Frederich, Tim on thu 28 feb 02
Hello all,
The opinion used by the people at Orton is that if the cone comes within
approximately 300 degrees F of the end point of that cone, it should not be
reused. The heatwork that has taken place can affect the true end point of
the cone. It might take a higher temperature to finally bend that cone
which would result in an overfiring.
My opinion is that if the firing went beyond 1063 degrees F (quartz
inversion), I would replace all of the cones just as a safety factor. I
definately would replace the cone or bar in the kiln-sitter. Cones are not
that expensive ( I have to buy mine even though I work here) and I would
rather be safe than sorry.
Best regards,
Tim Frederich
Dave Finkelnburg on thu 28 feb 02
Tim,
If I understand this correctly, what you are saying is if the firing has
gone much past the quartz inversion it is not possible to tell the correct
endpoint from using your cones, period.
You say restarting the firing without changing cones will result in
overfiring.
Since some heatwork has accumulated, more the higher the firing went, it
seems logical to me that firing with a new cone will also guarantee some
overfiring of the ware.
Arrrrghh! Like most of pottery, I get a headache trying to contemplate
all of this. :-)
Dave Finkelnburg, hoping I don't have to deal with an interrupted
firing
From: Frederich, Tim
>The opinion used by the people at Orton is that if the cone comes within
>approximately 300 degrees F of the end point of that cone, it should not be
>reused. The heatwork that has taken place can affect the true end point of
>the cone. It might take a higher temperature to finally bend that cone
>which would result in an overfiring.
Gavin Stairs on fri 1 mar 02
Hi again,
I had a little think about what Tim reported concerning twice fired cones,
and I can understand the observation this way: As the cone begins to melt,
it absorbs energy called the latent heat of melting. In the first
instance, with a new cone, this occurs over an extended region of the
firing profile, and the perturbation of the local temperature is not
great. In the second instance, when the cone has melted somewhat in
previous firing, that region of the profile which is twice fired will not
melt the cone, since that region of heat work is already done, and the
resulting melt is higher melting point than the original material, so the
temperature is not perturbed at all. The cone is effectively bit ahead of
a normal firing in respect of its sensed temperature. However, when it
reaches the point at which it begins to melt again, the situation reverses,
as the cone must absorb all the latent heat that it did not absorb on the
way up, in order to remelt the material that was melted before. So the
local temperature is heavily perturbed, and the cone stalls at that
point. If the firing continues, the previously melted material will all
melt, and the cone will revert to its normal behaviour, but will still take
some time to catch up. If the previous firing got too close to the end
point, the cone will not have time to recover, and the result will be an
under fired cone, or an over fired load.
The trouble with this is that the load is also going through the same
process, but with different end point and different critical
temperatures. Consider a bisque load. The bisque is not going to vitrify
nearly as much as the cone, and its local temperature perturbations will be
much less than the cone. So the bisque load will indeed overfire, as it
will be dominated by the ordinary heating load (little latent heat) and
will see the cone as stalled. However, consider a glaze load. The
vitrification process must proceed well beyond that of the cone, to the
puddle stage. So the glaze will be experiencing the same stalling
described above, though not necessarily before the cone, since the fluxing
system is probably completely different. From the point of view of simple
kinetics, the glaze is probably going to stall more than the cone, but the
glaze is sitting on a heat mass, the pot, which will mitigate the effect
somewhat. Except that the pot is also vitrifying, since it is going well
past the bisque stage. The pot is in fact undergoing the same stalling as
the cone, but again, not necessarily with the same critical points and
kinetics. As a result, it may turn out that the pots and glaze are before
or after the cones, or even right on, depending on a lot of factors.
So Orton says replace the cone pack and fire as usual. This will probably
work well with bisque. But with a glaze firing, I wouldn't be so
confident. If the firing stopped early, either the old or the new should
work well. If the firing stopped late, the old cone pack may well work
better than a new one, but that is not certain. It depends on too many
factors to predict. The glaze load will be before the new cones and with
the old cones for all the firing up to the previous stopping point, and
will then begin to fall back, along with the old cones, while the new cones
carry on as before. If the firing proceeds long enough, the old cones will
fall behind the new ones, and the glaze may do so as well. Then both will
begin to catch up with the new cones, but may never recover completely. So
both outcomes, over and under fire, are possible. Possibly the glaze will
follow the old cones better than the new ones. I don't know.
Sorry to go on about this. The problem intrigues me, even though what I
have contributed probably doesn't greatly help someone in the fix. If I
were confronted with a late firing failure, I'd probably replace the cones
in a bisque firing, and maybe not in a glaze firing. Probably replace the
cones in an earthenware glaze firing, and not in a porcelain glaze
firing. Stoneware, somewhat of a toss up. Maybe replace in mid fire, not
in high fire, or depending on the end point density expected. Replace if
dense, not if not. Helpful, huh?
Gavin
At 08:20 PM 28/02/2002 -0700, you wrote:
>Tim,
> If I understand this correctly, what you are saying is if the firing has
>gone much past the quartz inversion it is not possible to tell the correct
>endpoint from using your cones, period.
Frederich, Tim on fri 1 mar 02
Dave and anybody else who is interested,
I would not worry about interrupted firings. Almost any load can be refired
without any terrible side effects based on my experience over 35 years. One
must use common sense when refiring. If the kiln has not been off for very
long, it can probably be restarted without any problem. If it has been off
for a longer period of time, I would probably let it cool to a low
temperature and then restart using my normal firing schedule. I stated in
my earlier post that I would probably change my cones just to be safe
depending on how high the kiln had reached in temperature.
As Gavin stated in his post most of the heatwork takes place toward the
final part of the firing. The temperature swing of a few degrees one way or
the other probably will not make much difference in your bisque or glaze
firing. Most kilns have a larger difference top to bottom and front to back
than what you might see between 2 different sets of cones, one from an
interrupted firing and one not.
Dave, don't develop a headache. We are not dealing with something that has
to be that exact. We have variables that we deal with every day. We just
need to maintain some balance of control to the best of our ability.
Just keep making pots.
Best regards,
Tim Frederich
vince pitelka on fri 1 mar 02
> I would not worry about interrupted firings. Almost any load can be
refired
> without any terrible side effects based on my experience over 35 years.
One
> must use common sense when refiring. If the kiln has not been off for
very
> long, it can probably be restarted without any problem. If it has been
off
> for a longer period of time, I would probably let it cool to a low
> temperature and then restart using my normal firing schedule. I stated in
> my earlier post that I would probably change my cones just to be safe
> depending on how high the kiln had reached in temperature.
Tim and others -
This is not always true. If a firing is interrupted just as it reaches red
heat, the glaze has begun to sinter, and yet has not established a strong
bond with the claybody. If the firing is interrupted at that point, the
glaze will sometimes crack upon cooling, and in refiring it can peel away
from the claybody in big flakes, falling on the kiln shelves. This can be a
real disaster, and can ruin an entire kilnload of wares. Whenever a firing
is interrupted, if you can catch it before it drops below red heat, ALWAYS
turn it back on and complete the firing.
Sometimes sloppily-made cone packs will explode in a gas kiln, but are not
noticed until the kiln reaches red heat. In that event, it is far better to
continue to fire the kiln using a pyrometer, and take it up to bisque
temperatures (cone 08) and then shut it off and cool it in order to place
new cone packs. At that temperature, the clay/glaze bond is strong enough
to prevent flaking and separation. Don't ever stop a firing below that
temperature if you can avoid it.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
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