Fredrick Paget on sun 28 sep 08
Kendra,
Sorry to say that it will not go back but will just sag some more.
Different porcelains will take different maximum temperatures before
sagging. You don't say what kind you used or how hot it was fired so
???
I have recently been working with Southern Ice which is the whitest
and most translucent of porcelains that I have ever tried. The bad
news is that it is hell to work with and making paper thin large
pieces will try your patience and result in plenty of rejects. It
sags horribly at cone 10 does pretty well at 9 and still vitrifies.
To get large bowls that are thin I have been hand building in plaster
molds that support the form and fit on my Jepson wheel so after
lining the form with a 1/8 inch thick slab I can finish up by
throwing on added parts.
Then prebisqueing at cone 015 so the bisque is soft and chalky so it
is easy to carve and sand.
Then I do my thing with soluble salts and refire the prebisque to
cone 04 so I can glaze it for the high fire . It works best if dry
footed on an advancer shelf that has a thin coating of alumina
hydrate rubbed on.Stilts are no good as you saw.
Have at it. There is a big learning curve and I am just at the
beginning of it myself. There are ways to make supports to hold the
ware in shape during firing but I don't know how to do that and keep
the ware from sticking to the support. Perhaps another potter will
tell us how to do that.
Fred
>I just recently did my first glaze firing at my new college. I was
>excited to see that my most important piece came out without any
>problems- the good part. The bad part of this firing was that my work
>was put on those small 3 prong kiln stilts, and apart from making weird
>black marks where the prongs were, my porcelain boxes sagged into weird
>shapes.
>Instead of being perfectly square before firing, the sides lean in a
>little and the bottoms are no longer flat. This is a digital kiln and we
>didn't use a cone pack. The glaze came out perfect, exactly like it is
>supposed to. If I fire these in the same kiln again without kiln stilts,
>will these boxes go back to their original or close to their original shape?
--
Fred Paget
Twin Dragon Studio
Mill Valley, CA, USA
fredrick@well.com
Charter Member Potters Council
Ann Brink on sun 28 sep 08
It might work. A few months ago I had a similiar problem- I fired a plate
on a kiln shelf that was slightly narrower than the base of the plate, and
it warped out of round. It was a B-mix body, by the way. So in my next
glaze firing, I placed it on a shelf where it had even support, and it
corrected itself during the firing.
If you do try refiring, you may want to place a layer of alumina on the
shelf, to prevent it sticking as it moves. Good luck!
Ann Brink in Lompoc CA
(mostly about pottery)
----- Original Message -----
From: "Digital Studio"
To:
Sent: Sunday, September 28, 2008 8:12 PM
Subject: Wobbly Porcelain - Refiring
>I just recently did my first glaze firing at my new college. I was
> excited to see that my most important piece came out without any
> problems- the good part. The bad part of this firing was that my work
> was put on those small 3 prong kiln stilts, and apart from making weird
> black marks where the prongs were, my porcelain boxes sagged into weird
> shapes.
> Instead of being perfectly square before firing, the sides lean in a
> little and the bottoms are no longer flat. This is a digital kiln and we
> didn't use a cone pack. The glaze came out perfect, exactly like it is
> supposed to. If I fire these in the same kiln again without kiln stilts,
> will these boxes go back to their original or close to their original
> shape?
>
Digital Studio on sun 28 sep 08
I just recently did my first glaze firing at my new college. I was
excited to see that my most important piece came out without any
problems- the good part. The bad part of this firing was that my work
was put on those small 3 prong kiln stilts, and apart from making weird
black marks where the prongs were, my porcelain boxes sagged into weird
shapes.
Instead of being perfectly square before firing, the sides lean in a
little and the bottoms are no longer flat. This is a digital kiln and we
didn't use a cone pack. The glaze came out perfect, exactly like it is
supposed to. If I fire these in the same kiln again without kiln stilts,
will these boxes go back to their original or close to their original shape?
Digital Studio on mon 29 sep 08
Here's a little background. The person that loaded the kiln was just the
studio assistant, and I didn't glaze the bottoms. I wasn't there when
the kiln was loaded or I would have told the assistant why porcelain
can't be fired on stilts.
This was a body that was bisqued to cone 08 and glazed to cone 6.
I'm going to check and see about doing a test with one piece in the kiln
with a cone pack. Should I use more than one to check the evenness of
firings?
William & Susan Schran User wrote:
> On 9/28/08 11:12 PM, "Digital Studio" wrote:
>
>
>> The bad part of this firing was that my work
>> was put on those small 3 prong kiln stilts, and apart from making weird
>> black marks where the prongs were, my porcelain boxes sagged into weird
>> shapes.
>>
> Why were they placed on stilts? Did you apply glaze to the bottom?
> Generally speaking, stilts are used for low-fire glaze fire applications.
>
>
>> Instead of being perfectly square before firing, the sides lean in a
>> little and the bottoms are no longer flat. This is a digital kiln and we
>> didn't use a cone pack. The glaze came out perfect, exactly like it is
>> supposed to. If I fire these in the same kiln again without kiln stilts,
>> will these boxes go back to their original or close to their original shape?
>>
> Porcelain and clays with little or no grog will be prone to warping if
> supported by stilts in the usual fashion and fired to a high enough
> temperature where the clay becomes proto-plastic and vitrified.
> If you re-fire, the boxes will not return to their geometric shapes, but
> will probably warp/distort even further.
>
> Please suggest to your teacher that cones should be used in every firing,
> bisque or glaze. Relying on a thermocouple, that degrades over time, is not
> a reliable or accurate way to fire. You may show him/her this response and I
> would be happy to explain in greater detail.
>
> Bill
>
> --
> William "Bill" Schran
> wschran@cox.net
> wschran@nvcc.edu
> http://www.creativecreekartisans.com
>
>
>
William & Susan Schran User on mon 29 sep 08
On 9/28/08 11:12 PM, "Digital Studio" wrote:
> The bad part of this firing was that my work
> was put on those small 3 prong kiln stilts, and apart from making weird
> black marks where the prongs were, my porcelain boxes sagged into weird
> shapes.
Why were they placed on stilts? Did you apply glaze to the bottom?
Generally speaking, stilts are used for low-fire glaze fire applications.
> Instead of being perfectly square before firing, the sides lean in a
> little and the bottoms are no longer flat. This is a digital kiln and we
> didn't use a cone pack. The glaze came out perfect, exactly like it is
> supposed to. If I fire these in the same kiln again without kiln stilts,
> will these boxes go back to their original or close to their original shape?
Porcelain and clays with little or no grog will be prone to warping if
supported by stilts in the usual fashion and fired to a high enough
temperature where the clay becomes proto-plastic and vitrified.
If you re-fire, the boxes will not return to their geometric shapes, but
will probably warp/distort even further.
Please suggest to your teacher that cones should be used in every firing,
bisque or glaze. Relying on a thermocouple, that degrades over time, is not
a reliable or accurate way to fire. You may show him/her this response and I
would be happy to explain in greater detail.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Mary Driever on mon 29 sep 08
have you tried a fine layer of sand on the shelve, since porcelin shrinks
the sand draws in like ball bearings and keeps the bottoms flat. doll makers
use it all the time.
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Christine Campbell on mon 29 sep 08
Fred wrote:
> There are ways to make supports to
> hold the ware in shape during firing
> but I don't know how to do that and keep
> the ware from sticking to the support.
> Perhaps another potter will tell us how
> to do that.
Many porcelain people high fire first, then
low fire with a glaze.
Low bisque fire only if you need to clean or carve.
Fire high with supports ... unglazed or just
glaze the unsupported surfaces.
Fire low to glaze other surfaces.
Some shapes are self supporting so they do not slump
but you still need Alumina hydrate on bottoms so they
do not stick to the shelf. I put it in my wax resist
and paint the bottoms with it.
Once you get beyond cylinders and round the shape out
you have to support the ware.
With Southern Ice you can increase your success rate by
drying it extremely slowly and evenly. By slowly I mean
by the week, not days or hours.
Then fire down the kiln after high fire.
I know many people will say they cannot wait that long
for something to dry, etc ... but you just put the work
aside and keep making more ... all that happens is you end
up working on a longer time frame than others.
The same company makes a Cone 6 porcelain called Frost.
It is not quite as white as Southern Ice but it is easier
to work with and fires lower.
Hope this helps some
Chris Campbell - in North Carolina
Chris Campbell Pottery LLC
9417 Koupela Drive
Raleigh NC 27615-2233
Designs in Colored Porcelain
1-800-652-1008
Fax : 919-676-2062
website: www.ccpottery.com
wholesale : www.wholesalecrafts.com
Antoinette Badenhorst on mon 29 sep 08
Kendra, what you've experienced with your porcelain is one of the main characteristic of porcelain: It moves in the kiln, due to the glass like nature of porcelain. If you do not want for it to move, you have to restrict it with obstacles in its way. The beauty though is that if you want to go with the movement, you learn to judge it like two dance mates would in a dancing act. Large flat surfaces simply do not work with porcelain unless it is supported by a flat basis. Porcelain should not be fired on stilts ( for that matter stoneware should also not be stilted......only earthenware should be on stilts because it needs glaze on the bottom). I assume your teacher let you do that to protect the kiln shelves and I can not blame her.
Suggest to her that you make flat discs, a little larger than your piece that you protect with alumina hydrate or even silica ( as others described it) and place your work onto that in the kiln.If your porcelain have little feet, create a heap of silica and nest the piece into it, of cause there should not be glaze on the bottom.
Remember silica and any other raw materials should be treated with respect and responsibility, since it can create health problems.
--
Antoinette Badenhorst
www.clayandcanvas.com
www.studiopottery.co.uk
-------------- Original message ----------------------
From: Digital Studio
> I just recently did my first glaze firing at my new college. I was
> excited to see that my most important piece came out without any
> problems- the good part. The bad part of this firing was that my work
> was put on those small 3 prong kiln stilts, and apart from making weird
> black marks where the prongs were, my porcelain boxes sagged into weird
> shapes.
> Instead of being perfectly square before firing, the sides lean in a
> little and the bottoms are no longer flat. This is a digital kiln and we
> didn't use a cone pack. The glaze came out perfect, exactly like it is
> supposed to. If I fire these in the same kiln again without kiln stilts,
> will these boxes go back to their original or close to their original shape?
William & Susan Schran User on tue 30 sep 08
On 9/30/08 12:02 AM, "Digital Studio" wrote:
> I wasn't there when
> the kiln was loaded or I would have told the assistant why porcelain
> can't be fired on stilts.
I still don't understand why it was fired on stilts. Are the stilts, actual
triangular shaped hard ceramic devices with little points or was clay/kaolin
pads of clay used?
> This was a body that was bisqued to cone 08 and glazed to cone 6.
> I'm going to check and see about doing a test with one piece in the kiln
> with a cone pack. Should I use more than one to check the evenness of
> firings?
Bisque and glaze firing temperature is not an issue - the issue is the clay
body and it's vitrification temperature.
How big is the kiln? Unless it's a very small test kiln, it makes no sense
to fire a single small piece.
I would suggest placing a cone pack of ^5, ^6, ^7 at the top, middle and
bottom of kiln to see how even or uneven the temperature is.
If temperature is even throughout the kiln, then one cone pack at the center
would be fine.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Patty Kaliher on tue 30 sep 08
I recently talked to a potter who said that since changing to advancer kiln
shelves he has not had to stilt large porcelain platters. Either the
platters are able to shrink across the shelf or the shelves hold less heat
and platters can cool more evenly top and bottom.
Just passing along the comment, I use a kiln wash on the advancer shelves
when firing porcelain but I also do not make large platters.
Patty Kaliher
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