs only as an increase in thickness. However, when
the clay starts to dry, it shrinks in all directions, and "S" cracks form.
I hope this all makes sense. I've been watching and thinking about these
phenomena for 35 years now, and I think I am starting to get a pretty good
handle on platelets, water, and grain structure. I went through lots of
theories over the years, and kept running head-on into contradictions. Now,
the theories seem to align more closely with the physical evidence, which is
what any scientist hopes for. Hey, does that mean I are a scientist?
My mother Dr. Dorothy Pitelka was a fine scientist, the first on the West
Coast to be trained in the use of the electron microscope. I wish she was
around now to slip in a few clay images.
- 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 sun 16 oct 05
Dear Craig Martell,=20
No arguments with either yourself or Vince. But I am a little confused =
about ascribing the problem to the Kaolinite mineral crystals and wonder =
to what extent they actually do conform to our concept of alignment, =
that is rotating so that they are normal to the direction of the applied =
pressure.
The things that I worry about are scale and volume. Give that a clay =
body with a bit of tooth to it has particles as coarse as 1 millimetre =
and that these are blocky fragments, whereas the average dimensions of a =
particle of clay is 1mu (micron or 1/1000 mm) across the face and 0.1mu =
thick then you would be dealing with about ten billion clay crystals if =
every cubic millimetre was Kaolin mineral and about half as many in =
pastes that are based on the 50-25-25 mixture such as porcelain bodies. =
The other thing is that the ratios of the ingredients of a plastic body =
generally show that the major proportion of the material, expressed as =
volume rather than mass, is Water. The values become even more =
pronounced if you make an adjustment for depression of the density of =
the water when it form the "Water Hull" described by Lawrence and West =
or the " Structured Solid Water" alluded to by Grim. I agree, Frank =
Hamer's illustrations of cracks are a good education in circumventing =
these things or explaining them when they occur but the science does not =
hold up.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Bonnie Staffel on sun 16 oct 05
I used to get S cracks on my "off the hump" pots quite often. However,
when I saw the description of the coning process of getting the inner
clay up to the top and not just moving the outside clay, my S cracks
have mainly disappeared. I throw with very little water and mainly use
the slip that accumulates on my hands for lubrication on these small
pieces. I also had the theory that on them, the sides dried much faster
than the bottoms so turned them over as quickly as I could to offset
this problem. For whatever reason, my problem has disappeared.
Bonnie Staffel
http://webpages.charter.net/bstaffel/
Neil Grant on mon 17 oct 05
I always cut my lids etc from the hump nwith a blunt side of a bamboo tool =
so that the 3-4 mm blunt edge is forced into the clay. this has the =
effect of comprssing the clay and a spin off is that the liitle cone left =
in the centre lifs the article off the ware board and it dries evenly. =
Touch wood but I never get S cracks any more!!!
=20
Neil Grant
Otago polytechnic
Dunedin New Zealand
>>> bstaffel@CHARTERMI.NET 10/17/05 7:46 a.m. >>>
I used to get S cracks on my "off the hump" pots quite often. However,
when I saw the description of the coning process of getting the inner
clay up to the top and not just moving the outside clay, my S cracks
have mainly disappeared. I throw with very little water and mainly use
the slip that accumulates on my hands for lubrication on these small
pieces. I also had the theory that on them, the sides dried much faster
than the bottoms so turned them over as quickly as I could to offset
this problem. For whatever reason, my problem has disappeared.
Bonnie Staffel
http://webpages.charter.net/bstaffel/
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Craig Martell on thu 20 oct 05
Ivor was saying:
>But I am a little confused about ascribing the problem to the Kaolinite
>mineral crystals and wonder to what extent they actually do conform to our
>concept of alignment, that is rotating so that they are normal to the
>direction of the applied pressure.
Hello Ivor:
I didn't mean to let this linger but I've been splitting wood and firing kilns.
Who of us is putting the blame on the kaolinite crystals? I reread Hamer
and I don't think that's the case with his explanations in the
book. Although I was more than non specific about a lot of things in my
two posts about S cracks, I was considering the claybody as a whole. What
I learned from Harry Davis more than 25 years ago was to subject the pot,
base and walls, to the same sort of forming pressures to reduce the amount
of cracking. When we form pots on the wheel the walls of the pot are
worked much more than the base so it makes sense to give that foundation
some thoughtful attention before and perhaps after raising the walls. I
think that this does have a bearing on particle orientation but as to how
much we affect that condition I have not a clue.
As a matter of record, I work with porcelain bodies that contain no grog or
calcined materials. The non plastics are feldspar and ground
quartz. Theses bodies have not shown a tendency to S crack. I mix them by
blunging the plasticizer, vee gum T, in the exact amount of added water and
adding this to the dry mixed body in the clay mixer. I mix the clay wetter
than suitable for use and dewater and pug before use. I also wedge the
clay and then spiral knead before throwing. I invert the spiral to the
"up" position and the "point" is placed on the wheelhead. Also on advice
from Harry Davis.
I don't have anything to say or add about the water hull concept or the
solid particle/water ratios other than one odd case. I will sometimes
throw plates and bowls with very soft clay which would contain a higher
amount of water. When I center and depress and expand the mass the edges
will sometimes split in three or four places. If I apply pressure these
splits will heal and the piece will come out of the kiln with nary an edge
crack. The main thing is that this will happen consistently with all the
lumps I've prepared for throwing unless I stop and dewater the clay a bit
more and then throw the flatware pieces. The stiffer clay does not
fracture at the edges. Obviously, this isn't a statement containing any
molecular level explanation, mainly just problem and remedy.
for what it's worth, Craig Martell Hopewell, Oregon
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