Pamela Jo Stamper on thu 20 nov 97
Hello,
I hope you won't mind revisiting this subject briefly. I went
through many hundreds of not very well orginized saved files and did find
several posts on preventing cracks during firing of large platters. I
learned that drying is important because uneven drying creates stresses in
the clay that can show up as a crack later and that it is important to fire
so that heat can evenly distribute and the center or edges are not hotter.
One of you suggested sand (on the kiln shelf) to allow movement during
firing.
I remember that someone suggested bisque firing large platters on
edge. Does anyone remember this hint? Why would you do it and how would
it work? I have had several large shallow bowls and platters crack
recently and I am ready to try any bisque firing technique that might work.
Thanks.
Pamela Jo (on the wet North Oregon coast) stamps@seasurf.com
Ray Carlton on fri 21 nov 97
hi pamela.....i had a problem with large platters [500mm 20-24"] with
radial cracks// i lost dozens and finally tracked the problem down to the
clay body//the claymaker [i was told by a reliable informant] had changed 1
of the ingredients of the body as it was no longer available//the claymaker
emphatically denied this citing my inexperience in firing large plates in a
fibre kiln [i had been doing it for around 15 years at that point] any how
the punch line is that i changed the clay body and miraculously the problem
dissapeared. I now use a raku body that is designed naturally enough to
withstand the rigours of that technique so it has a good resistance to
thermal shock//
it also happily easily fires to cone 13 with no distortion or slumping//it
throws really well and is not too course for table ware//it dries like a
dream. are you using a high silica body??? they usually have more problems
with cracking
good luck
At 03:33 20/11/97 EST, you wrote:
>----------------------------Original message----------------------------
>Hello,
> I hope you won't mind revisiting this subject briefly. I went
>through many hundreds of not very well orginized saved files and did find
>several posts on preventing cracks during firing of large platters. I
>learned that drying is important because uneven drying creates stresses in
>the clay that can show up as a crack later and that it is important to fire
>so that heat can evenly distribute and the center or edges are not hotter.
>One of you suggested sand (on the kiln shelf) to allow movement during
>firing.
> I remember that someone suggested bisque firing large platters on
>edge. Does anyone remember this hint? Why would you do it and how would
>it work? I have had several large shallow bowls and platters crack
>recently and I am ready to try any bisque firing technique that might work.
>Thanks.
> Pamela Jo (on the wet North Oregon coast) stamps@seasurf.com
>
>
raycarlt@valylink.net.au
Ray Carlton
McMahons Creek Victoria Australia 3799
Don Prey on fri 21 nov 97
Pamala Jo,
If you get Ceramics Monthly, it might pay to review the response to a similar
question in the Oct. 97 issue, page 96. Also, in my somewhat limited
experience with large platters, I have concluded that this problem is quite
"clay body specific". Some clay bodies are probably inherently wrong for this
application.
Don Prey in Oregon
Ron Roy on fri 21 nov 97
Hi Panela,
I assume the "dunting" is happening during the cooling of the bisque - It
is always the same reason - the free silica in the body goes through the
beta to alpha quartz inversion at about 573C - the beta quartz suddenly
gets smaller when it changes to alpha. If one part of a pot gets smaller
and the rest doesn't then something has to give if the clay isn't strong
enough. The platter rim gets smaller but because the mass of clay at the
foot, in contact with the kiln shelf, goes through the quartz inversion
later. The cure is slow the cooling of the kiln during that period - seal
up any leaks - close spy holes, leave an element on low - best of all - if
you have a controller - program a cooling ramp from 700C down to 500C. The
other solution is to bisque higher - say 04 - this soaks up more if the
quartz - turns more of it into amorphous silica - which does not go through
an inversion and makes the clay stronger. You will have to thicken up your
glazes a bit because the clay will also be less absorbent.
Bisque dunts - cracks - are always closed when you open the kiln - if you
could open the kiln when they are happening you would see them open. They
are closed because after quartz in the inner part of the pot goes through
the inversion the stress between the inner and outer clay is gone.
>----------------------------Original message----------------------------
>Hello,
> I hope you won't mind revisiting this subject briefly. I went
>through many hundreds of not very well orginized saved files and did find
>several posts on preventing cracks during firing of large platters. I
>learned that drying is important because uneven drying creates stresses in
>the clay that can show up as a crack later and that it is important to fire
>so that heat can evenly distribute and the center or edges are not hotter.
>One of you suggested sand (on the kiln shelf) to allow movement during
>firing.
> I remember that someone suggested bisque firing large platters on
>edge. Does anyone remember this hint? Why would you do it and how would
>it work? I have had several large shallow bowls and platters crack
>recently and I am ready to try any bisque firing technique that might work.
>Thanks.
> Pamela Jo (on the wet North Oregon coast) stamps@seasurf.com
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
Sue Hintz on sat 22 nov 97
I had this problem for a while and got some suggestions from everyone. Here is
what I now do. When drying, I cover my rims and let me platter sit on drywall
to dry. I bisque with the platter on its own shelf with nothing else on it.
I was trying to prevent my rims from coming up as they dry so I tried putting
weight on them but ended up with a crack all the way around the bottom edge.
I'm not having problems now. I do cut my platters off the bats and do this
about daily until the platter is dry enough to remove and put on the drywall.
Good luck.
WardBurner@aol.com on sat 22 nov 97
Hey Pamela,
One other trick that I used to use on large, thin lipped platters was to
support the platter off the shelf with several small pieces of soft brick.
This helps alleviate the uneven cooling and phase change rates that Ron
refered to. One caveat; be sure the soft brick feet are identical with no
rocking or you'll have a warped platter.
Marc Ward
Ward Burner Systems
PO Box 333
Dandridge, TN 37725
USA
423.397.2914 voice
423.397.1253 fax
wardburner@aol.com
Pierre Brayford on sat 22 nov 97
Firing platters on their rim works well, the first platter must be leant
against something - supporting it at an angle so that the rim on the
shelf (that is taking the weight) is vertical. Subsequent platters can
be leant against the first - until you run out of room, or platters, or
it becomes impossible to maintain the rims vertical.
You get more platters in the space, heat can circulate around each
platter, no problem with the kiln shelf keeping the centre of the
platter cool/hot, no problem with shelf restricting movement.
It won't work with bowls except the most shallow because you can't get
the rim vertical and it will bend.
--
Pierre Brayford
Old School Pottery, Edgton
Shropshire, U.K.
Pamela Jo Stamper wrote:
>
> ----------------------------Original message----------------------------
> Hello,
> I hope you won't mind revisiting this subject briefly. I went
> through many hundreds of not very well orginized saved files and did find
> several posts on preventing cracks during firing of large platters. I
> learned that drying is important because uneven drying creates stresses in
> the clay that can show up as a crack later and that it is important to fire
so that heat can evenly distribute and the center or edges are not
hotter.
> One of you suggested sand (on the kiln shelf) to allow movement during
> firing.
> I remember that someone suggested bisque firing large platters on
> edge. Does anyone remember this hint? Why would you do it and how would
> it work? I have had several large shallow bowls and platters crack
> recently and I am ready to try any bisque firing technique that might work.
> Thanks.
> Pamela Jo (on the wet North Oregon coast) stamps@seasurf.com
Sharon Armann on sat 22 nov 97
Pamela,
When I had this problem a year ago, someone suggested that
I place hard bricks, kiln shelf supports, or coils of dried
clay tightly around the outside rim and walls of the
platter when I bisqued. I opted for the hard bricks and
have not lost a platter since. The theory is that the rim
and body of the platter were cooling faster than the
foot and thus cracking. I fire to 06 in an electric kiln
and then to 10 in gas. I usually bisque 3 platters at a
time, or I make sure I have a full load of other pots to
bisque, so I have a lot of mass I am heating and cooling. I
fire the bisque slowly and cool slowly and I do put sand
under the foot in both the bisque and high fire. My
platters range from 17 to 20 inches in diameter. Its worth
a try. Good luck.
Sharon Armann
Phoenix, Az.
--------------------
Sharon Armann
SArmann@asu.edu
Carolynn Palmer on sun 23 nov 97
Recently, I had a firing of Bird Feeder tops that are shallow bowls about 10
inches in diameter, I glaze the one side only, and I fired these upside down
over a shelf support - I have made 1000's of these in the past without
incident - this time, I opened the electric kiln, after a cone 8 firing, and
every one had cracked exactly in two, but had not moved enough to even fall
off from the support.
This was right after I had looked at that article on in Ceramics Monthly with
the kiln load of large platters that had "dunted" - I had never heard that
word before, but it was like some kind of esp transference! I had never had
this problem before either.
And I cannot blame the clay company because I mix my own clay - maybe it was
the cooling down cycle in the kiln - I am glad to have another explanation of
why this happened.
Carolynn Palmer, Somerset Center, Michigan
Ron Roy on mon 24 nov 97
I feel an additional comment would be helpful here. All the kilns I have
ever used cool faster at the bottom. Keeping platters in the top half of a
kiln will slow the cooling and will help avoid dunting. Keeping rimes
thicker will help also.
I make sure my biggest plates are closest to the top in bisque firings.
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
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