Craig Martell on thu 31 aug 00
Hi:
Cristobalite doesn't start to develop until around cone 6 so it's not a
problem with cristobalite. I would think it's a quartz inversion dunt
caused by a larger piece inverting unevenly. If you raise the bisque temp
to 04 you will give the body more strength and the pitchers stand a better
chance of coming out whole. Your idea of downfiring will help to. Try one
cure at a time to isolate the cause. It may very well be underfired bisque.
later, Craig Martell in Oregon
Robert Santerre on thu 31 aug 00
Clayarters,
I've encountered a strange problem and I'd like to tap your collective expertise to solve it. I make tall pitchers (14"-16") with long, curving strap-like handles. You can see an example of one at this web site: http://www.silverhawk5.com/santerre/index.html. Recently I bisque-fired 8 of these to cone 06 - all nicely packed together on one level. The firing schedule was: dry for 8 hrs @ 200 F, fire to 700 F @ 100 degrees/ hr, fire to 1850 F @ 200 degrees/hr and then just shut the kiln off and cool at it's own rate. It's a small Duncan
kiln and cools fairly rapidly (down to 300 F in ~6-8 hrs). This is my normal firing schedule for all types of vessels. Never have any problems -except this time. The handles of 6 out of 8 of the pitchers blew off the body of the pitchers. The bodies of all the pitchers were unharmed suggesting to me that the explosion occurred after the pots had been fired rather than while firing up. The handles exploded into a mass of small fragments, none larger than 3 inches. Other vessels on upper shelves in the same firing were perfect.
My current theory is that during the cooling phase when these pots were passing (rather rapidly) through the crystal (cristobalite ?) phase change at ~650 F, a lot of tension was developed in the long handle versus the body of the pitcher (clearly the pitcher body would cool and shrink more slowly than the long, exposed handle) and KABOOM! Operating on that theory I'm planning to make a bunch more and THIS TIME I'll program the firing to fire down and go very slowly (maybe 50 degrees/hr) down through 800 - 500 degrees.
Help me please. Is this likely to remedy my problem or do you have a different theory and/or solution? I really don't want to change the pot. I want to learn how to make it - with a whole lot more success.
Thanks, Bob
rfsanterre@iquest.net
ferenc jakab on fri 1 sep 00
> My current theory is that during the cooling phase when these pots were
passing (rather rapidly) through the crystal (cristobalite ?) phase change
at ~650 F, a lot of tension was developed in the long handle versus the body
of the pitcher (clearly the pitcher body would cool and shrink more slowly
than the long, exposed handle) and KABOOM! Operating on that theory I'm
planning to make a bunch more and THIS TIME I'll program the firing to fire
down and go very slowly (maybe 50 degrees/hr) down through 800 - 500
degrees.
Robert,
You're theory may have some validity, but I suspect your drying cycle was
hotter or quicker than you think and moisture trapped between the body and
the handle was the culprit. Are you absolutely sure of the accuracy of your
controller?
Feri.
Tom Wirt/Betsy Price on fri 1 sep 00
> > My current theory is that during the cooling phase when these pots
were
> passing (rather rapidly) through the crystal (cristobalite ?) phase
change
> at ~650 F, a lot of tension was developed in the long handle versus
the body
> of the pitcher (clearly the pitcher body would cool and shrink more
slowly
> than the long, exposed handle) and KABOOM! Operating on that
theory I'm
> planning to make a bunch more and THIS TIME I'll program the firing
to fire
> down and go very slowly (maybe 50 degrees/hr) down through 800 - 500
> degrees.
Robert
If it is the phase change of silica that's causing the problem as the
pots cool, you need to slow down from about 1100 degF. to maybe 900
degF. The phase change is considered to take place at 1063 degF
(range 1022-1067deg). And it is possible the phase change is causing
it since the handle hanging out there is cooling much faster than the
body. Maybe just stacking so the handles are closer to more massive
pieces to hold some heat might help.
Tom Wirt
Ron Roy on fri 1 sep 00
Hi Bob,
Can't be cristobalite if they were only fired to 06 - won't happen unless
you are using hi talc bodies and even then not much.
I'll bet the farm they blew up at the beginning of the firing. Fast drying
in a kiln can be tricky - was the lid open or closed? If closed the water
vapor traped inside can prevent the drying.
Fast drying can close pores up and slow down drying. I have noticed - when
I have not used my electric kiln for a while - moisture can build up in the
bricks - takes longer to dry things out when that happens.
Test with something at a spy hole to see if there is still water vapor
present - a mirror or shiny metal - then proceed.
RR
>I've encountered a strange problem and I'd like to tap your collective
>expertise to solve it. I make tall pitchers (14"-16") with long, curving
>strap-like handles. You can see an example of one at this web site:
>http://www.silverhawk5.com/santerre/index.html. Recently I bisque-fired 8
>of these to cone 06 - all nicely packed together on one level. The firing
>schedule was: dry for 8 hrs @ 200 F, fire to 700 F @ 100 degrees/ hr,
>fire to 1850 F @ 200 degrees/hr and then just shut the kiln off and cool
>at it's own rate. It's a small Duncan
>kiln and cools fairly rapidly (down to 300 F in ~6-8 hrs). This is my
>normal firing schedule for all types of vessels. Never have any problems
>-except this time. The handles of 6 out of 8 of the pitchers blew off the
>body of the pitchers. The bodies of all the pitchers were unharmed
>suggesting to me that the explosion occurred after the pots had been fired
>rather than while firing up. The handles exploded into a mass of small
>fragments, none larger than 3 inches. Other vessels on upper shelves in
>the same firing were perfect.
>
>My current theory is that during the cooling phase when these pots were
>passing (rather rapidly) through the crystal (cristobalite ?) phase change
>at ~650 F, a lot of tension was developed in the long handle versus the
>body of the pitcher (clearly the pitcher body would cool and shrink more
>slowly than the long, exposed handle) and KABOOM! Operating on that
>theory I'm planning to make a bunch more and THIS TIME I'll program the
>firing to fire down and go very slowly (maybe 50 degrees/hr) down through
>800 - 500 degrees.
>
>Help me please. Is this likely to remedy my problem or do you have a
>different theory and/or solution? I really don't want to change the pot.
>I want to learn how to make it - with a whole lot more success.
>
>Thanks, Bob
>rfsanterre@iquest.net
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Louis Katz on sat 2 sep 00
Hi,
Two suspicions.
One is that your kiln may say it is at 200 degrees but is actually above that
temperature and the handles are blowing then.
Two is that they are blowing as your clay is loosing its chemical water. Someone
might be able to help with more exact guidelines, but I slow my electric bisques to
75 degrees F. an hour between 800 and 1125. I fire at 150 degrees per hour between
225 and 800 and at 200 degrees an hour above that. This is with nromal thickness
student work with a fairly open stoneware body.
When handles blow off the pot it is often from air trapped underneath them as they
are attached. Instead of pressing the handle straight on, Make contact with one
side and slowly roll the hand toward the other as you press it on the pot hopefully
pushing the air out as you go.
Usually this kind of expolsion leaves a rim of the handle on the pot and a crater
in the middle, looking a bit like a volcano.
Hope it helps
Louis
Ron Roy wrote:
> Hi Bob,
>
> Can't be cristobalite if they were only fired to 06 - won't happen unless
> you are using hi talc bodies and even then not much.
>
> I'll bet the farm they blew up at the beginning of the firing. Fast drying
> in a kiln can be tricky - was the lid open or closed? If closed the water
> vapor traped inside can prevent the drying.
>
> Fast drying can close pores up and slow down drying. I have noticed - when
> I have not used my electric kiln for a while - moisture can build up in the
> bricks - takes longer to dry things out when that happens.
>
> Test with something at a spy hole to see if there is still water vapor
> present - a mirror or shiny metal - then proceed.
>
> RR
>
> >I've encountered a strange problem and I'd like to tap your collective
> >expertise to solve it. I make tall pitchers (14"-16") with long, curving
> >strap-like handles. You can see an example of one at this web site:
> >http://www.silverhawk5.com/santerre/index.html. Recently I bisque-fired 8
> >of these to cone 06 - all nicely packed together on one level. The firing
> >schedule was: dry for 8 hrs @ 200 F, fire to 700 F @ 100 degrees/ hr,
> >fire to 1850 F @ 200 degrees/hr and then just shut the kiln off and cool
> >at it's own rate. It's a small Duncan
> >kiln and cools fairly rapidly (down to 300 F in ~6-8 hrs). This is my
> >normal firing schedule for all types of vessels. Never have any problems
> >-except this time. The handles of 6 out of 8 of the pitchers blew off the
> >body of the pitchers. The bodies of all the pitchers were unharmed
> >suggesting to me that the explosion occurred after the pots had been fired
> >rather than while firing up. The handles exploded into a mass of small
> >fragments, none larger than 3 inches. Other vessels on upper shelves in
> >the same firing were perfect.
> >
> >My current theory is that during the cooling phase when these pots were
> >passing (rather rapidly) through the crystal (cristobalite ?) phase change
> >at ~650 F, a lot of tension was developed in the long handle versus the
> >body of the pitcher (clearly the pitcher body would cool and shrink more
> >slowly than the long, exposed handle) and KABOOM! Operating on that
> >theory I'm planning to make a bunch more and THIS TIME I'll program the
> >firing to fire down and go very slowly (maybe 50 degrees/hr) down through
> >800 - 500 degrees.
> >
> >Help me please. Is this likely to remedy my problem or do you have a
> >different theory and/or solution? I really don't want to change the pot.
> >I want to learn how to make it - with a whole lot more success.
> >
> >Thanks, Bob
> >rfsanterre@iquest.net
> >
> >______________________________________________________________________________
> >Send postings to clayart@lsv.ceramics.org
> >
> >You may look at the archives for the list or change your subscription
> >settings from http://www.ceramics.org/clayart/
> >
> >Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
>
> Ron Roy
> 93 Pegasus Trail
> Scarborough
> Ontario, Canada
> M1G 3N8
> Evenings 416-439-2621
> Fax 416-438-7849
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Robert Santerre on sun 10 sep 00
A couple of weeks ago I sent out a plea for help with a bisque firing
problem (see below). I want to thank all who responded and especially
Bonine Hellman, Craig Martell, Dave Finkelnburg, Ferenc Jakab, Tom Wirt
and Ron Roy. The consensus of their thoughts was that (a) it couldn't
be cristobalite because that forms at much higher temperatures, (b) 4-2,
the handles were blowing up on the temperature rise because of trapped
water, (c) 1-5, it could be that the explosion happened on cooling
because of differential shrinkage between the handles and the pitcher
bodies. My added note: the handles that exploded were intact after the
8 hr drying period @200 F (I peeked, the kiln lid was partially open
during the entire drying time). Truthfully, these pitchers were not
dried on the shelf as long as I normally dry them - as usual, hurrying
the process a little to meet a show deadline - but they also were not
really wet or even cool to the touch, just not as fully shelf-dried as
normal (appx. 2 weeks).
One of the experts noted that trying to hurrying a drying process can
actually result in slowing things down because the rapid outside surface
drying actually blocks the migration and surface evaporation of water
deep in the middle of the structure. While this idea has merit, in this
particular case these pitcher handles are the most exposed part of the
vessel, the first area to dry. The base of the pitcher is much thicker
and always the last to dry. The bases didn't explode, the handles
did???????
OK, on the advice of the experts here's the firing schedule I tried (one
pitcher, fully shelf-dried, by itself, that's all I was willing to
sacrifice - once bitten , twice cautious):
8 hrs @ 190 F (kiln door partially open)
100 degrees/hr to 700 F
200 degrees/hr to 1850 F
200 degrees/hr down to 1000 F
75 degrees/hr down to 400 F
shut down
GOOD NEWS! The pitcher survived. In fact, we glaze fired and sold it
yesterday at the Penrod Society Art Fair here in Indianapolis, hopefully
gracing it's new owner's table for this Sunday's brunch.
So gurus, Clayarters, I humbly thank you all for furthering my
education in clay and rescuing me from a deep depression - losing 6 out
of 8 pitchers in a bisque firing is enough to drive ya to Prozac. This
has to be, hands down, the greatest support group in all of Artdom.
Bob
rfsanterre@iquest.net
///////////////////////////////////////////////////////////////////////////////////////////////////
Robert Santerre wrote:
> Clayarters,
>
> I've encountered a strange problem and I'd like to tap your collective
> expertise to solve it. I make tall pitchers (14"-16") with long,
> curving strap-like handles. You can see an example of one at this web
> site: http://www.silverhawk5.com/santerre/index.html. Recently I
> bisque-fired 8 of these to cone 06 - all nicely packed together on one
> level. The firing schedule was: dry for 8 hrs @ 200 F, fire to 700 F
> @ 100 degrees/ hr, fire to 1850 F @ 200 degrees/hr and then just shut
> the kiln off and cool at it's own rate. It's a small Duncan kiln and
> cools fairly rapidly (down to 300 F in ~6-8 hrs). This is my normal
> firing schedule for all types of vessels. Never have any problems
> -except this time. The handles of 6 out of 8 of the pitchers blew off
> the body of the pitchers. The bodies of all the pitchers were
> unharmed suggesting to me that the explosion occurred after the pots
> had been fired rather than while firing up. The handles exploded into
> a mass of small fragments, none larger than 3 inches. Other vessels
> on upper shelves in the same firing were perfect.
>
> My current theory is that during the cooling phase when these pots
> were passing (rather rapidly) through the crystal (cristobalite ?)
> phase change at ~650 F, a lot of tension was developed in the long
> handle versus the body of the pitcher (clearly the pitcher body would
> cool and shrink more slowly than the long, exposed handle) and
> KABOOM! Operating on that theory I'm planning to make a bunch more
> and THIS TIME I'll program the firing to fire down and go very slowly
> (maybe 50 degrees/hr) down through 800 - 500 degrees.
>
> Help me please. Is this likely to remedy my problem or do you have a
> different theory and/or solution? I really don't want to change the
> pot. I want to learn how to make it - with a whole lot more success.
>
> Thanks, Bob
> rfsanterre@iquest.net
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