[Clayart] Star crack in large flat porcelain bisque

[kst...@aol.com]

So someone at our art center made a foot long oval porcelain flat bottomed casserole. Sides and bottom were equal 1/4 inch thickness. A large star shaped crack developed in the base during bisquing.

Thoughts- if I were in charge and ruled the world ( evil laughter).

I suggested firing the piece on its side or with small pieces of firebrick underneath so to lessen thermal shock if the shelf is hot and the piece cools first.

We all suggested a white stoneware but this person is fixated on using porcelain.

We used to have molochite around which perhaps could have been wedged in? Does that help?

This person for some reason decided to put rice in the piece while it was drying under plastic to keep it flat. The black mold was horrendous. I am not sure if she put the casserole on a piece of newspapering the bottom I don't think so. Could the inability of the floor to arch then flatten while shrinking ( I don't know where I heard that happened). Or the inability to slide across the wood surface cause a large star crack?

I was not there to see how much she compressed the base in different directions, or how dry the pieces were before joining wall to base.

Our experienced mentor has a great amount of skill and knowledge about stoneware but detests porcelain. He therefore has little experience with its unique problems.

Advise or corrections on my thoughts?

Thanks,

Kathy Stecker


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[douglas fur]

Kathy
I guess she was following the " cut a willow leaf segment from the bottom"
method. If you don't compress the bottom well it tends to crack, splitting
along the seam with cracks perpendicular to the seam, which could look like
a star. After ovaling the dish the leaf is put on top of the seam, pressed
and smoothed down. This usually supplies enough compression. (Was this
step done?)
The rice sounds like a pastry technique, a reverse crossover too as ceramic
beans are sold to use in precooking pie crusts to keep them from puffing up.

DRB
Seola Creek

[Kathi]

I would have bisque this piece on edge, long side leaned against a prop of some kind. This how I fire all of my large platters.

Kathi

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[Dannon Rhudy]

Kathy said: .....So someone at our art center made a foot long oval

porcelain
flat bottomed casserole. Sides and bottom were equal 1/4 inch thickness.

A large star shaped crack developed in the base during bisquing.........


I believe that such a problem is not unique to
porcelain. But porcelain is, ummm, fussy.

However, the bisque crack sounds to me, from
the description, to be from uneven heating/cooling.
A lot of surface sitting on a hot shelf, while the
sides are cooling/shrinking. It might have happened
heating up or cooling down, but since I can't see
it - not sure. But the problem is the same, heating
or cooling: part of the piece staying hotter, or
cooler, while the sides heat/cool much more quickly.

Needs to be up off the shelf for more even temp. change.

Drying with rice in the bottom was not a wise choice. If
anything were needed, then I'd suggest making a bunch of bisque
clay "chips" for such purpose.
If it was already being dried under plastic,
seems like overkill to me. Also, just as an aside -
if such a casserole died during bisque firing, imagine
what might happen in the oven, when said casserole is partially
full of some substance, heating up and cooling down
unevenly over and over. Worse than what might happen in
the oven, what might happen while it was being lifted OUT
of the oven. Things to consider.


regards

Dannon Rhudy

[David Woof]

*****Kathy said: <"So someone at our art center made a foot long oval porcelain flat bottomed casserole. Sides and bottom were equal 1/4 inch thickness. A large star shaped crack developed in the base during bisquing. ......... This person for some reason decided to put rice in the piece while it was drying under plastic to keep it flat. The black mold was horrendous. I am not sure if she put the casserole on a piece of newspapering the bottom I don't think so. Could the inability of the floor to arch then flatten while shrinking ( I don't know where I heard that happened). Or the inability to slide across the wood surface cause a large star crack?">**********

Hi Kathy, More likely from your description of process, the crack did not develop during the bisque firing, as you suggest, but conditions were set up during the fabrication and drying of the piece. Technically she did everything to insure failure which caused internal stresses and fractures to develop that didn't visually become manifest until unloading the bisque.

Was the piece thrown and altered, slab or coil built or a combination? It sounds like her vessel's bottom dried last, what with soggy news print and rice absorbing and then holding moisture.

When any area dries ahead of the entire piece, the material matrix in that area becomes inflexibly bonded/locked, and all other areas must shrink and pull(read stress) against that initial prematurely dried area. Something then, the weakest link, must begin to give and fracture to relieve that stress.

When ever, even creatively possible, i invert the vessel for drying as this creates a humidity chamber with-in the interior which helps even-out the escape and evaporation of moisture. I also use plastic wraps if needed, and wax resist problematic joins, joints, rims, and protruding appendages to mask and so retard drying of these areas for more even overall drying. Complex pieces need creative and sometimes complex solutions.

Getting back to simple: A flat bottom piece whose bottom is dried to leather hard on canvas covered plaster, then placed on a discarded oven or refrigerator shelf, elevated by 2X4's, with small fan moving air underneath, and with the top of vessel plastic sheeted will usually dry splendidly. Wax resisting the rim is buying insurance. Again you are creating that humid interior space. Lock that bottom first and let the sides and rim area shrink down to meet it. Rim last.

I could go on and on about this with descriptions, pictures and diagrams, but then I would have a book and there are already enough books published and archives available. Enough stupid also!!! One must educate one's self deeply enough to recognize good advice. Ask self, how committed am I to the craft? Am I willing to work at the self education side of this to become knowledgeable and proficient? Fast and easy answers are superfluously just that and rarely take one to the next level.

So suffice to say that your friend needs to study the physics of the craft until she understands, and can demonstrate by practical applications. I wish her the best.

Love to all,
David Woof......... We have seen the clay, it is us.) <](:>0)==={--------<

> Date: Sun, 3 Nov 2013 09:54:41 -0500
> From: kst...@aol.com
> To: cla...@ceramicist.org
> Subject: [Clayart] Star crack in large flat porcelain bisque

[Snail Scott]

On Nov 3, 2013, at 8:54 AM, kst...@aol.com wrote:
> So someone at our art center made a foot long oval porcelain flat bottomed casserole. Sides and bottom were equal 1/4 inch thickness. A large star shaped crack developed in the base during bisquing...


A star-shaped crack suggests that the bottom dried last.
The stresses were there, left over from drying unevenly,
but didn't manifest until firing.

Dry rice will absorb moisture, but if you don't remove the
rice then the moisture is still there in close proximity, and
air is prevented from contacting the clay directly. You gotta
swap the stuff out regularly, to remove the moisture it
absorbs! Daily, if possible. Set one day's damp rice aside
to dehydrate and replace it with dry rice, then the next day
when the first batch is dry, you can swap them out. The fact
that it grew mold is a good sign that it got left to just sit, damp.

This holds true of anything (like paper) that you put inside the
plastic to absorb moisture. Paper, however, is usually loose
and fluffy, not sitting right up against one spot on the clay. If
it were, you'd have the same problem: holding the absorbed
moisture unequally, likely right where you don't want it!

Absorbent materials don't remove moisture from inside the
plastic, they just provide an additional home for the water to
take up residence. It can be a great way to facilitate slow,
even drying, but only if you actually remove the moisture!
(Think of the paper, or rice, like mousetraps. You gotta toss
'em out, or you've still got the mouse inside, becoming
nastier by the day!)

I'd either dry such forms upside-down (on foam if the top isn't
level) or else with loose paper swapped out, or on drywall
with the drywall sitting on an open-slat shelf. Doesn't hurt to
swap out the drywall, either, especially if the shelf is an
impervious material. Drywall's gotta dry out, too!

-Snail

[kst...@aol.com]

Thank you thank you for all your comments. I will take printouts to share with my friend and look for more comments. It is also nice to clarify thoughts on how to fire these types of pieces in the kiln and that the rice just kept the whole thing wet which makes alot of sense. I will suggest to her to use drywall and make sure that she flips or dries the floor of the piece more evenly

The piece seemed nicely made and handbuilt. I do remember Sylvie Granatelli telling us that she had many problems with splitting slabs and finally started to throw tall cylinders then slice them into slabs to make them properly compressed as compared to rolled out slabs.

Hopefully a couple of these changes should help out!

Thanks so much!

Kathy Stecker



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[Benjamin Kant]

Kathy Stecker wrote:
"I do remember Sylvie Granatelli telling us that she had many problems with
splitting slabs and finally started to throw tall cylinders then slice them
into slabs to make them properly compressed as compared to rolled out
slabs."

Huh? This makes no sense to me, in particular when the thickness of the
cylinder may vary, in particular toward the base, making the "split slab"
uneven in thickness.
How can a thrown wall be more compressed than a rolled slab? Am I missing
something?

Ben

[Michael Wendt]

Ben,
I agree a thrown wall would be fairly uneven when cut and laid flat.
Further, my experience shows that the wall would have an oblique or spiral
structure that would skew a square form into a parallelogram shape after
firing.
Tile maker Janet Starr reported to me that her loss rate on tiles dropped
significantly after she started using the stack and slam wire wedging method
shown in my YouTube video. Further, she reduced the doublings from 30 to 10
or less and still got vastly lower warping, the main issue with flat work.
I have my own theories about clay memory but I will keep them to myself
while I continue my research.
Regards,
Michael Wendt

[Eva Gallagher]

One of the best methods to stop flat pieces from warping is to do what Dan
Hill showed us in a workshop - lay the flat slab on a drywall piece and drop
from waist height on the floor several times. Also once the item is put
together he uses a small bag filled with sand to tamp down on the bottom
several times during the drying .
If I do the above, I never have to rotate the slabs through the slab
roller - After pounding the clay into a roughly rectangluar shape and about
1 - 2 inches high I just put it through once or twice - all in the same
direction.

Eva Gallagher
Deep River, Ontario
http://newfoundoutpotter.blogspot.com/