Ceramic Design Group on tue 5 apr 05
Platters crack in the bisque kilns because of differential heating and
mainly, differential cooling, all can be referred to as a "T1 / T2
temperature differential."
Remember there are three ways of heat transfer in a kiln ....
conduction, convection, and radiation
The feet the body of the piece, and the rims all heat and cool
differently. The kiln shelves retain heat and transfer it to the foot
of the pot.(conduction) The other parts of the platter heat and cool
faster as they receive heat by convection, radiation, and to a very
lesser degree, conduction from the already heated foot.
Upon cooling, the foot area will retain more heat than the rim. The rim
will be the first to cool and it sets up a stress that needs to be
relieved. It relieves itself by cracking. Referred to as cooling dunts.
Sometimes, the foot area cracks to relieve the stress, If there are
radical changes in direction of the ware or radical changes in cross
section, eg thick and thin, all will show cracking in one form or
another.
Big platters and plates, big rimmed bowls, all are quite susceptible to
cracking in the bisque.
Also remember that electric kilns are notoriously under insulated. The
cool really quickly from the outside towards the inside.
There are many solutions to eliminate cracks of this sort, of which
many have been well described in previous posts.
Other ways can include programming a cooling cycle into the computer,
building a saggar with IFB around the rim of the platter, elevating the
piece off of the kiln shelf with wadding, grog, or other refractory
like setters, firing on edge, the list goes on and on.
You find a way that works and you keep doing it. Dr. Lawrence's Ceramic
Science for the Potter offers real good, understandable, and well
illustrated examples of exactly what many of you are experiencing, as
well as good examples of other cracking problems due to forming. I must
have read his book a thousand times and will probably re-read it 100
more as there is always some wisdom lurking betwix the lines.
Best
Jonathan
Jonathan Kaplan
Ceramic Design Group
PO Box 775112
Steamboat Springs CO 80477
(970) 879-9139
Plant location for commercial deliveries excluding USPS
1280 13th Street Suite K
Steamboat Springs CO 80487
info@ceramicdesigngroup.net
www.ceramicdesigngroup.net
Liz Harris on thu 7 apr 05
Hi Cindy,
I agree with you. I don't think the moist clay coils caused the bowls to crack. I did
just receive two new clays from my supplier, both of which are highly recommended
for large ware; both have alot of grog. My other clay was evidently only
recommended for small and medium-sized pieces. Threw some big bowls today
with the new stuff, and I'll have some results in a few weeks after they're trimmed,
dried, and fired. Will post results...wish me luck!
Liz
Linda Ferzoco on fri 8 apr 05
I've seen this term, wadding, used many times. I understand it conceptually
from the context, but please, what the heck is it? Specifically, what material?
Linda
A newbie (I know, you coulda guessed)
On Tue, 5 Apr 2005 21:02:05 -0600, Ceramic Design Group
wrote:
>
>Other ways can include programming a cooling cycle into the computer,
>building a saggar with IFB around the rim of the platter, elevating the
>piece off of the kiln shelf with wadding,
Earl Brunner on fri 8 apr 05
Wadding can be made out of a variety of materials, but
what works about as good as anything is kiln wash
mixed to the consistency of clay, use balls of it
under the foot.
--- Linda Ferzoco wrote:
> I've seen this term, wadding, used many times. I
> understand it conceptually
> from the context, but please, what the heck is it?
> Specifically, what material?
>
> Linda
> A newbie (I know, you coulda guessed)
>
Earl Brunner
e-mail: brunv53@yahoo.com
Liz Harris on sat 9 apr 05
Dear Earl,
Am I right in assuming that wadding made of kiln wash could be used to elevate a
large glazed piece above the kiln shelf during firing? Could the kiln wash then be
reconstituted and reused?
Liz in CT, where Spring is finally beginning to show herself.
Earl Brunner on sat 9 apr 05
Yes, it can be used that way. As far as recycling it, if it has any clay in
it, maybe not. The old kiln wash I used in college was kaolin and silica
mostly. The stuff I use now is Alumina Hydrate, kaolin and bentonite.
Haven't made kiln putty out of it. We used to use kiln putty to "level"
kiln shelves and we used four posts, one in each corner. Now I use three,
and they self level. Someone who wood fires and uses the shells could
probably give you more of a definitive answer.
Earl Brunner
Las Vegas, NV
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Liz Harris
Sent: Saturday, April 09, 2005 6:45 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Large Platters Cracking
Dear Earl,
Am I right in assuming that wadding made of kiln wash could be used to
elevate a
large glazed piece above the kiln shelf during firing? Could the kiln wash
then be
reconstituted and reused?
Liz in CT, where Spring is finally beginning to show herself.
____________________________________________________________________________
__
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.
| |
|
