Alisa and Claus Clausen on sun 23 apr 00
------------------
What have I done now?
I have sucessfully fired my old kiln Dagny for several bisques now and a =
handful
of glazes.
I am using cone 6 clay and glazes.
The kiln is set to fire with a electronic controller.
My glaze firing takes ca. 14 hours to fire to 1250c.
Results of last firing.
The kiln had a mechanical failure at 800c.
It shutoff and cooled to 600c.
I thought that there was an interruption in the power against my resident
electrician's view,
and I ran the firing program again (from 600c) and it reached only ca. =
1000c.
I let it cool down and the wiring was repaired.
The kiln was fired up again and reached the 1250c in an appropriate amount =
of
hours.
At least 60 precent of the pots are full of bloats and pimples =EDn the =
claybody.
The glazes are fine.
What happened?
I have these three things I did that were maybe the culprits but I do not =
know
and would like to know what
the fault could be:
1. I used waste clay, that was a mixture of three different types of slop =
from
all the clays I have been
using to find one that fits my preference.
All were cone 6, but three different bodies.
2. The kiln was fired up twice.
3. I stacked the kiln very high and tight, with the last shelf nearly at the
very top of the top vent.
I probably made a mistake to think that I could put these clay bodies =
together,
thinking that they are all cone 6,
so there should not be problem. Even if they are all designed for the same
temp. can a bloating problem be created
by mixing different (same temp.) bodies together?
With an electronic controller, can the kiln after the soak and shut off =
actually
reach a higher temp. than the shut off
temp. due to compact stacking?
If the kiln was overfired (do not know if it can with the controller) =
wouldn't
the glazes also show signs of being overfired?
I would like to know in a basic way why clay bodies of the same firing range
cannot or should not be mixed together?
Does it also alter the clay's ability to become vitrified? I have marbled
different clays and thrown them with no problem.
These clays were really mixed together from slop so were actually made into =
a
new body consisting of the old 3 bodies.
What have I done? Do not want to do this again. Have to be more careful =
with
experiements.
I have decided on one clay body now so I will not have different sorts of =
slop
in the future to mix up.
I am puzzled that the clay body is looking overfired but not the glazes. Or=
can
bloating be the effect of the mixed
clays despite the kiln reaching the correct temp.?
Best regards,
Alisa in Denmark
Cindy Strnad on mon 24 apr 00
Alisa,
Glazes tend to be a bit more forgiving of overfiring than claybodies are.
They may run off, or lose color, but glazes can usually be fired higher than
their designated cone, in my experience.
I'm no expert, but if you truly mixed only ^6 bodies, I doubt that was your
problem. If I'm wrong, I'll gladly concede the point.
It's never a good idea to pack your kiln too tightly, but this generally
results in lower, rather than higher firing temperatures, and slower
firings.
Fourteen hours is a long time to take getting to ^6. Is your kiln capable of
reaching temperature faster, and you just fire slowly because you want to?
Mine used to take that long and longer and still never quite make
temperature, but this is not a normal or acceptable situation. Slow firing
can make some beautiful pottery, but it should be a choice rather than a
necessity.
I suspect that the pre-firing did more heat-work than you realized, and that
your clay body may have reached more like the equivalent of ^7 or even ^8.
Many ^6 glazes can take this in their stride, but a true ^6 body won't like
it at all.
Finally, did you use a witness cone? It's possible that your kiln reached a
higher temperature than the thermocouple was "willing" to admit. These
gadgets are wonderful, great, excellent tools to have, but you can't take
them too seriously.
Cindy Strnad
earthenv@gwtc.net
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
Paul Taylor on mon 24 apr 00
------------------
Dear Alisha
My understanding is that cone six bends at just below 1220 centigrade,
but maybe you are using a different cone manufacturer.
Some bodies do not mix well but most should. The iron in the clays
can cause them to collapse or vitrify very suddenly and a bit low fired
porcelain thrown in with it will also not help.
Firing twice to biscuit should inhibit bloating not encourage it but
if your glaze fluxed over a bit in the failed firing, especially at the
glaze body interface, that could cause bloating.
Now =21 My pyrometer has lied about its temperature since birth - 100.=
I
make allowances and I have a suspicion that your pyrometer could have been
doing the same. However when you had the repairs the shock to its system
resulted in an honest reading and thus your stuff is over fired.
I do not know what glazes you are using if they are matt or
crystalline glazes. If they appear clearer and less matt that is a sign of
over firing. If your clear glaze has any sign of crazing it is also an
indication of over firing. If the clay that you know is a warmer color than
usual that is also a sign of over firing.
So many apparent faults in pottery are an accumulation of slightly
broken rules. Taking these chances is all right, because they make little
difference by them selves, but taken all together they make for a fault
which is difficult to pin down.
Regards Paul Taylor
----------
=3EFrom: Alisa and Claus Clausen =3Caliskin=40mail.dk=3E
=3ETo: CLAYART=40LSV.UKY.EDU
=3ESubject: Toil and kiln trouble
=3EDate: Mon, Apr 24, 2000, 1:33 am
=3E
=3E----------------------------Original message----------------------------
=3E------------------
=3EWhat have I done now?
=3E
=3EI have sucessfully fired my old kiln Dagny for several bisques now and a
handful
=3Eof glazes.
=3E
=3EI am using cone 6 clay and glazes.
=3E
=3EThe kiln is set to fire with a electronic controller.
=3EMy glaze firing takes ca. 14 hours to fire to 1250c.
=3E
=3EResults of last firing.
=3EThe kiln had a mechanical failure at 800c.
=3EIt shutoff and cooled to 600c.
=3EI thought that there was an interruption in the power against my resident
=3Eelectrician's view,
=3Eand I ran the firing program again (from 600c) and it reached only ca. =
1000c.
=3E
=3EI let it cool down and the wiring was repaired.
=3EThe kiln was fired up again and reached the 1250c in an appropriate =
amount of
=3Ehours.
=3E
=3EAt least 60 precent of the pots are full of bloats and pimples =EDn the =
claybody.
=3EThe glazes are fine.
=3E
=3EWhat happened?
=3E
=3EI have these three things I did that were maybe the culprits but I do not=
know
=3Eand would like to know what
=3Ethe fault could be:
=3E
=3E1. I used waste clay, that was a mixture of three different types of slop=
from
=3Eall the clays I have been
=3Eusing to find one that fits my preference.
=3EAll were cone 6, but three different bodies.
=3E
=3E2. The kiln was fired up twice.
=3E
=3E3. I stacked the kiln very high and tight, with the last shelf nearly at =
the
=3Every top of the top vent.
=3E
=3E
=3E
=3EI probably made a mistake to think that I could put these clay bodies =
together,
=3Ethinking that they are all cone 6,
=3Eso there should not be problem. Even if they are all designed for the =
same
=3Etemp. can a bloating problem be created
=3Eby mixing different (same temp.) bodies together?
=3E
=3EWith an electronic controller, can the kiln after the soak and shut off
actually
=3Ereach a higher temp. than the shut off
=3Etemp. due to compact stacking?
=3E
=3EIf the kiln was overfired (do not know if it can with the controller) =
wouldn't
=3Ethe glazes also show signs of being overfired?
=3E
=3EI would like to know in a basic way why clay bodies of the same firing =
range
=3Ecannot or should not be mixed together?
=3EDoes it also alter the clay's ability to become vitrified? I have =
marbled
=3Edifferent clays and thrown them with no problem.
=3EThese clays were really mixed together from slop so were actually made =
into a
=3Enew body consisting of the old 3 bodies.
=3E
=3EWhat have I done? Do not want to do this again. Have to be more =
careful with
=3Eexperiements.
=3EI have decided on one clay body now so I will not have different sorts of=
slop
=3Ein the future to mix up.
=3E
=3EI am puzzled that the clay body is looking overfired but not the glazes. =
Or
can
=3Ebloating be the effect of the mixed
=3Eclays despite the kiln reaching the correct temp.?
=3E
=3EBest regards,
=3EAlisa in Denmark
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