John Rodgers on mon 13 sep 04
Sue,
I use Laguna ^5 B-Mix and make 22-24 inch platters with ornate
decoration on the rim. Rarely do I lose a platter to cracks.
In the beginning, I used flat bats that were made from particle board
covered with plastic laminate. This was ok, but the clay sticking to the
bat was a big problem. I would cut it free with a wire, from several
different directions, but it would still stick, and I had to repeat the
process over and over. The sticking was a real problem, and sometimes if
the clay decided to stick in one place and remain free in another the
piece would crack. If I stayed after it would dry successfully
eventually but would take two to three weeks, as the clay had to dry
through the top of the piece. The plastic beneath inhibited drying.
Failure rate was fairly high.
I finally moved over to using plaster bats. I made some 24-26 inch bats,
and now use them exclusively. The plaster bat will draw the moisture out
of the clay in just a few hours to the point of being able to slide the
new platter off the bat and onto a couple of sheets of newspaper on a
piece of plywood. The paper wrinkles up and moves under the piece as it
drys, so there is no cracking.
I still hated centering and pulling that much clay out onto the bat, so
I developed a new method. I now roll a slab of NEW clay rolling it first
one way, then rotating the clay 90 degrees then rolling it again. I work
it down to a slab 1/2 to 3/4 inch thick. I remove any fabric marks with
a sheetrock mud trowel. I then turn the bat over onto the slab, and flip
the whole thin over, clay side up. Once I put the bat on the wheel, I
start it turning and with a sponge wet the surface, start pulling clay
from the center to the edge of the bat, creating a roll of clay at the
rim, and thinning the bottom until it is about 3/8 inch thick. The roll
at the rim eventually becomes the platter rim. After this is done, the
rim is decorated. I allow to dry for a while - a couple or three hours
- time varies - I watch the piece closely and when it is ready, I slide
it onto paper with a bat under it.
Once the piece is on paper, I cover it with plastic and allow to dry
very slow. Finally, when I think it is ready, I will put the piece in
the kiln, and I will heat it at 150 degrees over night and next morning
complete the firing to bisque.
That slow, low fire is absolutely essential. With all the various
thicknesses in my platter work, it must be dried slowly under heat
immediately before firing. I have learned the hard way that clay will
entrain enough moisture from the atmosphere to cause a piece to explode
in the kiln, even if that piece has been drying for three months, unless
it is dried in the kiln under heat just before firing to above 212
degrees F.
To finish a platter, I spray glaze only. No brushes, no pouring of
pitchers.
This is my method and it works for me. I might lose two a year to cracking.
Regards,
John Rodgers
Chelsea, AL
>them to cracking. Sometimes the crack shows after the bisque firing and sometimes after the glaze firing. Sometimes it is a smallish "s" type crack, but often it is larger & all the way through the piece.
>
>My husband thinks it is usually recycled clay in the pieces that crack, but I think it happens more often than that.
>
>I dry my pieces slowly, thoroughly & usually upside down.
>
>I'm a mostly self-taught potter and so would really appreciate any suggestions that might help cure this problem. It has gotten so bad that I am discouraged from throwing large pieces since 95% of the time, they end up being hammered.
>
>Thanks.
>
>Sue Beach
>in Muncie, Indiana, USA where it is a glorious late-summer day (but I'm at the office - sigh)
>Potters Council Member
>
>
>
>______________________________________________________________________________
>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.
>
>
>
Beach, Sue on mon 13 sep 04
The recent threads about throwing large pieces reminded me of a question =
I've been wanting to ask.
When I throw largish bowls or platters, I often lose them to cracking. =
Sometimes the crack shows after the bisque firing and sometimes after =
the glaze firing. Sometimes it is a smallish "s" type crack, but often =
it is larger & all the way through the piece.
My husband thinks it is usually recycled clay in the pieces that crack, =
but I think it happens more often than that.
I dry my pieces slowly, thoroughly & usually upside down. =20
I'm a mostly self-taught potter and so would really appreciate any =
suggestions that might help cure this problem. It has gotten so bad =
that I am discouraged from throwing large pieces since 95% of the time, =
they end up being hammered.
Thanks.
Sue Beach
in Muncie, Indiana, USA where it is a glorious late-summer day (but I'm =
at the office - sigh)
Potters Council Member
=20
Scott Paulding on mon 13 sep 04
There was a thread on here earlier in the summer htat I found to be very
helpful for s-cracks...
After wedging the clay, we usually have a cone shaped lump of clay. I used
to put the flat end of the cone down on the wheelhead. Well, the
suggestion was to turn the clay on its side, and reform it into a rough
cone. Then throw as normal. This has helped me immensly, since I no longer
seem to get s-cracks, even on pieces throuw off the hump, and even when
throwing off the hump with reclaim. The only thing I have noticed is tha
tthe clay is a bit harder to center.
Hope this helps you too,
-scott
--- "Beach, Sue" wrote:
> The recent threads about throwing large pieces reminded me of a question
> I've been wanting to ask.
>
> When I throw largish bowls or platters, I often lose them to cracking.
> Sometimes the crack shows after the bisque firing and sometimes after
> the glaze firing. Sometimes it is a smallish "s" type crack, but often
> it is larger & all the way through the piece.
>
> My husband thinks it is usually recycled clay in the pieces that crack,
> but I think it happens more often than that.
>
> I dry my pieces slowly, thoroughly & usually upside down.
>
> I'm a mostly self-taught potter and so would really appreciate any
> suggestions that might help cure this problem. It has gotten so bad
> that I am discouraged from throwing large pieces since 95% of the time,
> they end up being hammered.
>
> Thanks.
>
> Sue Beach
> in Muncie, Indiana, USA where it is a glorious late-summer day (but I'm
> at the office - sigh)
> Potters Council Member
>
>
>
>
______________________________________________________________________________
> 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.
>
=====
"I should have been a plumber."
-Albert Einstein
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William Melstrom on mon 13 sep 04
Hamer has an excellent section on cracks. Hamer says that you are not
sufficiently compressing the bottom.
I would add that the problem can be exacerbated if you leave the piece
attached to the bat too long. Wire early, and wire often. Don't plan on
letting the piece get dry enough to "pop" the piece off the bat. When you
finally do get the wired piece off of the bat, put it on a fresh bat and
re-compress.
Here's a link to a huge, 19" platter. I have about a 100% success ratio
with big platters and bowls this size.
http://www.handspiral.com/images/platttopsm.jpg
William Melstrom
The recent threads about throwing large pieces reminded me of a question
I've been wanting to ask.
When I throw largish bowls or platters, I often lose them to cracking.
Sometimes the crack shows after the bisque firing and sometimes after the
glaze firing. Sometimes it is a smallish "s" type crack, but often it is
larger & all the way through the piece.
My husband thinks it is usually recycled clay in the pieces that crack, but
I think it happens more often than that.
I dry my pieces slowly, thoroughly & usually upside down.
I'm a mostly self-taught potter and so would really appreciate any
suggestions that might help cure this problem. It has gotten so bad that I
am discouraged from throwing large pieces since 95% of the time, they end up
being hammered.
Thanks.
Sue Beach
in Muncie, Indiana, USA where it is a glorious late-summer day (but I'm at
the office - sigh)
Potters Council Member
sdr on mon 13 sep 04
Sue said:
.....When I throw largish bowls or platters, I often lose them to cracking.
Sometimes the crack shows after the bisque firing and sometimes after the
glaze firing. Sometimes it is a smallish "s" type crack, but often it is
larger & all the way through the piece.......
The first thing I would suspect would be the clay body.
Some clays are great for throwing small items, but because
of shrinkage/warpage characteristics simply do not suit
large pieces.
I've been throwing for some years now, and have never had
cracking problems - until. I got some clay that I really liked,
had used before, used for workshops - threw like a dream. But,
when I got into the studio with it, and used it for large/wide items, it
cracked 80% of the time during drying or bisquing. Nothing I did would
prevent it, so I used that clay up on smaller items, and will never
purchase it again. Disappointing, I liked the small company that
made it, etc. Likely it could have been saved by a judicious
addition of grog, but I'd better things to do with my time.
Try a clay with more tooth and see if the problems remain the
same.
regards
Dannon Rhudy
foxpots on mon 13 sep 04
Dear Sue,
This isn't the first time this subject has arisen, and I know there will be
many useful tips for you. For myself, when making large thin platters (12
pounds of clay, 19 - 22" d), I cover them with newspaper, topped by a single
sheet of dry cleaning bag. Every day I undercut them again with a wire and
change the newspaper to dry paper and turn over the cleaning bag. It takes
a long time to dry, but now I rarely have cracks. I, also, do this with all
my plates and pie pans.
By the way, you are welcome to come visit me at Fox Hollow.
Jean Wadsworth Cochran
http://www.foxhollowpottery.com
When I throw largish bowls or platters, I often lose them to cracking.
Sometimes the crack shows after the bisque firing and sometimes after the
glaze firing. Sometimes it is a smallish "s" type crack, but often it is
larger & all the way through the piece.
I dry my pieces slowly, thoroughly & usually upside down.
Brant Palley NMCLAY.com on tue 14 sep 04
Cracks that show up in the glaze fire can be caused by the "rare roast
beef syndrome". That is a large flat piece fired in a kiln not too much
larger. The outside edge of the clay is much hotter than the inside
resting on the kiln shelf, the edge shrinks more than the middle causing
stresses.
Fire much slower and hold at the end (if you have a digital kiln).
Brant Palley
www.nmclay.com
David Hendley on tue 14 sep 04
Sue, judging by your description of the cracks, I think the problem is
created during forming, not drying or firing.
I think you are not adequately centering, opening, and compressing
the clay. It is much harder and takes a lot more strength to adequately
compress large amounts of clay, which is why the problem shows
up only in large pieces.
I suggest that you spend an extra moment centering the clay - cone it
up and down a couple of times, to make sure it is well centered. After
opening the ball and setting the floor, spend a moment to go back and
forth really compressing the bottom.
David Hendley
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
When I throw largish bowls or platters, I often lose them to cracking.
Sometimes the crack shows after the bisque firing and sometimes after the
glaze firing. Sometimes it is a smallish "s" type crack, but often it is
larger & all the way through the piece.
Veena Raghavan on wed 15 sep 04
If I may tag onto David Hendley's great advice, I have also found that
compressing the bottom from the outside with a rubber rib or a teflon
slider (the type sold in hardware stores for putting under furniture to
slide the piece of furniture), also helps compress the bottom and avoid
cracks.
Veena
Message text written by Clayart
> =
Sue, judging by your description of the cracks, I think the problem is
created during forming, not drying or firing.
<
Veena Raghavan
75124.2520@compuserve.com
Eleanor on wed 15 sep 04
I don't make really huge pieces--my plates average 10 inches--but I
rarely get cracks and I think this method would be applicable to
larger sizes.....
This is not my idea; it was posted to Clayart some years ago and
unfortunately I can't remember by whom:
Center a large lump and flatten it a bit. Using a length of PVC pipe,
longer than the diameter of the wheelhead, generously lubricated with
slip (or water), hold it firmly at each end and apply it to the clay
disk with downward pressure, equal on each side. Re-slip the pipe
frequently. A 3 inch diameter pipe seems to work best and the wheel
can turn fairly fast.
This is best done standing up; you get more leverage.
When you get the diameter you want, check and correct the centering,
if necessary; lift up the edge of the disk and form the rim. Maybe on
a larger platter, this could be done with an added coil.
I throw on a Hydrobat; when the plate is leather hard, it will come
away from the bat. I leave it on the bat until I can handle it
without distortion; then I turn the footring(s). Then it goes back
onto the bat right side up and dries slowly-- slow drying methods
have already been adequately covered here.
Whenever I open to make anything larger than a mug, I invariably wind
up with an uneven bottom, higher in the center. Evening this out
requires compression--so no cracks.
Compression seems to be the key. Using the pipe for plates or my
hands for bowls works.
Eleanor Kohler
Centerport, NY
Thanks, Lili--Happy New Year to all. May the coming year be free of
disasters, natural and man-made.
Ron Roy on fri 17 sep 04
Hi Sue,
better check out the "cracks" section in the Hamer book - find the cracks
that look like yours and it will be explained.
It is most important to identify the kind of crack to find the cause - is
it open or closed after the bisque firing?
Am I right in assuming these cracks are across the bottom of the piece?
That usually means the foot area needs more compression (inside and out)
during throwing and/or more even drying up to and after trimming.
RR
>The recent threads about throwing large pieces reminded me of a question
>I've been wanting to ask.
>
>When I throw largish bowls or platters, I often lose them to cracking.
>Sometimes the crack shows after the bisque firing and sometimes after the
>glaze firing. Sometimes it is a smallish "s" type crack, but often it is
>larger & all the way through the piece.
>
>My husband thinks it is usually recycled clay in the pieces that crack,
>but I think it happens more often than that.
>
>I dry my pieces slowly, thoroughly & usually upside down.
>
>I'm a mostly self-taught potter and so would really appreciate any
>suggestions that might help cure this problem. It has gotten so bad that
>I am discouraged from throwing large pieces since 95% of the time, they
>end up being hammered.
>
>Thanks.
>
>Sue Beach
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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