Lynn Goodman Porcelain Pottery on thu 24 jun 10
Hi Randy,
How are you trimming the bowls? I used to get cracks like that on my
big bowls when I was securing the bowl onto the bat with little clay
wads. When I switched to using a foam pad on top of a bat, the cracks
disappeared. I use carpet padding foam--it's thin, dense, and you can
cut big pieces to go on top of big bats. The weight of the bowl, plus
friction, holds the bowl in place.
I have seen smaller versions sold in some ceramic supply stores, with
the foam glued on; I don't glue down my foam.
Lynn
On Jun 24, 2010, at 12:56 PM, Randy McCall wrote:
> What is an acceptable pot loss during bisque? I have been firing
> 6lb bowls and have been losing a little over 10% of the bowls due to
> straight cracks from the rim inward. With this clay I never have
> much problems with small pieces or large vases 6 to 8lbs.
>
> Me or the clay? Smaller bowls are ok.
>
> Randy
>
>
Lynn Goodman
Fine Porcelain Pottery
Cell 347-526-9805
www.lynngoodmanporcelain.com
David Hendley on thu 24 jun 10
Acceptable loss during bisque should be close to 0.
Your bowls are cracking because of uneven cooling.
The rims of the bowls are thinner and closer to the
outside wall of the kiln. The bases are thicker and
sitting on thick and dense kiln shelves. The rims
therefore cool faster and go through phase inversion
before the bases. It's not a problem with smaller items
but plates and platters can also be susceptible.
There are two ways to prevent or at least reduce this.
First, cool slower by turning the kiln back on during
cooling. The critical time is at dull red heat.
About 3 to 3 1/2 hours after the kiln shuts off, I turn
all the elements on high for 45 minutes or so. This serves
to heat up the outside walls of the kiln, getting them back
to a temperature closer to the thick shelves and bowl
bottoms towards the middle of the kiln.
A second strategy is to surround the bowl rims with
smaller pieces. This extra clay heats up and serves as
heat sinks, maintaining the heat and slowly radiating it
to the bowl rims during cooling.
David Hendley
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
What is an acceptable pot loss during bisque? I have been firing 6lb bowls
and have been losing a little over 10% of the bowls due to straight cracks
from the rim inward. With this clay I never have much problems with small
pieces or large vases 6 to 8lbs.
Me or the clay? Smaller bowls are ok.
Randy
Randy McCall on thu 24 jun 10
What is an acceptable pot loss during bisque? I have been firing 6lb bowls=
=3D
and have been losing a little over 10% of the bowls due to straight cracks=
=3D
from the rim inward. With this clay I never have much problems with small=
=3D
pieces or large vases 6 to 8lbs.
Me or the clay? Smaller bowls are ok.
Randy
=3D
D. Michael Coffee on thu 24 jun 10
For me the only acceptable loss rate is 0%. Larger pieces need to be
supported off the shelf to allow even heating and cooling, and to allow for
thermal expansion and contraction, particularly wide bottom pieces (large p=
lates
and platters).
D. Michael Coffee
http://dmcarts.com
http://dmichaelcoffee.wordpress.com
Ron Roy on thu 24 jun 10
Hi Randy,
Are the cracks closed or open.
I suspect closed which means the cracks are happening because of the
beta to alpha quartz change at 573C.
Probably happens more to the pots on the bottom of the kiln - because
cooling is faster there.
Rims go through quartz inversion sooner than feet because of contact
with hot shelves.
Cures are higher bisque (04) will help, thicker rims and/or thinner
feet, slowing cooling down between 700C and 550C, using clay coils to
keep feet off the shelves a bit to help faster cooling. Keep the bowls
in the top half of the kiln.
What kind of kiln do your use?
Clays with a lot of silica are the worst - porcelain for instance.
RR
Quoting Randy McCall :
> What is an acceptable pot loss during bisque? I have been firing
> 6lb bowls and have been losing a little over 10% of the bowls due to
> straight cracks from the rim inward. With this clay I never have
> much problems with small pieces or large vases 6 to 8lbs.
>
> Me or the clay? Smaller bowls are ok.
>
> Randy
>
>
>
jeanette harris on thu 24 jun 10
>What is an acceptable pot loss during bisque? I have been firing
>6lb bowls and have been losing a little over 10% of the bowls due to
>straight cracks from the rim inward. With this clay I never have
>much problems with small pieces or large vases 6 to 8lbs.
>
>Me or the clay? Smaller bowls are ok.
>
>Randy
Is your clay tolerant of a lot of stress? Because the further out
clay is 'pushed' at the rim, the more stress there is on it. Is it an
open or a dense clay? How do your plates fare? Do you have the same
problem? Maybe switching to another clay would help.
Are your bowls a lot thicker at the bottom than the rim? Are the
rims' compressed? Have you allowed enough clay to throw to the top of
the form or are you forcing the clay up and out to get the height and
diameter? How are you opening out the bowl? Do you begin with a
cylinder?
Here's some suggestions: Weigh your clay out to make sure you are
allotting enough to reach the height. Aim at making the thickness of
the walls consistent. A heavy base and thin rim will create a lot of
stress when the bowl is fired.
Here's a throwing suggestion I learned from Ken Turner: Start with a
uniform cylinder (a heavy base can be trimmed to size when the bowl
is leather hard, but you must have a good rim for it to rest on when
you do trim.) Open the cylinder outward slowly from the top down
using a curved rib and ONLY from the inside downward. (A blowtorch is
very helpful here to gently strengthen the outside wall, but you can
allow the vessel to stiffen naturally too, just takes longer.) Take
the opening action all the way down to the bottom of the bowl when it
is opened enough to allow it. Work slowly.
And STOP at the first hint of stress.
--
Jeanette Harris in Poulsbo WA
http://www.jeanetteharrisblog.blogspot.com
http://www.washingtonpotters.org/WPA_Gallery.htm
Ann Brink on thu 24 jun 10
Hi Randy,
I don't think that's acceptable at all! Are the rims of your bowls too
thin? If you have a little extra at the top, and then compress the rim, you
shouldn't have cracking. Or are you nesting them in the bisque & not
allowing enough room? ...just a few thoughts.
Ann Brink in Lompoc CA, all geared up for my best selling weekend of the
year-Flower Festival Arts/Crafts show.
(mostly about pottery)
----- Original Message -----
From: "Randy McCall"
To:
Sent: Thursday, June 24, 2010 9:56 AM
Subject: Acceptable pot loss during bisque?
> What is an acceptable pot loss during bisque? I have been firing 6lb
> bowls and have been losing a little over 10% of the bowls due to straight
> cracks from the rim inward. With this clay I never have much problems
> with small pieces or large vases 6 to 8lbs.
>
> Me or the clay? Smaller bowls are ok.
>
> Randy
>
>
>
---------------------------------------------------------------------------=
-----
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23:35:00
Vince Pitelka on thu 24 jun 10
Randy McCall wrote:
"What is an acceptable pot loss during bisque? I have been firing 6lb bowl=
s
and have been losing a little over 10% of the bowls due to straight cracks
from the rim inward. With this clay I never have much problems with small
pieces or large vases 6 to 8lbs."
Randy -
Every now and then there is a fluke where a piece cracks in the bisque for
no identifiable reason, but usually it is pretty easy to track down. When =
I
was throwing full time I made large numbers of mixing bowls and serving
bowls of many sizes, and initially I had a lot of trouble with the kind of
cracks you talk about - from the rim inwards toward the center. I solved
the problem when I started stacking the bowls with shims beneath and betwee=
n
bowls. I used small softbrick shims about 1/4" thick, but you can also rol=
l
balls from any claybody and flatten them to about 1/4". Obviously, they ca=
n
be used over and over again. If you stack multiple bowls inside each other=
,
be sure to use the shims like you would use kiln posts in stacking shelves,
with each set of three shims placed directly above the previous set, throug=
h
the whole stack. If the bowls diminish in size slightly with each one, the=
n
the placement of shims can diminish in diameter slightly with each bowl. I
used the same system with plates and platters with equal success.
The kind of crack you describe is usually caused by uneven cooling and
shrinkage between the rim and the center. This can happen with a large bow=
l
or platter that is just sitting on the kiln shelf without other bowls or
platters stacked inside it, because the bottom of the piece against the
shelf heats up slower during firing, and preserves the heat during cooling.
The problem is far worse when bowls are stacked inside each other, but the
shims will solve the problem (if this is the problem). Once I started usin=
g
the shims, I would stack a full series of mixing or serving bowls inside
each other - from 6" to 16" by 2" increments, and failure was extremely low=
.
I hope this helps -
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
Ron Roy on fri 25 jun 10
Lee,
Wood kilns cool slow - try that in a fast cooling kiln. How did you =3D20
know the clay the clay had a lot of free quartz in it?
RR
Quoting Lee Love :
> I bisqued fired in my wood kiln to 012 and below, high silica clay,
> with almost no loss. My teacher bisqued to red heat in the hottest
> part of the kiln, in his noborigama, with no loss.
>
>
> --
> Lee, a Mashiko potter in Minneapolis
> http://mingeisota.blogspot.com/
>
> =3DE2=3D80=3D9CObserve the wonders as they occur around you. Don't claim =
them. F=3D
eel
> the artistry moving through and be silent.=3DE2=3D80=3D9D --Rumi
>
David Woof on fri 25 jun 10
Hi Randy=3D2C All=3D2C
as Ron=3D2C David et.al. have said=3D3B 0% or near is where you should come=
in.
=3D20
The often attending problem when addressing even cooling/cracking is folks =
=3D
making "door stops" and dog dishes and trying to pass them off as "sturdy b=
=3D
owls for lifetime service of utility". Bad form!!! a bowl that is too bo=
=3D
ttom thick will look like two different pots when seen from inside and outs=
=3D
ide. Translates: Clunky Thermal Mass. =3D20
=3D20
Learn to throw well and trim as needed to refine the form. The outside of =
=3D
the bowl should not have extraneous love handles or a bird perch ledge "sho=
=3D
reline" near the bottom inside. You don't need two bottoms on the same piec=
=3D
e. Study pottery form!!! Does the vessel both visually and physically ris=
=3D
e from the table into your hand in a gesture of grace and balance or does t=
=3D
he uneven bottom weight bend and twist your wrist slopping it's contents? =
=3D
A bowl can be thrown "sturdy" for everyday utility and still have a pleasa=
=3D
nt balance when handled.=3D20
Be willing to put it back on the wheel for a tad more trimming a dozen time=
=3D
s if that's what It Takes to get it right. You will learn and develop by ho=
=3D
lding your self ever to a higher standard.=3D20
=3D20
The other error of beginners is to throw rims too thin in relation to the r=
=3D
est of the piece because they get caught up in the fascination of the clay =
=3D
moving thru their hands and with no clear vision of what they want anyway u=
=3D
ntil they run out of clay and then try to justify keeping and firing that b=
=3D
utt ugly monument???
=3D20
An acceptable pot loss concern should start with learning to throw well and=
=3D
only putting your best into the bisque. Then take the good and excellent a=
=3D
dvice posted on this list and go for that 0%. You will get there!! =3D20
=3D20
David Woof...still refining what I have done pretty damn good for a lifetim=
=3D
e.
________________________________________________________________-
1a. Re: Acceptable pot loss during bisque?=3D20
----- Original Message -----
What is an acceptable pot loss during bisque? I have been firing 6lb bowls
and have been losing a little over 10% of the bowls due to straight cracks
from the rim inward. With this clay I never have much problems with small
pieces or large vases 6 to 8lbs.
Me or the clay? Smaller bowls are ok.
=3D20
Randy
=3D20
=3D20
=3D20
_________________________________________________________________
Hotmail has tools for the New Busy. Search=3D2C chat and e-mail from your i=
nb=3D
ox.
http://www.windowslive.com/campaign/thenewbusy?ocid=3D3DPID28326::T:WLMTAGL=
:O=3D
N:WL:en-US:WM_HMP:042010_1=3D
Lee Love on fri 25 jun 10
I bisqued fired in my wood kiln to 012 and below, high silica clay,
with almost no loss. My teacher bisqued to red heat in the hottest
part of the kiln, in his noborigama, with no loss.
--
Lee, a Mashiko potter in Minneapolis
http://mingeisota.blogspot.com/
=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi
steve graber on fri 25 jun 10
when i trim larger bowls, 6, 7 pounders and up, i make sure i build a foam =
=3D
pile under the bowl to support it mainly at the center-foot area instead of=
=3D
letting all the weight sit on the rim while trimming.=3DA0 =3D0A=3D0Ai als=
o let =3D
the bowl dry on it's bat, on it's foot.=3DA0 that way even if i move the bo=
wl=3D
from shelf to shelf i only touch "it" once more to load into the kiln.=3DA=
0 =3D
=3D0A=3D0Aa friend used my studio for several years and had tons of trouble=
wit=3D
h cracks - and he is a retired pottery teacher!=3DA0 i swear his cracks cam=
e =3D
from his process of drying a lot of the bowls on their rims.=3DA0 that and =
hi=3D
s use of b-mix.=3DA0 he even wanted to fire a lot of the bowls on their rim=
s.=3D
=3DA0 and nested them too tightly in my mind.=3DA0 (rim-to-rim nested bowl =
cont=3D
act sets of "hoop stresses".=3DA0 the rim is put into slight=3DA0tension an=
d cl=3D
ay has weak tensile properties so it cracks).=3DA0 =3D0A=3D0Athe climate of=
my st=3D
udio was certainly=3DA0diferent than his old classroom, and perhaps he coul=
dn=3D
't montitor dryness as often.=3DA0 but i have no problems with the same stu=
di=3D
o and clay by using the foam support and drying on the bat and=3DA0foot pro=
ce=3D
ss.=3DA0 =3D0A=3DA0Steve Graber, Graber's Pottery, Inc=3D0AClaremont, Calif=
ornia US=3D
A=3D0AThe Steve Tool - for awesome texture on pots! =3D0Awww.graberspottery=
.com=3D
steve@graberspottery.com =3D0A=3D0A=3D0AOn Laguna Clay's website=3D0Ahttp:=
//www.la=3D
gunaclay.com/blogs/ =3D0A=3D0A=3D0A=3D0A----- Original Message ----=3D0A> F=
rom: Randy=3D
McCall =3D0A> To: Clayart@LSV.CERAMICS.ORG=3D0A> Sent: Thu=
, Ju=3D
ne 24, 2010 9:56:38 AM=3D0A> Subject: Acceptable pot loss during bisque?=3D=
0A> =3D
=3D0A> What is an acceptable pot loss during bisque?=3DA0 I have been firin=
g 6l=3D
b bowls =3D0A> and have been losing a little over 10% of the bowls due to s=
tr=3D
aight cracks from =3D0A> the rim inward.=3DA0 With this clay I never have m=
uch =3D
problems with small =3D0A> pieces or large vases 6 to 8lbs.=3D0A=3D0AMe or =
the cl=3D
ay?=3DA0 Smaller bowls are =3D0A> ok.=3D0A=3D0ARandy=3D0A=3D0A=3D0A=3DA0 =
=3DA0 =3DA0 =3DA0 =3DA0 =3D
=3DA0 =3DA0 =3D0A> =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =3DA0 =
=3DA0 =3DA0 =3D0A> =3DA0 =3DA0 =3DA0=3D
=3D0A=3D0A=3D0A=3D0A
Rimas VisGirda on fri 25 jun 10
I used to get a hairline in a bowl or two from the bisque. I figured it cam=
e from trimming when the rim, even though leather hard, became inflexible; =
the rim of a bowl that has been allowed to dry to leather is NEVER perfectl=
y flat and when placed upside down on a flat metal wheel head can have a st=
ress applied and cause a crack that is not visible at the green state. I do=
n't use a damp-room and air dry all my work some of my larger bowls become =
bone dry at the rim before the bottom gets firm enough to trim, I dislike t=
rimming mush, and that exasperates the situation... After some cumbersome a=
ttempts, I solved MY problem by either trimming on a foam covered batt or b=
y using an interior wet-chuck. -Rimas
Randall Moody on fri 25 jun 10
On Thu, Jun 24, 2010 at 12:56 PM, Randy McCall wrote:
> What is an acceptable pot loss during bisque? I have been firing 6lb bow=
ls
> and have been losing a little over 10% of the bowls due to straight crack=
s
> from the rim inward. With this clay I never have much problems with smal=
l
> pieces or large vases 6 to 8lbs.
>
> Me or the clay? Smaller bowls are ok.
>
> Randy
>
>
>
I think that it may be the weight of the bowl on the rim during trimming. A=
t
least that is where I would start. Use some upholstery foam on your bat or
center a tall mound of clay that will support the center of the bowl but no=
t
touch the rim and go that route. If you still have the same problems it is
the firing and you can go with the great advice given for that.
--
Randall in Atlanta
http://wrandallmoody.com/home.html
Paul Haigh on fri 25 jun 10
10% sounds like a lot. I bisque in wood fire and don't think I have 10% los=
s. If all the cracks are the same, this points to something that may be fix=
able. Tapping greenware in loading, or holding by one side of the rim so th=
at the full weight of a bowl like that applies torque should be looked at. =
Are you compressing rims with a rib inside and out? Finally, I dry bowls un=
til not quite leather hard, put a bat or board on top first, then flip it o=
ver to dry. usually uneven drying shows as bottom cracks/S cracks, but ya c=
an't be too careful.
-pH
Web: http://wileyhill.com
Facebook: http://tinyurl.com/28ggv3w
etsy: http://www.etsy.com/shop/WileyHillMudworks
Vince Pitelka on sun 27 jun 10
I previously posted a reply to this question, mentioning my use of shims
under larger bowls and platters, and between stacked bowls and platters, to
allow even circulation of air, eliminating the uneven heating and cooling
(thus expansion and contraction) that can occasionally cause the kinds of
cracks mentioned in the original post.
Several people have mentioned that the cracks might result from stresses
caused while trimming the bowls resting on their rims, and I agree that thi=
s
could be the problem. Some people have mentioned trimming on a foam pad,
but the only way I can see this helping is if the foam pad is on a bat, and
the bat is placed upside-down on the right-side-up bowl, and the whole thin=
g
is turned over and placed on the wheel.
Steve Graber mentioned trimming large bowls on a foam "pile," and my
equivalent is a fresh-thrown chuck, with the slurry removed with a metal
rib. It can be thrown to fit the largest bowl or platter you are trimming,
and then re-thrown to fit smaller ones as you get all your trimming done.
The chuck should fit the inside of the bowl or platter at approximately the
diameter of the foot, and should be at least an inch and a half taller than
the depth of the platter or bowl. This allows you to grasp the bowl or
platter with your fingers considerably down the walls from the rim and flip
it over, thus avoiding any stress on the rim. Leather-hard pots cannot
stand any flex at all, and if flexed will develop stress zones that will
likely turn into cracks. I would guess that a large percentage of the
problems people have with cracks coming in from the edge of platters or
large trimmed bowls results from mishandling during trimming.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
David Hendley on sun 27 jun 10
Vince, Steve, and all,
I am still convinced that Randall's large bowl cracking problem is
the result of uneven cooling.
However, since the conversation has turned to stress on the rims
and foam trimming batts, I would like to recommend Hank
Morrow's foam "ring bats" system. With this set up, you will
have what you need for trimming just about size bowl, all using
a system that puts absolutely no weight or stress on the rim.
There are photos of the system in use on the "Clayart"
page of my website:
http://www.farmpots.com/oven.htm
Check it out - it is a great system.
David Hendley
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
>I previously posted a reply to this question, mentioning my use of shims
> under larger bowls and platters, and between stacked bowls and platters,
> to
> allow even circulation of air, eliminating the uneven heating and cooling
> (thus expansion and contraction) that can occasionally cause the kinds of
> cracks mentioned in the original post.
>
> Several people have mentioned that the cracks might result from stresses
> caused while trimming the bowls resting on their rims, and I agree that
> this
> could be the problem. Some people have mentioned trimming on a foam pad,
> but the only way I can see this helping is if the foam pad is on a bat,
> and
> the bat is placed upside-down on the right-side-up bowl, and the whole
> thing
> is turned over and placed on the wheel.
>
> Steve Graber mentioned trimming large bowls on a foam "pile," and my
> equivalent is a fresh-thrown chuck, with the slurry removed with a metal
> rib. It can be thrown to fit the largest bowl or platter you are
> trimming,
> and then re-thrown to fit smaller ones as you get all your trimming done.
> The chuck should fit the inside of the bowl or platter at approximately
> the
> diameter of the foot, and should be at least an inch and a half taller
> than
> the depth of the platter or bowl. This allows you to grasp the bowl or
> platter with your fingers considerably down the walls from the rim and
> flip
> it over, thus avoiding any stress on the rim. Leather-hard pots cannot
> stand any flex at all, and if flexed will develop stress zones that will
> likely turn into cracks. I would guess that a large percentage of the
> problems people have with cracks coming in from the edge of platters or
> large trimmed bowls results from mishandling during trimming.
> - Vince
>
Steve Slatin on mon 28 jun 10
Vince -- Perhaps I don't understand the issue
here, but I leave the foam bat on the wheel
head and put the bowl onto the foam to trim.
Steve Slatin --=3D20
--- On Sun, 6/27/10, Vince Pitelka wrote:
=3DA0 Some people have mentioned
> trimming on a foam pad,
> but the only way I can see this helping is if the foam pad
> is on a bat, and
> the bat is placed upside-down on the right-side-up bowl,
> and the whole thing
> is turned over and placed on the wheel.
>=3D20
=3D0A=3D0A=3D0A
David Finkelnburg on tue 29 jun 10
Randy,
Do you have this sorted out yet? You've had a lot of feedback on
possible fixes.
My experience with cracks in bisque has usually been that they occur in
drying but show up in the bisque. To detect fine cracks in greenware may
take close inspection with a magnifying glass.
Radial cracks usually indicate the rim is drying too fast. On wider
forms I have had to compensate for the dry climate here by covering the rim
to prevent such cracks, or by inverting the piece onto a bat or flat
surface.
As others have said, zero cracks in bisque firing is achievable.
Good potting,
Dave Finkelnburg
http://www.mattanddavesclays.com
-----------------------------
Date: Thu, 24 Jun 2010 12:56:38 -0400
From: Randy McCall
Subject: Acceptable pot loss during bisque?
What is an acceptable pot loss during bisque? I have been firing 6lb bowls=
=3D
and have been losing a little over 10% of the bowls due to straight cracks=
=3D
from the rim inward. With this clay I never have much problems with small=
=3D
pieces or large vases 6 to 8lbs.
Me or the clay? Smaller bowls are ok.
Randy
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