Ben Morrison on sun 19 feb 12
I've found nothing but failure trying to make plates with the method mentio=
ned here. The only success I found in slab plates at all was in using a por=
ous hump or slump mold and compressing it on the mold with a rib or even pu=
tting it on the wheel and compressing the clay on the mold. With plates I w=
ait until it's self supportive and then, flip it out of the porous mold, fa=
ce down on a smooth board and cover it for a day to allow the pot to come t=
o an even moisture content. Then uncover and let dry in my cool garage flip=
ping as needed to dry evenly.
-Ben
________________________________
From: William & Susan Schran User
To: Clayart@LSV.CERAMICS.ORG
Sent: Sunday, February 19, 2012 8:48 AM
Subject: Re: warping slab-build plates
On 2/19/12 10:49 AM, "Flo Fahrenheit" wrote:
>I am finally making a set of slab-built dinner plates for my OWN kitchen.
>I am using Laguna ^5 B-mix clay. I am using the back side of a large
>fiesta ware plate as a hump mold. I am not adding a foot. After the
>plates are removed from the mold and begin to dry, they tend to curl up a
>bit on the sides, so the bottoms are no longer completely flat. Anyone
>have tips on how to prevent this??
>
>Later during the cone 6 glaze firing, the rims tend to "droop" a bit.
Flo,
Try drying your plate upside down and cover the edges with plastic leaving
the middle of the bottom uncovered. Don't say how thick you make the
slabs, but using the B-Mix with no grog, you may need to go a bit thicker.
When you reach the vitrification point of the clay, the clay becomes
protoplastic (begins to soften) and any horizontal surface that is too
long will droop/slump. Two ways to resolve this - raise the edge from
horizontal to more of an angle up. After removing from mold, put some
coils of clay under the rims, raising them at an angle.
The other way would be not to glaze underside of the rims and support them
during the glazes firing.
If you can get this clay with grog or switch to another clay with grog you
will have less issues with warping. And no, no warping in the bisque
firing because the clay does reach vitrification at that temperature.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
>
Flo Fahrenheit on sun 19 feb 12
I am finally making a set of slab-built dinner plates for my OWN =3D
kitchen. I am using Laguna ^5 B-mix clay. I am using the back side of a =3D
large fiesta ware plate as a hump mold. I am not adding a foot. After =3D
the plates are removed from the mold and begin to dry, they tend to curl =
=3D
up a bit on the sides, so the bottoms are no longer completely flat. =3D
Anyone have tips on how to prevent this??
Later during the cone 6 glaze firing, the rims tend to "droop" a bit. I =
=3D
have a Scutt 1027, and have been firing it on the pre-programmed cone =3D
04-slow setting for bisque, and the cone-6-medium setting for glaze. I =3D
watch for the witness cone to drop, and shut the kiln off at that point. =
=3D
Something Rimas said recently made me think I should use the ramp-hold =3D
option on the kiln. Rimas said " I use Ramp-Hold for large plates and =3D
slabs to slow the kiln past quartz inversion". I researched the temp for =
=3D
quartz inversion and learned that it happens at 1060 F degrees - that =3D
means it would take place during the bisque firing. Is my problem =3D
beginning in the bisque stage? the plates don't look droopy at that =3D
point. Would someone please share their firing schedule, start to =3D
finish, for the hold during quartz inversion. Also should I be firing =3D
the kiln down for the glaze firing in order to cool the kiln more =3D
slowly after the vitrification stage?
Thanks
Flo Fahrenheit
Grass Valley, CA
William & Susan Schran User on sun 19 feb 12
On 2/19/12 10:49 AM, "Flo Fahrenheit" wrote:
>I am finally making a set of slab-built dinner plates for my OWN kitchen.
>I am using Laguna ^5 B-mix clay. I am using the back side of a large
>fiesta ware plate as a hump mold. I am not adding a foot. After the
>plates are removed from the mold and begin to dry, they tend to curl up a
>bit on the sides, so the bottoms are no longer completely flat. Anyone
>have tips on how to prevent this??
>
>Later during the cone 6 glaze firing, the rims tend to "droop" a bit.
Flo,
Try drying your plate upside down and cover the edges with plastic leaving
the middle of the bottom uncovered. Don't say how thick you make the
slabs, but using the B-Mix with no grog, you may need to go a bit thicker.
When you reach the vitrification point of the clay, the clay becomes
protoplastic (begins to soften) and any horizontal surface that is too
long will droop/slump. Two ways to resolve this - raise the edge from
horizontal to more of an angle up. After removing from mold, put some
coils of clay under the rims, raising them at an angle.
The other way would be not to glaze underside of the rims and support them
during the glazes firing.
If you can get this clay with grog or switch to another clay with grog you
will have less issues with warping. And no, no warping in the bisque
firing because the clay does reach vitrification at that temperature.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
>
Gayle Bair on sun 19 feb 12
Hi Flo,
I have had similar problems with B-Mix. After trying just about everything
a good friend suggested firing it cooler. That was a DUH moment because
B-Mix is listed as ^5 not ^6.
My bowls and mugs because of the shape have more structural strength come
out fine when fired ^6 but not the plates and platters.
I haven't yet tried it but I'd bet firing them to ^5 would prevent the
slumping experienced with B-Mix when firing plates & platters.
Gayle
Gayle Bair Pottery
gayle@claybair.com
www.claybair.com
On Sun, Feb 19, 2012 at 8:49 AM, Flo Fahrenheit wrot=
e:
> I am finally making a set of slab-built dinner plates for my OWN kitchen.
> I am using Laguna ^5 B-mix clay. I am using the back side of a large fies=
ta
> ware plate as a hump mold. I am not adding a foot. After the plates are
> removed from the mold and begin to dry, they tend to curl up a bit on the
> sides, so the bottoms are no longer completely flat. Anyone have tips on
> how to prevent this??
>
> Later during the cone 6 glaze firing, the rims tend to "droop" a bit. I
> have a Scutt 1027, and have been firing it on the pre-programmed cone
> 04-slow setting for bisque, and the cone-6-medium setting for glaze. I
> watch for the witness cone to drop, and shut the kiln off at that point.
> Something Rimas said recently made me think I should use the ramp-hold
> option on the kiln. Rimas said " I use Ramp-Hold for large plates and sla=
bs
> to slow the kiln past quartz inversion". I researched the temp for quartz
> inversion and learned that it happens at 1060 F degrees - that means it
> would take place during the bisque firing. Is my problem beginning in the
> bisque stage? the plates don't look droopy at that point. Would someone
> please share their firing schedule, start to finish, for the hold during
> quartz inversion. Also should I be firing the kiln down for the glaze
> firing in order to cool the kiln more slowly after the vitrification sta=
ge?
>
> Thanks
> Flo Fahrenheit
> Grass Valley, CA
>
Vince Pitelka on mon 20 feb 12
Flo -
In that workshop in Nevada City last summer we talked about clay memory, an=
d
specifically about the unidirectional platelet grain structure created by
the slab roller, and how important it is to cross roll the slab, either by
running it through the slab roller again or with a rolling pin after the
slab roller. Are you doing that? I have found that I need to follow
multiple precautions in order to minimize the chance of warping in slumped
slab plates. 1) I cross-roll the slab at multiple angles to eliminate any
unidirectional grain structure. 2) I always apply a foot ring that is
absolutely concentric with the outer shape of the plate. The foot ring is
either made from a thin rolled coil, or a thin strip cut from the edge of a
long slab. 3) I always apply the foot ring right where the curvature of th=
e
plate rim starts to level out - never any further in so that a portion of
the level bottom of the plate extends out beyond the foot ring. 4) I make
sure the slumped slab plates and bowls dry without exposure to any air
currents (in a damp box or under a fabric tent). 5) I make sure that the
foot ring is evenly supported in the firing. If the shelves are not
absolutely flat, I fire the piece on lots of wads supporting the foot ring.
6) I try to avoid having one edge of a slab plate or bowl close to or
hanging over the firebox on a gas kiln, as the focused heat can cause
warping.
I hope that helps.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net
http://iweb.tntech.edu/wpitelka/
Rimas VisGirda on mon 20 feb 12
Flo, my reason for slowing the kiln past quartz inversion is to eliminate c=
racking. Nothing to do with drooping. And only when the clay is vitrified. =
In a bisque firing to 04 the clay is not yet vitrified as it goes past quar=
tz inversion both as it's going up and as it's coming down. In a glaze fire=
from bisque, the clay is soft going up and vitrified coming down so there =
may be a case for slowing the kiln past quartz inversion on the way down; h=
owever it has not been a problem for me during that first firing. I multifi=
re my work, sometimes up to 10-15 firings after the first high fire (10-11)=
and large plates and tiles tend to crack during these firings. This ONLY h=
appens to LARGE FLAT things, say 12" or wider plates and tiles; so for that=
type of work and ONLY that type of work I slow the kiln past quartz invers=
ion both going up and coming down. I also elevate the piece from the kiln s=
helf so that it is "suspended" and the kiln shelf can't act as a
"heat sink" I am not suggesting the only way to fire, I'm only relaying wh=
at works for me and trying to explain why it works for me -or not... So loo=
k at it like speeding on the highway; I tend to drive over the speed limit.=
When I had a Lotus, I would get nailed all the time and I figured out I ne=
eded to slow down; now that I drive an SUV I can't remember getting a speed=
ing ticket in it since we bought it in 2001 so I tend to push it 10-20 mph =
over the limit. And relating that to ceramics, I kept cracking my wide flat=
pieces, and ONLY the wide flat pieces) on consecutive firings , sometimes =
the 2nd sometimes the 10th. So my solution was to not make wide flat things=
. But in the last bunch of years I wanted to make large flat things so figu=
red out that quartz inversion was the problem and adjusted for it... Incide=
ntally the fact that (American) raku is possible is that the clay never get=
s over bisque temps; raku clay is pretty much any high fire clay
that is bisqued then glazed (or not) and refired to bisque temperature. At=
bisque the clay is "soft" in a sense, think a rubber ball or play-dough, i=
f you drop it it will bounce or deform but won't break. After a clay is vit=
rified, think of it as a glass ball or stone that is brittle, if you drop o=
ne of those they can break and so the shock of coming out of a hot kiln nee=
ds a "soft" clay to withstand breaking wheras a vitrified (hard, brittle) c=
lay will crack... Regards, -Rimas
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