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Loading and Firing Kilns: Part 2, Firing the Electric Kiln
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.
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.
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.
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.
By programming an electronic controller to turn the kiln on, up, and off when appropriate.
In order to address these different firing types, we will quickly review how a kiln sitter works, and how an electronic controller works.
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.
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).
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.
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 peephole plug is also removed during this time. After this time the kiln lid is closed. The top peephole plug remains out during the firing.
You may want to read our tip 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 peephole 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.
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.
Copyright 2003 Cindi Anderson, www.bigceramicstore.com. May be reprinted if credit is given.