Dave Finkelnburg on sat 31 mar 07
Good morning!
Why does a spout, or a whole pot, "unwind" in the
firing? This topic is a little like the daffodils in
the yard. It comes up in its season with dependable
regularity.
It is my OPINION that this "unwinding" is simply a
subset of warping. Warping, I have been told by
someone who really has studied that and who will
remain unnamed (but whose initials are Dr. W.C.
Carty), is caused by different density in different
parts of the formed ceramic. This causes different
shrinkage in places--less dense parts shrink more than
the denser parts do.
The classic example is a slab rolled vigorously in
one direction. A tile cut from the slab wants to curl
up from both ends in the direction the slab was
rolled. Form the slab by rolling it at right angles
and then at 45-degrees to that and, while it may curl
from ALL edges due to drying most rapidly there (a
separate issue) it doesn't just curl up in one
direction like the bow and stern of an old viking
ship.
I speculate that what's at work must be the forming
process literally pulling some of the clay particles
apart a tiny bit in the process of rolling the slab.
More of that pulling is going on in one direction than
the other.
I speculate, again, throwing with firm pressure on
a fast wheel must also literally pull some clay
particles apart a tiny bit and create an opportunity
for the clay, during drying and firing, to attempt to
return to uniform density. So a spout thrown firmly,
fast and thin tends to "unwind" in the kiln.
One can allow for this. If density is at work,
though, it may lead one to think about their forming
and prevent the problem in the first place.
Good potting!
Dave Finkelnburg
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Vince Pitelka on sat 31 mar 07
Dave Finkelnberg wrote:
> It is my OPINION that this "unwinding" is simply a
> subset of warping. Warping, I have been told by
> someone who really has studied that and who will
> remain unnamed (but whose initials are Dr. W.C.
> Carty), is caused by different density in different
> parts of the formed ceramic. This causes different
> shrinkage in places--less dense parts shrink more than
> the denser parts do.
Dear Dave -
Oh oh. Have we lost you to the dark side? Are you going to trust the word
of the academics over your own seat-of-the-pants studio experience now?
Please see the smile on my face. I am always interested in the work of
ceramic scientists.
I've written on Clayart a number of times on the idea of linear-grain
structure in clay and the effects it has on shrinkage and warping. Keep in
mind that everything I state is based on what I know from past experience
and research, none of which is based on scientific research in the strictest
sense of the concept. The example you cite of slabs is one of the most
common ones - that slab-rollers produce unidirectional linear grain
structure that shrinks more across the grain than along the grain.
I'd like to hear more about the scientific theory on linear grain structure.
We're dealing with flat particles, so it is easy to see how they will align
dependent on forces applied. In the case of throwing, the linear grain
structure is in the spiral represented by the torque applied by the hands
pulling the clay up, opposite the direction of wheel rotation. As is the
case with a slab, simple observation shows that shrinkage is greatest across
the grain rather than in the direction of the grain structure. Thus,
shrinkage is on a spiral in the same direction but diagonally opposite to
the throwing spiral. Or, to put it another way, if you look at a pot, on
the side facing you the throwing spiral is from lower right to upper left,
and the shrinkage will be in the same direction, but from upper right to
lower left, across the grain.
On larger diameter forms, I think that the spiral shrinkage across the grain
is neglible and rarely noticed, but it is quite noticeable on a teapot spout
with an asymmetrically-cut tip. I have never noticed the effects of this
spiral shrinkage on mugs, but I can see how it could be sufficient to offset
the handle on a tall, thin mug.
In my days as a full-time studio potter in Northern California I had a
rather severe lesson in such spiral shrinkage. I had a request from a
retailer to make porcelain candlesticks similar to the old jack-be-nimble
variety. I had never experienced this sprial shrinkage, so, instead of just
making a loop handle on the "plate" portion of the candle stick at the
bottom, I decided to connect the handle from the rim of the plate to just
below the rim of the candle bowl. These candlesticks had long stems
tapering down to about 3/4" in diameter and perhaps 5" tall overall. They
did fine through the drying and bisque-firing, but they all came out of the
glaze firing with the handle ripped partially or entirely off the rim of the
plate. The attachment point at the upper end was considerably more
substantial, like the handles on most mugs, and thus it held. Three or four
dozen candlesticks in the trash. So, I started just using a smaller loop
handle with both ends attached to the plate, and no more problems. They
looked better too. Gee, I wonder why?
- 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/
Duff bogen on sat 7 apr 07
Shrinkage due to differences in density makes sense. The out side of a spout gets more compression. In my experience I take a fettling knife and ream out the inside of spouts (to reduce weight). This stops the unwinding. By this theory I'm cutting away the less dense inner surface thereby making a uniform density and no unwinding. My image was I was cutting the "tendons" of tension leftover from stretching the clay.
Duff
Dave Finkelnburg wrote:
Good morning!
Why does a spout, or a whole pot, "unwind" in the
firing? This topic is a little like the daffodils in
the yard. It comes up in its season with dependable
regularity.
It is my OPINION that this "unwinding" is simply a
subset of warping. Warping, I have been told by
someone who really has studied that and who will
remain unnamed (but whose initials are Dr. W.C.
Carty), is caused by different density in different
parts of the formed ceramic. This causes different
shrinkage in places--less dense parts shrink more than
the denser parts do.
The classic example is a slab rolled vigorously in
one direction. A tile cut from the slab wants to curl
up from both ends in the direction the slab was
rolled. Form the slab by rolling it at right angles
and then at 45-degrees to that and, while it may curl
from ALL edges due to drying most rapidly there (a
separate issue) it doesn't just curl up in one
direction like the bow and stern of an old viking
ship.
I speculate that what's at work must be the forming
process literally pulling some of the clay particles
apart a tiny bit in the process of rolling the slab.
More of that pulling is going on in one direction than
the other.
I speculate, again, throwing with firm pressure on
a fast wheel must also literally pull some clay
particles apart a tiny bit and create an opportunity
for the clay, during drying and firing, to attempt to
return to uniform density. So a spout thrown firmly,
fast and thin tends to "unwind" in the kiln.
One can allow for this. If density is at work,
though, it may lead one to think about their forming
and prevent the problem in the first place.
Good potting!
Dave Finkelnburg
____________________________________________________________________________________
Looking for earth-friendly autos?
Browse Top Cars by "Green Rating" at Yahoo! Autos' Green Center.
http://autos.yahoo.com/green_center/
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