James Freeman on fri 25 jun 10
On Fri, Jun 25, 2010 at 3:09 PM, Chris Campbell w=
rote:
>>
> In order to see how the clay was moving through the rollers
> I decided to place some colored clay on both sides of a plain
> slice of clay. Then. I ran it through my Bailey slab roller.
>
> After compressing and stretching the clay, I found each
> surface was different. The pattern on the top of the clay had
> stretched 7" while the pattern on the bottom of the clay had
> only stretched 4 1/2". Not much size difference side to side..
>
Chris...
I too have a Bailey slab roller, and have noticed the same effect.
When I roll slabs, I flip the top canvas back over the rolling
mechanism before placing the clay, and let the top canvas feed from
that position (i.e, never covering the advancing slug). This prevents
the canvas from bunching up and ruining the slab. A side benefit of
this method is that one can observe exactly what is happening to the
slug of clay as it is being pulled through the machine. If you watch
from the side, you will see that the clay pretty much stays put on the
bottom, just gently smearing along the canvas, but the top of the clay
slug crests and is pushed away from the machine like a wave. This is
because as the excess clay is being pushed backwards by the rollers,
the bottom canvas, which is supported by the table, prevents the clay
from displacing downward, so the only way it can go is upward. As the
"upward" clay reaches the rollers, it is pushed backward too, but all
of this excess clay on top results in more movement of the top of the
slug than of the bottom. This is a hard thing to describe, but very
easy to see in "real life", and accounts for why your top colored clay
stretched so much farther than your bottom clay.
So why doesn't this excess top stretching over bottom stretching cause
warping? Because it is not really "stretching" at all. Rather, the
top clay is merely being displaced backward to a greater degree than
is the clay on the bottom. It's really more of an extrusion of the
clay than it is a stretching.
As an aside, when I roll slabs I do not let the slug of clay rest on
the bottom canvas at all. I bend the slug upward so that it can be
pushed straight back more evenly. If it is a particularly large slug
of clay, say half a pug, it tries to sag back onto the bottom canvas
as I am rolling. In this case I actually keep bending the slug upward
with my left hand as I roll the wheel with my right (a sometimes
difficult ballet). This prevents the wave of displaced clay discussed
above from flipping over itself like a cresting wave, which, on a
Bailey slab roller, results in an unhealed fault line across the slab.
This was never an issue when I used a Brent roller, because the
Brent's one-roller design results in more of a smearing action than a
rolling action (which is why one-roller slabs seem to warp more than
two-roller slabs), so any gap in the advancing slug tends to heal.
The reason I mention this is because I hypothesize that such a
procedure would result in a more even stretching of your top and
bottom colored clay veneers, though the top would still be stretched
somewhat more than the bottom.
Give it a try, and I think you will readily see the cause.
All the best.
...James
James Freeman
"All I say is by way of discourse, and nothing by way of advice. I
should not speak so boldly if it were my due to be believed."
-Michel de Montaigne
http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/resources
Chris Campbell on fri 25 jun 10
While presenting at the Potters Council Handbuilding
conference at the Spruill Center in Atlanta, my students
and I began discussing the effects we believed a slab roller
had on the clay it was compressing and extending.
We were wondering just what was happening to both sides
of the clay and what effect being compressed and stretched
mainly in one direction had on the clay.
I kept wondering about this after I got home and decided to
try a couple of simple experiments.
In order to see how the clay was moving through the rollers
I decided to place some colored clay on both sides of a plain
slice of clay. Then. I ran it through my Bailey slab roller.
Images : www.ccpottery.com
After compressing and stretching the clay, I found each
surface was different. The pattern on the top of the clay had
stretched 7" while the pattern on the bottom of the clay had
only stretched 4 1/2". Not much size difference side to side..
I decided to flip the piece and roll it again without changing
anything else.
Now the design is 8" on the original top and 6.25 on the
original bottom.
Obviously, the piece was compressed more on its length than
its width.
So, I wondered what this meant to the shrinkage of a piece of work.
I cut it into a 6" by 6" tile and let it rest in the open air.
After 30 minutes, it had shrunk 1/16th inch on all sides.
After 3 hours it had shrunk 1/8th inch on all sides.
Perfectly even shrinkage with absolutely NO warping.
SO ... why did something that appeared to compress and stretch
so unevenly shrink so evenly?
Is the conventional wisdom of rolling again across the grain really a
necessary step if this tile stayed perfectly flat without any help?
My husband, the engineer, speculates slab rollers have more to
do with compression than stretching so this is why it shrinks evenly.
Ideas or suggestions for another experiment are welcomed!
Chris Campbell - in North Carolina
Chris Campbell Pottery LLC
Designs in Colored Porcelain
www.ccpottery.com
SUMMER WORKSHOP
August 15-21, 2010
Touchstone Center for Crafts
1049 Wharton Furnace Road
Farmington PA 15437
1.800.721.0177
http://www.touchstonecrafts.com
MARCH 2011 -Seattle workshop
1 week, hands on, details soon!!
Michelle Williamson-Green on fri 25 jun 10
Hi Chris,
I have done the same thing, mixing two very different clay bodies with
my Bailey slab roller, and the tiles were not only beautiful due to the
difference in clays, but the because there was no warping! I do not
know why it worked, but I like the fact that it did.
-Michelle
On 6/25/10 3:09 PM, Chris Campbell wrote:
> While presenting at the Potters Council Handbuilding
> conference at the Spruill Center in Atlanta, my students
> and I began discussing the effects we believed a slab roller
> had on the clay it was compressing and extending.
>
> We were wondering just what was happening to both sides
> of the clay and what effect being compressed and stretched
> mainly in one direction had on the clay.
>
> I kept wondering about this after I got home and decided to
> try a couple of simple experiments.
>
> In order to see how the clay was moving through the rollers
> I decided to place some colored clay on both sides of a plain
> slice of clay. Then. I ran it through my Bailey slab roller.
>
> Images : www.ccpottery.com
>
> After compressing and stretching the clay, I found each
> surface was different. The pattern on the top of the clay had
> stretched 7" while the pattern on the bottom of the clay had
> only stretched 4 1/2". Not much size difference side to side..
>
> I decided to flip the piece and roll it again without changing
> anything else.
>
> Now the design is 8" on the original top and 6.25 on the
> original bottom.
>
> Obviously, the piece was compressed more on its length than
> its width.
>
> So, I wondered what this meant to the shrinkage of a piece of work.
> I cut it into a 6" by 6" tile and let it rest in the open air.
> After 30 minutes, it had shrunk 1/16th inch on all sides.
> After 3 hours it had shrunk 1/8th inch on all sides.
> Perfectly even shrinkage with absolutely NO warping.
>
> SO ... why did something that appeared to compress and stretch
> so unevenly shrink so evenly?
> Is the conventional wisdom of rolling again across the grain really a
> necessary step if this tile stayed perfectly flat without any help?
>
> My husband, the engineer, speculates slab rollers have more to
> do with compression than stretching so this is why it shrinks evenly.
>
> Ideas or suggestions for another experiment are welcomed!
>
> Chris Campbell - in North Carolina
>
>
> Chris Campbell Pottery LLC
> Designs in Colored Porcelain
> www.ccpottery.com
>
>
> SUMMER WORKSHOP
> August 15-21, 2010
> Touchstone Center for Crafts
> 1049 Wharton Furnace Road
> Farmington PA 15437
> 1.800.721.0177
> http://www.touchstonecrafts.com
>
> MARCH 2011 -Seattle workshop
> 1 week, hands on, details soon!!
>
>
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