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.
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.
Hank Murrow on sun 3 apr 11
On Apr 3, 2011, at 4:24 PM, Kristina Chadwick wrote:
> I have limited experience making plates, but due to a request I've =3D
been
> making them without much success. During the bisque fire the majority =3D
of the
> plates are cracking in half, right down the middle. There are no S =3D
cracks
> present. I am using Laguna B-mix, cone 5. I am bisque firing to cone =3D
03 in
> an electric kiln. Plates were stacked for firing (6 total, the bottom =3D
4
> cracked in half, top 2 survived intact).
>=3D20
> The plates are ~ 7-8" in diameter with a gradual rise along the edge =3D
like a
> traditional dinner plate. They are about 0.25" thick. The last time I =3D
made
> plates there was also a lot of cracking (mostly in half), but I =3D
thought it
> 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 =3D
same
> amount of time.
>=3D20
> I'd appreciate any advice, the low yield is utterly frustrating!
Dear Kristina;
I suggest stacking them on their rims, resting against the wall of the =3D
kiln and against each other. I regularly fire my dinner plates and =3D
platters this way, getting around fifty in two layers of my electric =3D
bisque kiln.
Cheers, Hank=3D
John Rodgers on sun 3 apr 11
Kristina,
First, I think firing your bisque B-mix to 03 is a little hot. I guess
some may do it, but through experience, I find that 06 works well for
Laguna's C-5 B-mix.
I use Laguna C-5 B-mix for nearly all my work. It is my core clay. When
I make plates, I do not stack them. I bisque the plate on a flat
surface, no stacking. Like you, I had cracking when I started out. Now I
begin by coating the surface of a shelf with loose alumina hydrate
spread thin, then put the plate on that, bisque firing it right side up
to cone 06. The alumina hydrate acts like little ball bearings allowing
the plate to slide as it shrinks during the firing. If any part of the
plate sticks while the rest shrinks - the plate will crack. So the
sliding action helps. This works for me. Others do it different.
One last thing .... absolutely DO NOT open your kiln until it is at room
temperature. It is very easy to set up a situation where the rim is
cooling faster than the center of the plate and this will guarantee
cracking every time. When I started out, in a full kiln load of plates,
I cracked every plate in there but one right through the middle into two
pieces trying to push it. Only the top plate survived. The plates down
through the stack of shelves were hot in the center and cooling much to
fast on the outside edge. CRACK!!! Lesson learned.
Good luck.
John
John Rodgers
Clayartist and Moldmaker
88'GL VW Bus Driver
Chelsea, AL
Http://www.moldhaus.com
On 4/3/2011 6:24 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 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.
>
>
Bonnie Staffel on mon 4 apr 11
Hi Kristina,
=3D20
From the description of your stacking practice, did you place shards =3D
between
each plate as you stacked them? If not, then the density of the fully
stacked plates did not allow the heat to get to the center of the plates
equally. I have stacked about 6 plates by placing 1/4 inch shards =3D
between
each and over one another as the stack grows so that the weight is
distributed evenly. Hope this suggestion will fix the problem. I even =3D
place
shards (3) under the bottom plate as well.
=3D20
Bonnie
http://webpages.charter.net/bstaffel/
http://vasefinder.com/bstaffelgallery1.html
DVD Throwing with Coils and Slabs
DVD Introduction to Wheel Work
Charter Member Potters Council
=3D20
David Woof on mon 4 apr 11
Hi kristina=3D2C
=3D20
In a stack of six the top two are getting the more even heat=3D2C and the b=
ot=3D
tom four are getting the rims hotter faster than the centers which causes s=
=3D
tress cracking. Added to this=3D2C the shelf is acting as a heat sink so t=
he=3D
bottom plate(s) must stay cooler longer which just exacerbates the problem=
=3D
.
=3D20
Why are you firing an 03 bisque for a b-mix 5 ? You might be tightening up=
=3D
the cone 5 body too much which can also contribute to thermal shocking in =
=3D
the stacking you are doing=3D3B and the plates could be cracking during coo=
li=3D
ng. Can you slow down your firing cycle=3D2C stack to no more than three=
=3D
=3D2C and try firing to 010 or 08. As a control: fire several plates as s=
in=3D
gles=3D3B no stack.
=3D20
On my large platters=3D3B fired in electric and gas=3D3B I fire them on ref=
ract=3D
ory balls attached to their feet such as we also use in wood and salt firin=
=3D
g=3D2C which lets heat circulate under the platter. Same goes for above t=
he=3D
platter: I allow an extra 2 inches of "head room" for better heat circula=
=3D
tion. The typical electric kiln supplies heat from out towards in=3D2C and=
e=3D
ven a vented electric still has deep interior circulation problems. Theref=
=3D
ore it is effective to "stagger" the shelving where ever you can=3D2C if yo=
u =3D
have half shelves=3D2C to allow more heat to circulate through the central =
in=3D
terior. Otherwise=3D2C slow the firing rise so that every part of the ware=
a=3D
nd furniture mass has time to saturate and so fire evenly.
=3D20
Best=3D2C=3D20
=3D20
David Woof
=3D20
17. Plates Cracking During Bisque Fire
Posted by: "Kristina Chadwick" kchadwick@GMAIL.COM=3D20
Date: Sun Apr 3=3D2C 2011 5:39 pm ((PDT))
=3D20
I have limited experience making plates=3D2C but due to a request I've been
making them without much success. During the bisque fire the majority of th=
=3D
e
plates are cracking in half=3D2C right down the middle. There are no S crac=
ks
present. I am using Laguna B-mix=3D2C cone 5. I am bisque firing to cone 03=
i=3D
n
an electric kiln. Plates were stacked for firing (6 total=3D2C the bottom 4
cracked in half=3D2C top 2 survived intact).
=3D20
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)=3D2C but I thought=
i=3D
t
was related to S cracks=3D2C so I threw these thicker.
=3D20
All plates in this last batch were thrown the same day and dried the same
amount of time.
=3D20
I'd appreciate any advice=3D2C the low yield is utterly frustrating!
=3D20
Thanks=3D2C
Kristina
=3D20
P.S. I can take a picture if that would help diagnose the problem.
=3D20
=3D20
Posted by: "Kristina Chadwick" kchadwick@GMAIL.COM=3D20
Date: Sun Apr 3=3D2C 2011 5:39 pm ((PDT))
=3D20
I have limited experience making plates=3D2C but due to a request I've been
making them without much success. During the bisque fire the majority of th=
=3D
e
plates are cracking in half=3D2C right down the middle. There are no S crac=
ks
present. I am using Laguna B-mix=3D2C cone 5. I am bisque firing to cone 03=
i=3D
n
an electric kiln. Plates were stacked for firing (6 total=3D2C the bottom 4
cracked in half=3D2C top 2 survived intact).
=3D20
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)=3D2C but I thought=
i=3D
t
was related to S cracks=3D2C so I threw these thicker.
=3D20
All plates in this last batch were thrown the same day and dried the same
amount of time.
=3D20
I'd appreciate any advice=3D2C the low yield is utterly frustrating!
=3D20
Thanks=3D2C
Kristina
=3D20
P.S. I can take a picture if that would help diagnose the problem.
=3D20
=3D20
=3D20
=3D
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