Dave and Pat Eitel on tue 15 apr 97
After 20-some years of working mostly in stoneware, I recently threw some
18" platters in porcelain. I understand what causes S-cracks and circular
cracks, but some of these cracked in relatively straight lines more or less
perpendicular to the rim. Any thoughts, those of you who throw a lot of
porcelain?
Later...Dave
Dave Eitel
Cedar Creek Pottery
Cedarburg, WI
pots@cedarcreekpottery.com
http://www.cedarcreekpottery.com
Dannon Rhudy on tue 15 apr 97
Dave,
What's in your porcelain body, and at what point do the cracks
appear? Green, bisque, glaze?
Dannon Rhudy
potter@koyote.com
----------------------------Original
message----------------------------
After 20-some years of working mostly in stoneware, I recently
threw some
18" platters in porcelain. I understand what causes S-cracks and
circular
cracks, but some of these cracked in relatively straight lines
more or less
perpendicular to the rim.
Malone & Dean McRaine on wed 16 apr 97
Aloha everyone:
I do a lot of work in procelain (Dave's Porcelain from Laguna).
Dave your problem may be from too much heat differential between your piece
and the shelf if your cooling cycle is too fast through the quartz changes
around 1050 F and 450 F. A better guess is too much friction between platter
and shelf, porcelain can grab onto a shelf and stick and pull itself apart
as it shrinks-dust shelf with something to make it slide, Alumina Hyd,
silica, sand, etc.
Good luck
Dean: beezer@aloha.net
At 07:05 AM 4/15/97 EDT, you wrote:
>----------------------------Original message----------------------------
>After 20-some years of working mostly in stoneware, I recently threw some
>18" platters in porcelain. I understand what causes S-cracks and circular
>cracks, but some of these cracked in relatively straight lines more or less
>perpendicular to the rim. Any thoughts, those of you who throw a lot of
>porcelain?
>
>Later...Dave
>
>Dave Eitel
>Cedar Creek Pottery
>Cedarburg, WI
>pots@cedarcreekpottery.com
>http://www.cedarcreekpottery.com
>
>
With Aloha blessings,
Malone
Tony Hansen on wed 16 apr 97
> After 20-some years of working mostly in stoneware, I recently threw some
> 18" platters in porcelain. I understand what causes S-cracks and circular
> cracks, but some of these cracked in relatively straight lines more or less
> perpendicular to the rim. Any thoughts, those of you who throw a lot of
> porcelain?
Our experience with plates seems to be the drying difficulty varies with
the
size squared. Twice the size is 4 times the cracks. Sometimes people
make
12" platters for years then go to 14 and they all crack. Then they
switch
to porcelain and they are ready to kill the clay manufacturer!
Porcelain has a much lower dry strength and
narrower particle size distribution. However the old rule still stands,
'you can dry anything if you do it evenly'. There are lots of things you
can
do to make it more even and there are lots of things you can do with
other
parts of the process. I'm sure suggestions will be forthcoming.
--
=================================================================
Tony Hansen, Plainsman
Jeremy/Bonnie Hellman on wed 16 apr 97
Dave-
Anytime I've had cracks like that it is from handling or stessing the
rim--the tendency to pick up a pot and anchor your finger over the rim.
Bonnie Hellman
>Subject: porcelain cracks
>Sent: 4/15/97 7:05 AM
>Received: 4/15/97 2:51 PM
>From: Dave and Pat Eitel, pots@cedarcreekpottery.com
>Reply-To: Ceramic Arts Discussion List, CLAYART@LSV.UKY.EDU
>To: Multiple recipients of list CLAYART, CLAYART@LSV.UKY.EDU
>
>----------------------------Original message----------------------------
>After 20-some years of working mostly in stoneware, I recently threw some
>18" platters in porcelain. I understand what causes S-cracks and circular
>cracks, but some of these cracked in relatively straight lines more or less
>perpendicular to the rim. Any thoughts, those of you who throw a lot of
>porcelain?
>
>Later...Dave
>
>Dave Eitel
>Cedar Creek Pottery
>Cedarburg, WI
>pots@cedarcreekpottery.com
>http://www.cedarcreekpottery.com
"Very funny, Scotty. Now beam down my clothes!" Capt. J. Kirk
"Outside a dog, a book is a man's best friend. Inside a dog, it's too
dark to read" Groucho Marx
" " Harpo Marx
"Time flies like an arrow. Fruit flies like an avocado" Att. to GM
Ron Roy on wed 16 apr 97
>----------------------------Original message----------------------------
>After 20-some years of working mostly in stoneware, I recently threw some
>18" platters in porcelain. I understand what causes S-cracks and circular
>cracks, but some of these cracked in relatively straight lines more or less
>perpendicular to the rim. Any thoughts, those of you who throw a lot of
>porcelain?
>
>Later...Dave
Hi Dave,
Porcelain can be a bitch after you are used to stoneware - I likes to dry
unevenly so -
If the crackes happened from wet to dry - compress those rims and dry
slowely - get em off the batt as soon as possible and turn em over as soon
as possible - and give em a kiss good night EVERY night.
If they cracked in the bisque it's a quartz dunt at 573C - fire bisque to
at least 04 and seal your kiln to slow the cooling down from 600C to 450C.
Make sure your damper is completely closed - you don't want any cold drafts
touching those rimes as the beta changes to alpha.
There is a wonderful section in the Hamer Dictionary on cracks. You find
the crack that looks like yours and you get the explanation of how it
happened.
Porcelain can drive you crazy - but your glazes are going to look better.
Let me know if you need more on this Dave.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
ronroy@astral.magic.ca
Jeff Lawrence on thu 17 apr 97
Hi Dave,
I've been very consistent in losing my 18" pendant fixtures (admittedly
low-fire) to exactly those kinds of cracks whenever I
(1) ramp up too fast
(2) cool down too fast
I think the effect of the silica phase shifts is somehow magnified in wide
pieces, because I get away with fast with the smaller ware. Or maybe the
inherent dignity of a larger piece requires a more stately progress through
the temperature range.
.... by no means to imply, of course, that you are so feckless as to try to
rush things the way I do ...
Best regards,
Jeff
Jeff Lawrence
jml@sundagger.com
Sun Dagger Design
Rt. 1 Box 394L
Espanola, NM 87532
vox/fax 505-753-5913
Craig Martell on thu 17 apr 97
At 08:11 AM 4/16/97 EDT, you wrote:
>----------------------------Original message----------------------------
>>----------------------------Original message----------------------------
>>After 20-some years of working mostly in stoneware, I recently threw some
>>18" platters in porcelain. I understand what causes S-cracks and circular
>>cracks, but some of these cracked in relatively straight lines more or less
>>perpendicular to the rim. Any thoughts, those of you who throw a lot of
>>porcelain?
>>
>>Later...Dave
Hello Dave:
All I do anymore is make porcelain and have been in that mode for the last
12 yrs, so I'll pass along what has worked for me. Maybe it will help.
Large flatware pieces need to be dried VERY slowly for a couple of reasons.
Porcelain dries fairly fast, and the rims will start to shrink first. They
will either raise up a bit, which I don't like, or they will crack.
Especially if the clay is short or not aged long enough. To minimize this
problem, I throw the rims out as much as possible instead of settling them
down with a rib from a more vertical orientation. This eliminates
stretching the rim too much and orients the clay platelets in a more
horizontal fashion. I then flatten the rim a bit more with a rib and then
compress it well. I do this with 12 inch dinner plates and up to 24 inch
platters and I never get cracks in greenware. The clay has to get some
credit for this success as well. I have a plastic covered damp rack that is
6' long, 2' wide, and 6' high. I place about 6 to eight large platters in
at a time, or about 24 pieces of dinnerware so the combined humidity really
slows the drying. I let the pieces lounge in the damp rack for 5 to 6 days
minimum before turning and slip decorating. The rims stay pretty flat, and
I get very little warping from uneven drying. When flipping the plates and
platters, I use the bat sandwich method to avoid any stress from handling,
and dry the plates on their feet, after turning the foot ring, until they
are bone dry and ready to bisque.
One thing to consider when you are formulating or buying a porcelain body
for throwing is the amount of added montmorillionitic plasticizers such as,
bentonite, vee gum "t", or macaloid. While small amounts of these materials
can be very beneficial to the working properties of the clay, too much can
cause problems by increasing the plastic to bone dry shrinkage and
interfering with the capillary drying action of the clay. Clays with good
capillary action will pass water from wetter to dryer areas resulting in
more even and uniform drying of the ware. Montmorillionitic clays have very
poor capillary action, so don't use any more than necessary, especially in a
porcelain body that is not "open" because of added sand or grog.
To avoid biscuit dunts, you will probably need to bisque fire large flatware
to at least cone 04, as Ron Roy has already stated. I bisque fire my
porcelain pieces to 03. When I bisqued to 06, I lost a lot of plates, all
sizes, to dunts. Porcelain clays are more refractory than stonewares and
need the extra temp to at least sinter or lighly soften the feldspar to
increase the strength of the ware so it will survive the quartz inversion on
the way down. I also box the plates and platters in the bisque instead of
stacking in a nesting manner to avoid a solid center mass that you get with
nested feet. When you nest plates, the rims will cool faster than the
central mass and increase the chances of dunts.
I hope this is of some use. If I can be of any further help, let me know.
I would also be glad to pass on the porcelain formula that I use if you want
it for comparsion or would like to try it.
Regards, Craig Martell-Oregon
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