Dai Scott on sat 27 oct 01
Llewellyn - I was taught that when you are finished with the actual wedging,
have slapped the ends in, and your clay is now is a sort of fat cylinder
shape, you should stand it with the right hand side UP, thus having a
from-left-over-to-right, or clockwise, swirl. This way, when you put it on
the wheel, and your wheel is running counter-clockwise, you aren't running
"against the grain", so to speak. It may be my imagination, but I find it
much easier to center if I follow these rules.
After suffering some small cracks in the bottom of things thrown off the
hump, I followed advice given on clayart to put your clay on "sideways"
(presumably that's in the same position as when you're wedging it), to
prevent these cracks from happening. I found it a little harder to center,
but I was also using a fairsized chunk of clay. Incidentally, changing the
way I put the clay on didn't eliminate the cracks. In fact the last batch
of sake cups I threw off the hump suffered about a 35% loss due to cracks.
I was pretty choked. I'm using Plainsman M370, very pricey, and also found
cracks in the bottom of mugs (not thrown off the hump). Don't know why---I'm
doing just about what I've always done for years! Very frustrating!
Dai in Kelowna, BC
"There is no right way to do the wrong thing."
potterybydai@shaw.ca
----- Original Message -----
From: "Llewellyn Kouba"
To:
Sent: Saturday, October 27, 2001 3:25 PM
Subject: [CLAYART] Still puzzling throwing
> RE: Question still puzzling about throwing
>
> I have thrown pots for some years but sometimes changes or discovery comes
> slow. Or shall I say coming to a new level or realization can be
> frustrating. All that aside.
>
> As the potter wedges clay he picks it up and it is 'swirled' in a certain
> motion....(I use what I was taught as the Oxtongue form)....now when you
go
> to the wheel - the wheel turns counterclockwise. So my question for myself
> now for too many years is: Does it make any difference which way or 'end'
> of the now wedged mass you slam down on the wheel head? I am beginning to
> think it is better to put the newly wedged piece 'swirl clockwise' . It
> seems if you put the mass the other way they both have been turned (wedge
> and wheel motion) in the same way and seems to fight everything......or
> does it make a difference or matter at all? Hope my description is clear
> enough. What has been your experience. Not a problem on small mass but
> the big ones more troubles.
>
> Llewellyn
>
>
____________________________________________________________________________
__
> 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.
Llewellyn Kouba on sat 27 oct 01
RE: Question still puzzling about throwing
I have thrown pots for some years but sometimes changes or discovery comes
slow. Or shall I say coming to a new level or realization can be
frustrating. All that aside.
As the potter wedges clay he picks it up and it is 'swirled' in a certain
motion....(I use what I was taught as the Oxtongue form)....now when you go
to the wheel - the wheel turns counterclockwise. So my question for myself
now for too many years is: Does it make any difference which way or 'end'
of the now wedged mass you slam down on the wheel head? I am beginning to
think it is better to put the newly wedged piece 'swirl clockwise' . It
seems if you put the mass the other way they both have been turned (wedge
and wheel motion) in the same way and seems to fight everything......or
does it make a difference or matter at all? Hope my description is clear
enough. What has been your experience. Not a problem on small mass but
the big ones more troubles.
Llewellyn
iandol on sun 28 oct 01
dear Llewellyn=20
This is one of those ideas that surfaces occasionally, usually related =
to the dreaded "S" crack.
I forget the details but the consensus seems to be to make sure the =
grain spiral in the clay is across the wheel head, not concentric with =
it to prevent this scourge.
Arguments are that if put on upside down it unwinds the spiral texture =
introduced by kneading either Rams head or Chrysanthemum style, put the =
right way it tightens the spiral and alleviates cracking.
Having done some tests, I tend to disbelieve these theories. One of the =
things which must occur during the kneading process is that the =
clay/water complex, which is disrupted or sliced by the shearing effects =
of moving the clay, should reweld to become a coherent continuous solid. =
This can only occur if there is sufficient water in the duplex =
clay/water structure. Without water to provide cohesion, the fractures =
or discontinuities may remain within the fabric and act as stress =
raisers as clay starts to dry. Hence, trying to gain additional height =
when throwing by using firmer clay may be defeating the intention. There =
are other factors which influence the initiation of basal cracks but =
they are not relevant in this discussion.
Best regards,
Ivor Lewis. Redhill, South Australia.=20
vince pitelka on tue 30 oct 01
> Now, for those of with pugmills comes the following question:-
> is there any benefit from continuing the swirl the pug mill has caused,
onto
> the wheel?
> i.e. should we note the rotation of the pug and continue it on the wheel?
> Or, is there any benefit in putting the cut pugs transversely on the wheel
> head?
Martin -
When I was doing production throwing in California I prepared all my clay
with a deairing pugmill, with no wedging at all. For small things I just
tore chunks off the pug and made balls, but for all other things I placed
the pug vertically on the wheel. I always wheel wedge (cone) the clay a few
times. In all my years in the studio in California, the only time I
occasionally had bottom cracks was while working with porcelain. In my
stoneware work I do not recall ever having any cracks appearing in the
bottoms during drying or firing.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Martin Howard on tue 30 oct 01
Dai Scott posted shape, you should stand it with the right hand side UP, thus having a
from-left-over-to-right, or clockwise, swirl. >
Now, for those of with pugmills comes the following question:-
is there any benefit from continuing the swirl the pug mill has caused, onto
the wheel?
i.e. should we note the rotation of the pug and continue it on the wheel?
Or, is there any benefit in putting the cut pugs transversely on the wheel
head?
I could plan my pugging, cutting and placing on the head in any of those
three ways, but have not done any controlled experiment to ascertain which
is best.
Martin Howard
Webbs Cottage Pottery
Woolpits Road, Great Saling
BRAINTREE, Essex CM7 5DZ
01371 850 423
martin@webbscottage.co.uk
http://www.webbscottage.co.uk
This web-site is being updated NOW!
Christena Schafale on tue 30 oct 01
Dai,
I think the "sideways" tip only works when you are throwing a single piece,
not for off-the-hump, since the cause of the s-cracks in hump-thrown pieces
is different. What I have found that does work for hump pieces is to
recompress the bottom from the inside before trimming. I use my Giffen
grip to hold the piece, then just compress firmly with a dry finger,
running it back and forth across the bottom. This, along with being
careful to trim so that the base is not a lot thicker than the side walls,
has completely eliminated s-cracks in my small hump-thrown bowls.
Chris
>After suffering some small cracks in the bottom of things thrown off the
>hump, I followed advice given on clayart to put your clay on "sideways"
>(presumably that's in the same position as when you're wedging it), to
>prevent these cracks from happening. I found it a little harder to center,
>but I was also using a fairsized chunk of clay. Incidentally, changing the
>way I put the clay on didn't eliminate the cracks. In fact the last batch
>of sake cups I threw off the hump suffered about a 35% loss due to cracks.
>I was pretty choked. I'm using Plainsman M370, very pricey, and also found
>cracks in the bottom of mugs (not thrown off the hump). Don't know why---I'm
>doing just about what I've always done for years! Very frustrating!
>Dai in Kelowna, BC
Sheryl VanVleck on tue 30 oct 01
Had not thought to ask this question myself, but I am curious to see the answer. I too find that after wedging, I must reform the ball or I end up with a swirl in my thrown form. Especially if I put the wedged clay on the wheelhead with the larger end down. Instead, I must take it and reform it so that I can use one of the sides as up and down.
Sheryl
Wyoming: where it is still nice and the front end of the car is fixed from being attacked by the antelope; who did not fare as well, I'm sorry to relate
Michael Wendt on tue 30 oct 01
Dai,
35% loss rate is a killer. Sorry. If the cracks start on the outside bottom,
try this: when the piece is still wet, but stiff enough at the lip to invert
on a smooth surface, take a metal rib and scrape out a slight concavity in
the bottom of the piece. Then burnish the entire bottom smooth as possible.
I burnish from 2 different directions. This seals the bottom and creates a
mechanically sound surface without stress concentration points. I dry in
this position too.
Ivor Lewis mentions this concept often and it applies not just to clay.
Metals exhibit a similar behavior. Tiny cracks or scratches in the surface
appear to cause stress concentrations and under deformation loads (in clay,
caused by drying shrinkage), cracks start at scratches.
Glass cutting relies on this same principle.
Let us know if this solves your problem. I rarely get more than 1-2% loss to
cracks and they happen because I fail to burnish cut off bottoms soon
enough.
Regards,
Michael Wendt wendtpot@lewiston.com
you wrote:
After suffering some small cracks in the bottom of things thrown off the
hump, I followed advice given on clayart to put your clay on "sideways"
(presumably that's in the same position as when you're wedging it), to
prevent these cracks from happening. I found it a little harder to center,
but I was also using a fairsized chunk of clay. Incidentally, changing the
way I put the clay on didn't eliminate the cracks. In fact the last batch
of sake cups I threw off the hump suffered about a 35% loss due to cracks.
I was pretty choked. I'm using Plainsman M370, very pricey, and also found
cracks in the bottom of mugs (not thrown off the hump). Don't know why---I'm
doing just about what I've always done for years! Very frustrating!
Dai in Kelowna, BC
John Jensen on wed 31 oct 01
I've been following this s-crack discussion and it seems that it doesn't
matter what you do to stop s-cracks...as long as you do something. Like
magic. I was throwing bowl forms in large numbers and when I went from hand
mixed and wedged clay to custom mixed and preboxed clay I started getting
small cracks in the bottoms of my bowls. Sometimes as high as 10%
loss...very disturbing as the cracks didn't show up until the firing. I
tried a few different solutions, but the problem went away when I started
giving each measured amount a wedge of about a dozen turns and put the cone
shape down point first. Point first results in an "unwinding" of the
presumed spiral. I don't know what is going on with the clay, theories
abound; but I do know the procedure solved my problem. I give all my pugged
clay the same wedging, and also no cracks.
John Jensen, Mudbug Pottery
mudbug@toad.net www.Toadhouse.com
| |
|
