Kristina Chadwick on sun 3 apr 11
I have limited experience making plates, but due to a request I've been
making them without much success. During the bisque fire the majority of =
=3D
the
plates are cracking in half, right down the middle. There are no S cracks=
=3D
present. I am using Laguna B-mix, cone 5. I am bisque firing to cone 03 i=
=3D
n
an electric kiln. Plates were stacked for firing (6 total, the bottom 4
cracked in half, top 2 survived intact).
The plates are ~ 7-8" in diameter with a gradual rise along the edge like=
=3D
a
traditional dinner plate. They are about 0.25" thick. The last time I mad=
=3D
e
plates there was also a lot of cracking (mostly in half), but I thought i=
=3D
t
was related to S cracks, so I threw these thicker.=3D20
All plates in this last batch were thrown the same day and dried the same=
=3D
amount of time.
I'd appreciate any advice, the low yield is utterly frustrating!
Thanks,
Kristina
P.S. I can take a picture if that would help diagnose the problem.
Vince Pitelka on sun 3 apr 11
Kristina -=3D20
The whole problem is stacking them in the bisque-firing. You are =3D
insulating the center of the plates from heating and cooling, resulting =3D
in differential expansion and contraction between the center and the =3D
rim. The top one didn't crack because the whole thing heated and cooled =
=3D
evenly. The second one was a fluke. =3D20
To solve the problem, make a big supply of little clay wads about 3/4" =3D
in diameter and 1/4" thick. Place the first plate on three of these =3D
wads. Place three wads in the plate directly above the first three =3D
wads, and then place the next plate, etc. Don't stack more than three =3D
or four plates in one stack. The wads allow heat and atmosphere to =3D
surround each plate, so that they expand and contract evenly during the =3D
firing. =3D20
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu=3D20
http://iweb.tntech.edu/wpitelka
Paul Herman on sun 3 apr 11
Hi Kristina,
When you stack plates in the bisque fire, they will heat up on the
outer edges faster than the middles, and when they cool the opposite
is true. The rims will cool faster than the center of the stack. This
can cause dunting cracks because of the large temperature
differential between the center and rim of the plates. It sounds to me
like that is what is happening to you.
To cure this problem, you have to figure out a way to heat and cool
the plates more evenly. When I stack plates in the bisque, I put three
small shards or bits or bisque fired clay under each successive plate,
which holds them apart about 1/4 inch. You could make special little
wads for this and prefire them, but I just use broken bisque from the
shard pile. This allows heat to penetrate the stack, and cooling to
take place more evenly. I think you should try that, or some other
method of heating and cooling your plates more evenly.
best wishes,
Paul Herman
Great Basin Pottery
Doyle, California US
www.greatbasinpottery.com/
On Apr 3, 2011, at 4:20 PM, Kristina Chadwick wrote:
> I have limited experience making plates, but due to a request I've
> been
> making them without much success. During the bisque fire the
> majority of the
> plates are cracking in half, right down the middle. There are no S
> cracks
> present. I am using Laguna B-mix, cone 5. I am bisque firing to cone
> 03 in
> an electric kiln. Plates were stacked for firing (6 total, the
> bottom 4
> cracked in half, top 2 survived intact).
>
> The plates are ~ 7-8" in diameter with a gradual rise along the edge
> like a
> traditional dinner plate. They are about 0.25" thick. The last time
> I made
> plates there was also a lot of cracking (mostly in half), but I
> thought it
> was related to S cracks, so I threw these thicker.
>
> All plates in this last batch were thrown the same day and dried the
> same
> amount of time.
>
> I'd appreciate any advice, the low yield is utterly frustrating!
>
> Thanks,
> Kristina
>
> P.S. I can take a picture if that would help diagnose the problem.
Kristina Chadwick on sun 3 apr 11
Thanks Vince! So obvious now you point it out. And such a simple fix. Will
make some more plates next weekend.
Kristina
On Sun, Apr 3, 2011 at 8:47 PM, Vince Pitelka wrote:
> Kristina -
> The whole problem is stacking them in the bisque-firing. You are
> insulating the center of the plates from heating and cooling, resulting i=
n
> differential expansion and contraction between the center and the rim. T=
he
> top one didn't crack because the whole thing heated and cooled evenly. Th=
e
> second one was a fluke.
>
> To solve the problem, make a big supply of little clay wads about 3/4" in
> diameter and 1/4" thick. Place the first plate on three of these wads.
> Place three wads in the plate directly above the first three wads, and t=
hen
> place the next plate, etc. Don't stack more than three or four plates in
> one stack. The wads allow heat and atmosphere to surround each plate, so
> that they expand and contract evenly during the firing.
> - Vince
>
> Vince Pitelka
> Appalachian Center for Craft
> Tennessee Tech University
> vpitelka@dtccom.net; wpitelka@tntech.edu
> http://iweb.tntech.edu/wpitelka
>
>
>
>
Ron Roy on sun 3 apr 11
Hi Kristina,
That would be bisque dunting - plate rims go through the quartz
inversion at 573C and try to get smaller - the feet - in contact with
a hot shelf will not get smaller till they cool more.
Keep the plates in the top half of the kiln where cooling is slower
and/or stack with something under the feet to keep them off the hot
shelf and/or slow the cooling down by firing down on low between 700
and 500C (1292 to 930F.) If you have a controller on your kiln you can
simply do a custom ramp to slow the cooling automatically.
Making the rimes thicker will help as well and stacking them rim to
rim will slow cooling.
RR
Quoting Kristina Chadwick :
> I have limited experience making plates, but due to a request I've been
> making them without much success. During the bisque fire the majority of =
the
> plates are cracking in half, right down the middle. There are no S cracks
> present. I am using Laguna B-mix, cone 5. I am bisque firing to cone 03 i=
n
> an electric kiln. Plates were stacked for firing (6 total, the bottom 4
> cracked in half, top 2 survived intact).
>
> The plates are ~ 7-8" in diameter with a gradual rise along the edge like=
a
> traditional dinner plate. They are about 0.25" thick. The last time I mad=
e
> plates there was also a lot of cracking (mostly in half), but I thought i=
t
> was related to S cracks, so I threw these thicker.
>
> All plates in this last batch were thrown the same day and dried the same
> amount of time.
>
> I'd appreciate any advice, the low yield is utterly frustrating!
>
> Thanks,
> Kristina
>
> P.S. I can take a picture if that would help diagnose the problem.
>
William & Susan Schran User on sun 3 apr 11
On 4/3/11 7:20 PM, "Kristina Chadwick" wrote:
> During the bisque fire the majority of the
> plates are cracking in half, right down the middle. There are no S cracks
> present. I am using Laguna B-mix, cone 5. I am bisque firing to cone 03 i=
n
> an electric kiln. Plates were stacked for firing (6 total, the bottom 4
> cracked in half, top 2 survived intact).
You've probably answered your own question - stacking 6 plates, bottom 4
cracked. If you're going to stack, go lip to lip or foot to foot, 2 or mayb=
e
3 plates, no more in the stack.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
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