This the second in a series of tips on firing your kiln. Whether you
are new to firing, or experienced, you are sure to find something that
will improve your firing results.
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Tip #49 Loading and Firing kilns: Part 2, Firing the Electric Kiln
Today we will talk about firing the kiln, both manual kilns (with Kiln
Sitters) and electronic kilns.
A. First some general firing tips:
* The worst thing you can do is fire low fire clay or glazes at high
fire. The clay and/or glaze will melt all over your kiln and can cause
major damage! If you have both low fire and high fire materials in your
studio, you might want to mark your pieces differently on the bottom so
you don't forget and get them mixed up!
* As mentioned in the previous tip, we recommend using witness cones in
the kiln even if you have an electronic controller or kiln sitter. This
will show you the Cone that was actually reached inside the kiln where
it matters.
* You should wear dark glasses with UV protection when looking into a
hot kiln.
B. There are 3 common ways to fire an electric kiln.
1. By manually turning the kiln on and up, and watching the cones inside
the kiln through a peephole to determine when to turn the kiln off.
2. By manually turning the kiln on and up, and using jr cones in a kiln
sitter to turn off the kiln when it reaches temperature.
3. By programming an electronic controller to turn the kiln on, up, and
off when appropriate.
In order to address these different firing types, wee will quickly
review how a kiln sitter works, and how an electronic controller works.
KILN SITTER
A kiln sitter works as follows. A jr cone of the appropriate number is
set inside the kiln sitter box. The cone is held by a retaining bar and
a moving rod. When the cone bends (because it has absorbed the correct
amount of heat), the rod falls. The rod activates the control mechanism
which turns off the current to the coils.
Tips:
* Since a kiln sitter works by gravity, it is important for the kiln to
be level.
* Kiln sitters can drift and need to be calibrated periodically. We will
discuss this more in a later tip.
* People often find that they need to put a slightly higher cone number
in the kiln sitter to get the kiln to fire to the correct temperature.
For example, to achieve a cone 6 inside the kiln, they must use a cone 7
in the kiln sitter. You will experiment with this for your individual
kiln. The firing log will assist you.
* Note that a kiln sitter does not assist in turning up the kiln, just
in turning it off.
* A little kiln wash dabbed on the top of new cone supports greatly
reduces the chance of a cone sticking to the new metal (which may cause
your kiln to over or under fire)
ELECTRONIC CONTROLLER
An electronic controller turns up the kiln, and also turns off the kiln.
It can even be used to do a controlled cool down. You can use a
pre-programmed Cone Fire mode, or program your own individual segments.
The pre-programmed modes automatically turn the kiln up when it is safe
to fire the pieces quickly, and down when the clay is at a point where
it should be fired slowly. To determine when to turn the kiln off, the
controller uses temperature charts to approximate when the appropriate
heat work is done (a certain cone level is reached.) This can vary
somewhat based on things such as the density of the load being fired. So
it is still important to monitor your firings at least periodically by
using witness cones inside the kiln. You will learn whether you need to
make adjustments to achieve the desired cone.
Tip:
Some people wonder if it is ok to fire a kiln during very cold weather.
It is, but Skutt in particular recommends warming the controller (if you
have one) to at least 40 degrees F with a space heater or hair dryer.
Your kiln will have to work a little longer to get to temperature.
C. BISQUE FIRING
For a bisque fire in particular, you need to drive off the water that is
left in the pot. If you fire too fast, the steam will cause the piece to
explode. (This is true even if the piece is very dry, because there is
still moisture inside the clay molecules.) So it is important to fire
bisque slowly.
If your pieces are not completely dry, you may want to candle them
first. Candling is done on a manual kiln by turning the bottom switch on
low and holding it there for several hours (6-10). With an electronic
kiln, you would program the kiln to remain at around 150 degrees F for
this time.
From there, the kiln is slowly turned up. You can read Tip
31 to remind yourself about the various stages clay goes through and
the critical temperatures to watch out for.
This is a typical firing schedule for a bisque firing in a manual kiln.
Bottom switch on low for several hours if necessary (candling).
Turn on all switches to low for 3-4 hours.
Turn all switches to medium for 3-4 hours.
Turn all switches to high until kiln has reached temperature.
(Note, if your kiln has multiple sections, you may turn them up
individually if you want slower heating.)
In contrast, this is the profile used by the Skutt electronic controller
for Slow, Cone 04.
Segment 1: 80 degrees / hour to 250 degrees F
Segment 2: 250 degrees / hour to 1000 degrees F
Segment 3: 150 degrees / hour to 1300 degrees F
Segment 4: 180 degrees / hour to 1685 degrees F
Segment 5: 80 degrees / hour to 1928 degrees F.
When you are firing bisque, it is very important that the steam have a
way to escape. If you are firing with a kiln vent, the moisture can
escape. If you don't have a vent, you must prop the lid open a few
inches (with a kiln brick or similar item) during candling and the first
few hours of firing. Usually the upper peep hole plug is also removed
during this time. After this time the kiln lid is closed. The top peep
hole plug remains out during the firing.
Tips:
* You may want to read Tip #39 on Choosing
a Bisque Temperature.
* Some people leave all the peep hole plugs out during the early stages
of firing. Skutt advises against this, saying that having multiple plugs
open creates a strong convection "jet-draft" which can easily
fracture ware and chill the cones in the kiln. Check your manual, and
experiment.
* Don't open the kiln until it is below 150-250 degrees F, or thermal
shock may hurt the ware and/or the kiln elements. You should be able to
touch the pieces before you unload them.
* It almost never hurts to fire a kiln slower rather than faster. (The
exception is some glazes that will look better if fired fast.)
D. GLAZE FIRING
* Glaze firings can generally be faster than bisque firings, because
most of the water has already been driven out of the clay. Some glazes
will look better when fired fast, and some when fired slow. This
requires experimentation. If unsure, start with slow firing.
* As in bisque firing, the kiln lid should be propped for the first few
hours, or until the kiln reaches 1000 degrees F. In addition the top
peep hole plug stay open during the whole firing. (These steps are both
unnecessary if firing with a vent).
* Mid to High fire glazes often look better if they are cooled slowly.
For this reason 3" brick is preferable for high firing. However, it
is possible to slow down the cooling by "firing down". With a
manual kiln, when you would normally turn the kiln off, instead turn the
switches down to medium. With an electronic kiln, you will want to
program this ahead of time. As an example, your last segments could
allow rapid cooling to 1950 degrees F, a 30 minute hold at that
temperature, then slow cooling at a rate of 150 degrees per hour down to
1100 degrees F. At that point the kiln would turn off.
* Having a soak (or holding the temperature) can be very useful at the
end of firing. A soak may last from 15 minutes to an hour or more. This
helps even out the temperatures throughout the kiln, and ensure all the
pieces have achieved the right temperature. This is particularly useful
if the kiln is densely packed. Soaking for too long can overfire ware,
so this must be taken into account.
* If something happens to stop the firing early, such as a power
failure, you can simply restart the kiln. If using cones, they will
continue to absorb heat and will still fall at approximately the correct
temperature. With an electronic kiln, the results will also be close
unless the kiln has shut off during the final hour or two of firing.
This is because most of the heat work happens during that time. If the
kiln shuts off toward the very end of firing, you should look at your
witness cones to determine when to turn the kiln off.
* If you are having trouble getting your kiln to reach temperature, or
the firing is taking extra long, the first thing to check is the power.
If your kiln is too far from the breaker box, you may be getting voltage
drops. Or if it is a hot, summer day when everyone is running their air
conditioning, the voltage on your line is probably low.
E. SAFETY
* You should be near the kiln while it is firing, especially toward the
end to make sure it goes off on schedule.
* A limit timer is a safety device which is set to turn the kiln off
after an amount of time that you set. For example, if you expect your
firing to last 8 hours, you may set the limit timer to 10 hours. At 10
hours the kiln will turn off. This can prevent a major catastrophe if
the kiln sitter or electronic controller fails.
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Copyright 2003 Cindi
Anderson, BigCeramicStore.com May be reprinted if credit is
given.
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