Dan Tarro on tue 15 sep 98
I have made an interesting find today and was wondering if my thinking is
on track or not. I have been making very large bowls that just fit into
my skutt kiln, I'm not sure what model off hand I think 1027. I have had
almost all the bowls in the past crack just inside the foot in a circle.
A week or so ago I was reading where someone was describing how electric
worked as compared to fuel kilns with radiant heat transfer, apposed to
heated gasses. I knew this but hadn't considered it with the bowls. On a
hunch, I ran a firing with eight of the larger bowls in the gas kiln for
bisque and they all made it, no cracks.
The hunch; The outside rim of the bowl took the heat first that was
radiating from the elements and came to temp before the center of the
bowl. Could this have caused uneven firing and dimensional stress during
the firing? Bisque firing for me takes a long time so I doubt that it was
forcing.
If you have any thoughts or experience on this, let me know.
Dan Tarro
Oak Tree Stoneware
Ham Lake, Minnesota
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Vince Pitelka on wed 16 sep 98
>The hunch; The outside rim of the bowl took the heat first that was
>radiating from the elements and came to temp before the center of the
>bowl. Could this have caused uneven firing and dimensional stress during
>the firing? Bisque firing for me takes a long time so I doubt that it was
>forcing.
Dan -
That is precisely the problem when firing large platters and bowls in an
electric kiln. I have completely eliminated the cracking problem when
bisque-firing my large platters and bowls in toploader electrics by placing
them on a starburst pattern of soft clay coils which supports the foot (or
feet). I just barely nestle the plate or bowl onto the coils, so that it is
completely evenly supported, but there is an airspace beneath. That does
the trick.
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Ron Roy on wed 16 sep 98
This could be related to the cooling - foot goes through the quartz
inversion at 573C after the sides - which cool faster. I wonder what would
happen if you fired them rim to rim or upside down - or even raised them
off the shelf a bit so the overall cooling is more even - RR
>----------------------------Original message----------------------------
>I have made an interesting find today and was wondering if my thinking is
>on track or not. I have been making very large bowls that just fit into
>my skutt kiln, I'm not sure what model off hand I think 1027. I have had
>almost all the bowls in the past crack just inside the foot in a circle.
>A week or so ago I was reading where someone was describing how electric
>worked as compared to fuel kilns with radiant heat transfer, apposed to
>heated gasses. I knew this but hadn't considered it with the bowls. On a
>hunch, I ran a firing with eight of the larger bowls in the gas kiln for
>bisque and they all made it, no cracks.
>
>The hunch; The outside rim of the bowl took the heat first that was
>radiating from the elements and came to temp before the center of the
>bowl. Could this have caused uneven firing and dimensional stress during
>the firing? Bisque firing for me takes a long time so I doubt that it was
>forcing.
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Barney Adams on thu 17 sep 98
Would there be a benefit to firing the bowl for bisque upside down? Again
I'm really new to this, but as I understand the heat in the kiln is more or less
line of sight to the elements. I imagine there may be another concern that I'm
not aware of.
Barney
>
> ----------------------------Original message----------------------------
> >The hunch; The outside rim of the bowl took the heat first that was
> >radiating from the elements and came to temp before the center of the
> >bowl. Could this have caused uneven firing and dimensional stress during
> >the firing? Bisque firing for me takes a long time so I doubt that it was
> >forcing.
>
> Dan -
> That is precisely the problem when firing large platters and bowls in an
> electric kiln. I have completely eliminated the cracking problem when
> bisque-firing my large platters and bowls in toploader electrics by placing
> them on a starburst pattern of soft clay coils which supports the foot (or
> feet). I just barely nestle the plate or bowl onto the coils, so that it is
> completely evenly supported, but there is an airspace beneath. That does
> the trick.
> - Vince
>
> Vince Pitelka - vpitelka@DeKalb.net
> Home 615/597-5376, work 615/597-6801, fax 615/597-6803
> Appalachian Center for Crafts
> Tennessee Technological University
> 1560 Craft Center Drive, Smithville TN 37166
>
Kelley Webb Randel on thu 17 sep 98
Vince? Are the soft clay coils wet clay? you are sitting your large platters
on wet clay? what other types of soft coils are there? am i showing my
stupid?
Kelley webb randel
Vince Pitelka on fri 18 sep 98
>----------------------------Original message----------------------------
>Vince? Are the soft clay coils wet clay? you are sitting your large platters
>on wet clay? what other types of soft coils are there? am i showing my
>stupid?
Kelley -
As I am fond of saying, the only stupid questions are the ones that do not
get asked.
Yes, just roll thin coils from soft clay - make them about 3/8"-thick, lay
out a starburst pattern, and gently nestle the foot of the platter or bowl
down onto these coils. It is only necessary to do this in the bisque-fire.
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Kelly Young on fri 18 sep 98
I'm curious about the use of soft clay coils. That would mean wet clay?
wouldn;t that be a problem during the firing?
Kelly
>Dan -
>> That is precisely the problem when firing large platters and bowls in an
>> electric kiln. I have completely eliminated the cracking problem when
>> bisque-firing my large platters and bowls in toploader electrics by
>placing
>> them on a starburst pattern of soft clay coils which supports the foot
>(or
>> feet). I just barely nestle the plate or bowl onto the coils, so that
>it is
>> completely evenly supported, but there is an airspace beneath. That
>does
>> the trick.
>> - Vince
>>
>> Vince Pitelka -
vpitelka@DeKalb.net
>> Home 615/597-5376, work 615/597-6801, fax 615/597-6803
>> Appalachian Center for Crafts
>> Tennessee Technological University
>> 1560 Craft Center Drive, Smithville TN 37166
>>
Vince Pitelka on sun 20 sep 98
>I'm curious about the use of soft clay coils. That would mean wet clay?
>wouldn;t that be a problem during the firing?
Kelly -
As long as you preheat the kiln overnight you will have no problem at all.
I normally only use this system (firing platters on a starburst pattern of
soft clay coils) in bisque-firings, but with some of my most complex inlaid
colored clay platters I also use it in the glaze firing to minimize warpage,
in which case I also preheat overnight. If you want to fire without an
overnight preheat, once the platter or bowl is nestled in place on these
coils, you can force dry them in an hour or two with the lid open and the
elements on low. As long as the coils are 3/8"-diameter or less, they will
dry extremely quickly without causing any problems. If you REALLY want to
make sure, you could make your coils from a claybody with lots of sand or
grog, which can dry out extremely fast with no problems.
Good luck -
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
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