Tom Wirt on mon 18 may 98
Re the blowing up of a refired and reglazed piece, I doubt that the blow up =
was
from moisture in the glaze. Stoneware would not soak up enough moisture to
explode. From your description of the blow up (shards throughout the kiln) =
I
would more suspect cristobalite was formed in the first firing (if you're =
going
to cone 8-10) and then when taken throught teh second firing, blew when you
passed the +/- 400 degree F mark. We had problems with this until Kyanite =
was
added to the clay body to prevent Cristobalite formation. Mullite, =
Pyrophyllite
will do the same.
If you're getting blowups like this on refire, better check out any oven =
ware
you make very carefully.
Hope this helps.
Tom Wirt
Clay Coyote Pottery
Hutchinson MN
claypot=40hutchtel.net
Ron Roy on tue 19 may 98
I am pretty sure we are talking about moisture in the clay responsible for
the blowup. Not all stoneware clays are tight enough to exclude water. This
has been discussed here before.
I have refired many pots which had plenty of cristobalite in them and all I
ever got was cracking.
It would not be difficult to design an experiment to prove it either way.
Just as a side note - I am looking at a dilatometer chart of a cone 10
stoneware clay - specifically at the increase in linear expansion due to
cristobalite. The curve starts up at 100C (212F) and levels out again after
220C (428F) - many of you assume this happens suddenly and at a higher
temperature - I would have if I had only read about it in books. I still
have trouble believing my eyes because none of the texts I use describe
what I see.
Some books describe the expansion of cristobalite as sudden and yet this is
not confirmed by my charts - ever. There are texts that describe the quartz
inversion as slow compared to the cristobalite inversion - on this (and all
such charts) the quartz inversion takes place over only 25C - in other
words - suddenly - compared to the cristobalite change.
I will be covering this subject at Cermatec - lots of graphs and explanations.
>----------------------------Original message----------------------------
>Re the blowing up of a refired and reglazed piece, I doubt that the blow up was
>from moisture in the glaze. Stoneware would not soak up enough moisture to
>explode. From your description of the blow up (shards throughout the kiln) I
>would more suspect cristobalite was formed in the first firing (if you're going
>to cone 8-10) and then when taken throught teh second firing, blew when you
>passed the +/- 400 degree F mark. We had problems with this until Kyanite was
>added to the clay body to prevent Cristobalite formation. Mullite,
>Pyrophyllite
>will do the same.
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
Vince Pitelka on tue 19 may 98
>Re the blowing up of a refired and reglazed piece, I doubt that the blow up was
>from moisture in the glaze. Stoneware would not soak up enough moisture to
>explode.
Actually, stoneware and porcelain can explode in spectacular fashion from
moisture absorbed into the clay and glaze. No claybodies we use in ceramics
have zero absorption, and while only a minute fraction of moisture can soak
into high-fired functional bodies, it is extremely difficult for that
moisture to escape when the temperature rises rapidly, as in a normal
firing. This is also the reason that high-fired functional bodies make
lousy claybodies for outdoor sculpture. Water invariably soaks into the
body, and when it freezes the water cannot escape easily, and the piece
disintegrates.
I have personally witnessed a number of frighteningly powerful explosions of
high-fire refires, and in all cases the cause was water soaking into the
body. In most cases it occurs when a student decides to refire a pot which
has been in daily use for food and/or beverages.
- 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
maggie j jones on wed 20 may 98
We had an amazing explosion when we tried to rescue a favored coffee mug
that had been dropped and neatly broke in 2 lengthwise. Freeman glazed
the 2 pieces together and balanced it on its side. Even with xtra preheat
we were surprised to find smitherines...nothing bigger than 1/4 inch
throughout the entire oval olympic...all levels..
Maggie Jones , Turtle Island Pottery, Black Mtn. NC
mrjj@juno.com
We do succeed in refiring something that just came out of the kiln with
an xtra preheat.
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Tom Wirt on thu 21 may 98
At great risk, I'm gioing to disagree with both Ron and Vince since they saw=
fit
to take exception with my comments re: Cristobalite explosion during refire.
When we first started building our gas kiln, we had been advised to save up
bisqued seconds to use as kiln filler for the first firing or two, putting =
in
just a few glazed pieces. That way you don't risk a whole load of finished
ware, something we were not in a positon to do.
The first firing went OK except we didin't reach temp after 15 hours. The
second try, with the same unglazed nfiller pots went the same. Then we =
added 4
feet oiof stack, loaded a third time with the same pots. NONE OF WHICH HAD =
BEEN
EXPOSED TO ANY MOISTURE, and fired the thirs time. Starting at about 350 =
deg F
on the pyrometer, and up to about 1000 deg. the muffled explosions in the =
kiln
were spectacular. ( I didn't stop because I was merely trying to see if we
could make temp). The range of the explosions supports Ron's measurements of
cristobalite expansion over a range of temp. I'd respecfully submit that it
could only have been cristobalite. Every pot that had been through 3 =
firings
blew. And the blow up was in small shards not the large cracks, (or a blown=
off
foot) that Ron described. Also, the sound of the blow ups was different =
from a
moisture explosion. Much sharper sound.
Again, if you lead this person to only believe that it was moisture in the =
pot
or glaze, you're dooming them to more ruined kiln loads since, as the =
original
post noted, the shards fill the entire kiln.
Tom Wirt
Clay Coyote Pottery
Hutchinson, MN
claypot=40hutchtel.net
Vince Pitelka on fri 22 may 98
>The first firing went OK except we didin't reach temp after 15 hours. The
>second try, with the same unglazed nfiller pots went the same. Then we added 4
>feet oiof stack, loaded a third time with the same pots. NONE OF WHICH HAD
BEEN EXPOSED TO ANY MOISTURE, and fired the thirs time. Starting at about
350 deg F
>on the pyrometer, and up to about 1000 deg. the muffled explosions in the kiln
>were spectacular. ( I didn't stop because I was merely trying to see if we
>could make temp). The range of the explosions supports Ron's measurements of
>cristobalite expansion over a range of temp. I'd respecfully submit that it
>could only have been cristobalite. Every pot that had been through 3 firings
>blew. And the blow up was in small shards not the large cracks, (or a
blown off
>foot) that Ron described. Also, the sound of the blow ups was different
from a
>moisture explosion. Much sharper sound.
Tom -
This seems like a compelling arguement in favor of the explosive power of
cristobalite shift. I am going to watch more carefully for it in the
future, but I must say that this is something I have never experienced in my
30 years in ceramics. As mentioned in my earlier post, I have seen some
spectacular explosions of wares due to moisture content, and I have
certainly seen lots of high-fire re-fires crack in the second or third
firing, but have never seen one explode from any other cause than moisture
content. And of course the problem of cracking in refiring high-fired wares
is not necessarily due to cristobalite. Bisque-fired wares are very porous
and have high thermal shock resistance, and therefore can be heated very
quickly. High-fired wares have little porosity and relatively low thermal
shock resistance (in comparison to bisque wares), and if placed near the
heat source in refiring are prone to crack early in the firing.
In the past I have on occasion re-fired high-fired pieces four, six, or
even eight times. One should avoid this, because there is there is little
advantage in it aesthetically, and almost certainly increasing cristobalite
with each re-fire (dependent on the available silica in clay and glaze).
But in my case it was for the sake of experiment. As long as I took these
pieces up to temperature gradually I had no problem. If I heated them too
fast, they cracked. I just have to go by my own experience. If there is
anyone else out there who has seen refired pots explode for reasons other
than moisture content, I would love to hear more about it.
- 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
Michael McDowell on sat 23 may 98
Tom,
I have only your report to go on here. But I'm supposing that your
observations and deductions are sound and that in the instance your pots
exploded after being refired three times it was indeed due to cristobalite
expansion. I do also have personal experience with such explosions that by
similar elimination of other possibilities would seem to confirm Vince's
suggestion that a similar occurrence in yet another situation, could have
been due to water trapped deep inside the body following use of a fired
piece to hold water, and then being refired. In my case there were other
refires in the same clay body that had had no use in between fires and
showed no problem. Fortunately I shut my kiln down after noticing the
explosions were occurring (blowing themselves to bits as you report). I was
able to unload, empty the kiln, other pots and burners tubes of shards and
then fired successfully.
This is not to deny your explanation of what happened in your circumstances.
Just to suggest to you that you are possibly wrong to generalize it to all
occurrences of explosions of refired pieces. Refiring works fine, sometimes.
Michael McDowell
Whatcom County, WA USA
http://www2.memes.com/mmpots
mmpots@memes.com
Bobbruch1@AOL.COM on wed 25 sep 02
One way to look at this, is that if you are unhappy with a pot and don't
consider it a first, then a refire could be like a large test tile. What do
you have to lose? You might even find some interesting effects might work
well for future work. There are some clay artists who routinely fire down
several times to create the surface effects that they desire.
There are several problems with refiring. One is getting the glaze to adhere
to the pot. There is material in the archives on that subject. Also, refires
may require slower firings than when firing glazed bisque work.
I have been using ash glazes at cone 6 and often find that my glaze sometimes
doesn't "take" on the entire piece. Refiring ash glazes is relatively easy,
since they seem to accept a second coating or even a new glaze overlaid on
top. I refire back to the same temperature that I fired to the first time. I
have also been experimenting with ash and other drippy glazes over the top of
already fired pieces. I am finding some differences between putting the
second glaze over the top of another glaze in the bisque state, and doing it
via a refire of an already glazed piece. Not all glazes will accept refiring
well, and not all glazes will work well with another glaze overlaid on top.
So, you are going to have to do some experimenting with tiles to find out
what works.
<<< >Question: If I refire to give it new life, do I need
to go to ^6 again? Do you think I could put it in with my ^06 bisque? I know
that my usual firings of ^10 - ^11 reduction could be bad news ..... and the
only other temp I use is the ^06. I'd appreciate any suggestions. I love
the lines of this pot...... round, strong,
simply stunning..... or stunningly simple .... it's part of the family. Joyce
Douglas Gorsline on sun 6 aug 06
One more question on refire...the clear glaze pcs were fired at cone 06 =
then refired to cone 6 and they appear milkey in spots. The milky spots =
came from the cone 06 fire and did not change with the hi fire.
Any suggestions...shame to lose the pc.
Thanks,
Doug Gorsline
thegors@earthlink.net
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