Joyce Lee, Jim Lee on thu 22 may 97
I don't know what's true and what's myth in clay. I'm still very much a
non-professional although I enjoy observing my growth and am glad to say
that there has been considerable improvement over the last two years.
However, due to Clayart, books, articles, videos teachers, mentors etc,
my intellectual understanding surpasses easily my motor capabilities,
which are improving with practice, thank goodness. BUT, my point, I have
only had two "S" cracks (and only a dozen or so cracks of any kind) in
my wheel-thrown pottery which numbers in the high hundreds (it must
because it lines all my walls and is now being placed outside in large
batches to greet visitors). I fire ^10 reduction.I never dry pots upside
down and until fairly recently I often forgot to wire my pots until it
was almost too late to do so. (Not proud of this but that's the way it
has been.) The one direction impressed over and over on my overloaded
brain by mentor and tachers was to compress, compress, compress. I do,
I do, I do. No cracks.
Joyce
The Mojave is heatin' up. Pretty soon I'll have to bring my pots inside
before they melt.
John Baymore on tue 27 may 97
------------------
--------------------------------------------------------------------------
=3Cclip=3E ......An example is a pot bottom that has a greater water content
than the walls of the pot. Greater water content, greater shrinkage, more
possibility of
cracking. =22Compressing=22 the bottom with a rib or sponge probably has =
the
effect of getting rid of the clay that has gained excess water. .....
=3Cclip=3E
-------------------------------------------------------------------------
This would not explain the following situation:
In Japan, in one particular place using one particular local clay body, the
potters must take a pot that has been thrown off the hump and =22slap=22
sharply the wire-cut bottom when the pot is not quite dry enough to trim
the foot ring............ otherwise they get =22S=22 cracks after they trim =
the
foot.
This practice doesn't sound like it would effect any significant particle
allignment. Any thoughts?
I have always understood the particle allignment issues already so aptly
addressed by folks like Jonathan Kaplan, based on such references as
=22Ceramic Science for the Potter=22- Sanders. That model fits most other
cases of forming techniques from throwing to ram pressing.
This one is an oddball.
Best,
............................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
Gavin Stairs on wed 28 may 97
At 09:11 AM 27/05/97 EDT, John Baymore wrote:
....
>In Japan, in one particular place using one particular local clay body, the
>potters must take a pot that has been thrown off the hump and "slap"
>sharply the wire-cut bottom when the pot is not quite dry enough to trim
>the foot ring............ otherwise they get "S" cracks after they trim the
>foot.
....
Gee you guys, you don't seem to be connecting. That little slap has the
same effect as slamming clay down during wedging. I does exactly the same
thing: it mobilizes water in the clay, water which has more or less
crystallized, and thereby permits internal stresses in the clay to relax.
Result: no cracks. It is not a question of "compression", but of
relaxation of the internal tensile stresses in the clay. Compression
replaces the tension with compression stress, which in most cases works
just fine. In this case, it is simply the removal of stress that does the
job.
Gavin
=================================
Gavin Stairs
http://isis.physics.utoronto.ca/
Karl P. Platt on wed 28 may 97
>I does exactly the same thing: it mobilizes water in the clay, water >which ha
What this says is that slapping the clay causes ice to form. Huh?
Gavin Stairs on tue 3 jun 97
At 09:44 AM 28/05/97 EDT, you wrote:
>----------------------------Original message----------------------------
>>I does exactly the same thing: it mobilizes water in the clay, water
>which ha
>
>What this says is that slapping the clay causes ice to form. Huh?
No, Karl, exactly the reverse. Gavin
=================================
Gavin Stairs
http://isis.physics.utoronto.ca/
James L Bowen on wed 23 feb 00
------------------
I found this in the Clayart Archives by my teacher Clyde Tullis.
-------------------------------------------------
When ever you push in from the outside,say to define the foot, you
=22decompress=22 the bottom. The clay sort of tornados down the center of =
the
hump. Therefore the =22S=22 shaped crack. Follow each push in with a =
compression
and comress the bottom the last thing you do before cutting the piece off.
Marek & Pauline Drzazga-Donaldson on sat 3 jun 00
Dear Elizabeth,
regarding fewer s-cracks in brown (iron rich) as opposed to white clays, =
could it be that the iron rich clay could be secondary or tertiary clay =
(deposited by water etc. and so ground down to a finer particle size) =
and has smaller platelets than the primary that is made up of larger =
platelets. As far as I can gather there seems no reason why magnetism =
should not be involved, but I have no idea as to how greatly this would =
affect the clay.
I do know that by proper compression of the clay bottom is essential, =
less so an even thickness of the base and walls, as if you place the =
freshly thrown piece onto newspaper this eradicates any stresses caused =
by base and wall thickness variations. Also, something that might upset =
those that use clay straight from the bag, pug mill etc., proper =
preparation of the clay (kneading and wedging, or just wedging if you =
are in the USA and use the same word to describe two different ways of =
clay prep.) will minimise s-cracks especially the Stoke-on-Trent method =
of wedging (cutting the clay in half and bringing one half down smartly =
onto the other half on your wedging surface, and repeating 5 - 6 times). =
I have only once suffered from s-cracks, and that was when I forgot to =
sponge out the water in a bowl.
Sometimes the strangest or oddest ideas turn out to have some validity.
Happy potting Marek http://www.moley.uk.com
Mark Issenberg on mon 24 jul 06
I would not sell s cracked pots,,,I compress the bottoms of all my wheel
work. I compress mugs, bottles, bowls.I spend time compressing the bottoms so i
dont have s cracks...I would be embarrased selling s crack ware.
compress,compress,compress.
Mark
Nancy Udell on fri 12 oct 07
Catherine, i went to a demo workshop with julia duff a few years
back. she has some forms she throws off the hump. After opening and
the first pull, she takes a screwdriver with a smooth round handle
end and bangs the handle end into the hump at the bottom of the form
to be a few times. According to her, this solves all s crack
problems. You could probably also use a pestal.
Nancy Udell
Santa Fe, New Mexico
505.984.9907
www.clayandcolor.net
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