Dave on tue 1 feb 05
I work in a cone 6 porcelain and utilize wheel and hand building techniques in
creating my sculptural forms. I notice that stress cracks seem to form in spots where I
had used a darting technique to alter the form or in a seam area where two pieces
of clay were blended together. What could be causing this? I am very careful to try
and work the clay back togther anytime I use the darting technique (just to verify
what I mean by this is I use an exacto knife to slice out a small piece and them push
the form in to seal this hole area). Also I like to create forms that have a lean to them
and I notice that several times, the form will come out bent over against the kiln wall
due to the form collapsing over (it's like the clay began to melt and lose its
strength?)
This doesn't show during the bisque firing (I fire to cone 04).
Wondering if anyone has some ideas on this or would recommend a porcelain clay
body that might be more durable and plastic to avoid this problem??
Thanks,
Dave
Bacia Edelman on tue 1 feb 05
Dave: Although I work in both porcelain and stoneware at cone 6, my modus
operandi is a combination of thrown and handbuilt parts for both pots and
sculpture. May I assume you are firing in an electric kiln?
As to the bent over results, make sure there is distance of more than an inch
(in your case) between the work and the elements.
Regarding the cracks where you form darts: First, porcelain is more
unforgiving
than other clays. Beyond that, the darting technique might require reinforcing
the dart with soft coil from the inside, if you can get a finger inside,
blended
to the walls very carefully. Also, are your slabs, coils or whatever too thin?
There are variables on all these counts and you may have to keep
experimenting.
Perhaps fire the clay slowly to cone 5 and hold at the top for twenty minutes.
Sometimes a hold at the top temperature can equal a higher cone, in heat work
terms. And cool slowly.
Now, Dave, let the gurus kick in with suggestions.
I would like to see a picture of your work if you use a digital camera and can
forward an image by email to me.
Bacia
unguru and constant clay worker for over fifty years
I work in a cone 6 porcelain and utilize wheel and hand building techniques
in creating my sculptural forms. I notice that stress cracks seem to form
in spots where I had used a darting technique to alter the form or in a
seam area where two pieces of clay were blended together. What could be
causing this? I am very careful to try and work the clay back togther
anytime I use the darting technique (just to verify what I mean by this is
I use an exacto knife to slice out a small piece and them push the form in
to seal this hole area). Also I like to create forms that have a lean to
them and I notice that several times, the form will come out bent over
against the kiln wall due to the form collapsing over (it's like the clay
began to melt and lose its strength?)
This doesn't show during the bisque firing (I fire to cone 04).
Wondering if anyone has some ideas on this or would recommend a porcelain
clay body that might be more durable and plastic to avoid this problem??
Thanks, Dave
______________
Bacia Edelman
Madison, Wisconsin
http://users.skynet.be/russel.fouts/bacia.htm
http://www.silverhawk.com/artisan/clay/edelman/index.html
John Rodgers on tue 1 feb 05
Dave,
Porcelains are notorious for the behavior you have described
.......cracking and slumping during hi fire.
For seams, joints, etc, you need to use a "glue" made up of the same
porcelain clay. Put a little in a container, add a dash of vinegar and
squish it up good. This often works. You can ramp this technique up by
adding a bit of Lana Wilson's Magic Water to it. There was a whole list
of various "magic waters" posted recently on this list, and they are in
the Clayart Archives for January of 2005. This "glue" will help.
One thing I have done in the past, was to add a frit to a porcelain
paste and use it for making my joint glue. The frit allows for movement
in the joint thus relieving stresses as the porcelain heats and cools,
thereby reducing cracking.
Another thing that will help ......! Sprinkle raw alumina hydrate on the
shelf and sit the porcelain piece directly on the Alumina Hydrate.
Reason being the porcelain moves a lot as it shrinks in firing. The
Alumina Hydrate will allow the foot of the porcelain piece to slide
freely during shrinking and avoid hanging up, which will cause cracks.
The Alumina Hydrate is a very fine powder and a mask is warranted during
it's use and handling.
For the slumping I have found it necessary to use support materials and
structures. I did a figure once where the extended arm always sagged. I
wound up making a tall cone (cones tend to be very stable when standing
on their base) nearly the height of the arm and then put a soft
refractory fiber called Proppitt between the arm and the cone. When it
fired, the cone shrank the same amount as the porcelain, and the
proppitt supported the arm so there was no sagging. It worked.
It's hard to tell, not actually seeing your work, but from your
description I would suggest that you will need to do something about
providing extra support to prevent the sagging and tilting effect you
described.
And of course be careful..... porcelain sticks to porcelain mightily
when fired touching.
Porcelain has to go to the point of vitrification - becomes glass like -
and in doing so reaches a point where much of it is in a liqidous state.
It is right on the cusp of liquefying, and as such begins to soften and
sag just before liquification. This is the realm of the porcelain
artisan. Always working of the cusp of disaster. It takes a little
practice to master it, and master it you can. I consider porcelain to be
the Queen of Clay and anyone who can make it do it's wonderful thing to
be a master artisan.
As for other porcelains - there are a variety, you can only try them to
find ones that are right for you. I use a Standard Cone 6 Grolleg that I
like. I have also used a couple of porcelains from Laguna Clay. Lately
there has been much talk of Southern Ice, now from Laguna Clay. I have
not used it, however, so cannot speak to it's qualities. Other's on this
list can, however.
One final note. The more translucent the porcelain, the greater the
tendency to sag. Something to think about!
Best of luck to you on you project.
Regards,
John Rodgers
Chelsea, AL
Dave wrote:
>I work in a cone 6 porcelain and utilize wheel and hand building techniques in
>creating my sculptural forms. I notice that stress cracks seem to form in spots where I
>had used a darting technique to alter the form or in a seam area where two pieces
>of clay were blended together. What could be causing this? I am very careful to try
>and work the clay back togther anytime I use the darting technique (just to verify
>what I mean by this is I use an exacto knife to slice out a small piece and them push
>the form in to seal this hole area). Also I like to create forms that have a lean to them
>and I notice that several times, the form will come out bent over against the kiln wall
>due to the form collapsing over (it's like the clay began to melt and lose its
>strength?)
>
>This doesn't show during the bisque firing (I fire to cone 04).
>
>Wondering if anyone has some ideas on this or would recommend a porcelain clay
>body that might be more durable and plastic to avoid this problem??
>
>Thanks,
>Dave
>
>______________________________________________________________________________
>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.
>
>
>
>
Dorothy Feibleman on wed 2 feb 05
What type of clay are you using and what is the plasticiser?
do you know what plasitciser is used?
YOu might want to try a clay with veegum tee or macaloid they are hectorites or
related to hectorites which are mined in Texas or produced synthetically in the
USA and are sold through Standard Ceramic supplies and others.
Best,
D.Quoting Dave :
> I work in a cone 6 porcelain and utilize wheel and hand building techniques
> in
> creating my sculptural forms. I notice that stress cracks seem to form in
> spots where I
> had used a darting technique to alter the form or in a seam area where two
> pieces
> of clay were blended together. What could be causing this? I am very
> careful to try
> and work the clay back togther anytime I use the darting technique (just to
> verify
> what I mean by this is I use an exacto knife to slice out a small piece and
> them push
> the form in to seal this hole area). Also I like to create forms that have a
> lean to them
> and I notice that several times, the form will come out bent over against the
> kiln wall
> due to the form collapsing over (it's like the clay began to melt and lose
> its
> strength?)
>
> This doesn't show during the bisque firing (I fire to cone 04).
>
> Wondering if anyone has some ideas on this or would recommend a porcelain
> clay
> body that might be more durable and plastic to avoid this problem??
>
> Thanks,
> Dave
>
>
______________________________________________________________________________
> 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.
>
>
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ccpottery@BELLSOUTH.NET on wed 2 feb 05
One thing I found that makes a big difference in the number
of cracks and flaws I get in my porcelain firings is to do as
Ron & John suggest and fire down the kiln.
An ordinary electric kiln cools way too fast to keep porcelain
from cracking under stress.
I use a slow controlled cooling to 1100 degrees.
I also found that I had to totally support wide bowls.
In the kiln load now cooling, all the work is being fired in
the form it was built in.
As John Rodgers previously mentioned, any form standing
directly on the shelf is on a dusting of alumina hydrate.
Porcelain is a diva .... hard to get along with .. . tempermental ...
but gorgeous and worth the aggravations.
Chris Campbell - in North Carolina - wishing my kiln was cool enough to
peek at my newest experiments in paper clay Southern Ice murrinis !!!!
They looked so beautiful before the firing ... which always makes me
extremely nervous. You hate to think it peaked during drying.
Chris Campbell Pottery, llc
9417 Koupela Drive
Raleigh, NC 27615
1-800-652-1008
1-919-676-2172
FAX : 919-676-2062
E Mail : chris@ccpottery.com
Website : www.ccpottery.com
Wholesale : www.wholesalecrafts.com
Dave on mon 7 feb 05
Dorthy-
This batch I am using was made a number of years ago and I don't remember the
recipe exactly but I do know that I used veegum. Once I am finished with this clay I
will have to abandon it anyway because 1- I don't even have the recipe anymore to
replicate it and 2- I value my lungs too much to deal with working with dry materials
to make it myself (don't use enough to warrant having a manufacturer make it for
me).
I have ordered a couple samples from suppliers such as Continetal Clay out of
Minneapolis as well as Great Lakes Supply. I guess I will have to see how these
bodies work for me.
Thanks for all your info!
Dave
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