Chris Campbell on thu 19 nov 09
Why try a down fire when there is no glaze?
My work is very thin ... paper thin sometimes.
I was getting a lot of cracking and warping in
each load which is money down the drain.
It is also discouraging since I put so much
time into each piece.
I have been slow cooling my kiln as per
Ron & John's suggestions for years now
with encouraging results.
Since I high fire, I was mostly guessing
at the rates and temps using their Cone 6
profiles as a guide.
Carol Marians e-mailed me off list with
information on how down firing might
affect quartz particle size in porcelain ...
she kindly proposed a schedule for down
firing my electric kiln for a load of
unglazed porcelain. I used this profile as
closely as I could.
I spent yesterday cleaning and sanding the
fired work so I had a chance to examine it
all closely.
The only cracks were ones I knew would
happen ... weak areas I had spotted pre firing.
I had repaired some with paper porcelain both
at the greenware and bisque stages and those
mostly held through the firing.
The warping was reduced and less severe ...
I guess you could say it looks charming rather
than awful.
The colors are very bright and crisp. I compared
the same colors with an earlier piece and the
difference is easily visible.
I am now convinced that there is more stress on
the way down than there is at the top and will
try to standardize my program for the slowest
cooling to 1100 F.
I want to thank Carol for her help with this as
my brain does not run down those highly
technical tracks!
Chris Campbell - in North Carolina
Chris Campbell Pottery LLC
Designs in Colored Porcelain
www.ccpottery.com
http://neriagechris.blogspot.com/
PRE-NCECA WEEKEND WORKSHOP
March 26, 27 & 28, 2010
Creative Ways With Colored Clays
Clay Art Center
40 Beech Street
Port Chester, NY 10573
914.937.2047 x222http://www.clayartcenter.org
Ron Roy on thu 19 nov 09
Hi Chris,
Sounds like it's the quartz inversion at 573C that is doing the damage -
unmelted quartz in the body reduces in size abruptly at 573C - so rims try
to get smaller but any clay in contact with a hot shelf stays the same -
cracks are typically at the rim.
The solution is to fire down from 700C (1300F) down to 500C (900F) - try 75=
C
per hour. or find a way to get your ware off the shelf - hard to do if the
porcelain is getting soft at top temp.
RR
On Thu, Nov 19, 2009 at 11:09 AM, Chris Campbell w=
rote:
> Why try a down fire when there is no glaze?
>
> My work is very thin ... paper thin sometimes.
> I was getting a lot of cracking and warping in
> each load which is money down the drain.
> It is also discouraging since I put so much
> time into each piece.
>
> I have been slow cooling my kiln as per
> Ron & John's suggestions for years now
> with encouraging results.
> Since I high fire, I was mostly guessing
> at the rates and temps using their Cone 6
> profiles as a guide.
>
> Carol Marians e-mailed me off list with
> information on how down firing might
> affect quartz particle size in porcelain ...
>
> she kindly proposed a schedule for down
> firing my electric kiln for a load of
> unglazed porcelain. I used this profile as
> closely as I could.
>
> I spent yesterday cleaning and sanding the
> fired work so I had a chance to examine it
> all closely.
>
> The only cracks were ones I knew would
> happen ... weak areas I had spotted pre firing.
> I had repaired some with paper porcelain both
> at the greenware and bisque stages and those
> mostly held through the firing.
>
> The warping was reduced and less severe ...
> I guess you could say it looks charming rather
> than awful.
>
> The colors are very bright and crisp. I compared
> the same colors with an earlier piece and the
> difference is easily visible.
>
> I am now convinced that there is more stress on
> the way down than there is at the top and will
> try to standardize my program for the slowest
> cooling to 1100 F.
>
> I want to thank Carol for her help with this as
> my brain does not run down those highly
> technical tracks!
>
> Chris Campbell - in North Carolina
>
> Chris Campbell Pottery LLC
> Designs in Colored Porcelain
> www.ccpottery.com
> http://neriagechris.blogspot.com/
>
>
> PRE-NCECA WEEKEND WORKSHOP
> March 26, 27 & 28, 2010
> Creative Ways With Colored Clays
> Clay Art Center
>
> 40 Beech Street
> Port Chester, NY 10573
> 914.937.2047 x222http://www.clayartcenter.org
>
--
Ron Roy
15084 Little Lake Road
Brighton, Ontario, Canada
K0K 1H0
Neon-Cat on thu 19 nov 09
Snail wrote: "I'm surprised that there is such a notable difference in
the colors, though - wouldn't have expected that. Any ideas why?
(Neon-Cat?)"
Thanks but no thanks, Snail -- I just got a new roof today (yes!!) and
am working on getting some heat in before a blue norther hits. Kitten
adoptions are up which makes me overall more optimistic about our
economy and art sales. I'm off on clay body research anyway although
after Chris' post I did review porcelain and cracking. There is just
too much unknown about Chris' deal for me to guess about crack
generation or color. I'll think on it in general as I construct and
test my native Texas porcelain. I'm of the mind-set that slow firing
is just as important as slow cooling or down firing...
I can celebrate Chris' more successful firing - that's great!
Marian
Neon-Cat
Snail Scott on thu 19 nov 09
On Nov 19, 2009, at 10:09 AM, Chris Campbell wrote:
> Why try a down fire when there is no glaze?
> I have been slow cooling my kiln...
> The warping was reduced and less severe ...
> The colors are very bright and crisp. I compared
> the same colors with an earlier piece and the
> difference is easily visible...
Good to know! I suppose that with a highly vitrified
body like yours, a slow cooling acts like annealing
does for glass - equalizing the stresses.
I'm surprised that there is such a notable difference
in the colors, though - wouldn't have expected that.
Any ideas why? (Neon-Cat?)
-Snail
Lee Love on thu 19 nov 09
On Thu, Nov 19, 2009 at 10:09 AM, Chris Campbell
wrote:
> Why try a down fire when there is no glaze?
Chris, how much silica in your porcelain? If you are slow
cooling a high silica body at higher temps, you will form
cristobalite. Euan took up crash cooling to 1100*C with his hanjiki
(half porcelain) to reduce quartz inversion cracking and the formation
of cristobalite.
--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/
"Ta tIr na n-=3DF3g ar chul an tI=3D97tIr dlainn trina ch=3DE9ile"=3D97tha=
t is, "T=3D
he
land of eternal youth is behind the house, a beautiful land fluent
within itself." -- John O'Donohue
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