Working with Clay 4: Bisque Firing
Kinds of Kilns
Kilns typically come in two varieties, barrel-shaped kilns and cabinet-shaped kilns. An important question with kilns is knowing when an appropriate temperature has been reached. There are two common methods:
- Kiln Sitter - a mechanical switch that turns off the kiln when a pyrometric cone melts. Kilns with this type of sitter may or may not have a thermostat to display the current interior temperature.
- Electronic Controls - a programmable processor and thermocouple to control the temperature of the kiln and how fast it heats and cools. Most (if not all) FCPS kilns use this method.
The Chemistry of Clay in a Kiln
Before firing, water evaporates from the clay, shrinking it. But there is still water present on a chemical level, combined with the alumina and silica. This water doesn’t evaporate until it is fired 1022F/550C. At this point, the vessel will shrink further. So, clay vessels shrink in two stages. Because the vessel shrinks, there must be a passage for air to escape, because hot air expands as the clay shrinks. If you make an enclosed shape, or if there are air bubbles in the clay (if you forget to wedge your clay), firing will cause your vessel to burst and explode, hurting not just your piece, but all the other pieces in the kiln.
Dunting is when two pieces of different thicknesses are fused together, and pull apart in the firing process, leading to large cracks and an unstable weak structure.
Recycled Clay - mixing different recycled clays can sometimes cause problems in firing. Any time you recycle clay, it's good to mix it with fresh clay, and don't lose the fine silt - it's crucial to the firing process.
Kiln Safety:
- Be sure not to set any inflammable objects within 3 feet of your kiln. You always want the area around your kiln clear. If, during a firing, your kiln sets anything on fire, don't go near it, call for help.
- Never open a kiln while it is firing! Allow a kiln to cool fully before opening the door (the process takes a day). The kiln should show that its temperature is below 125°F before it is safe to open.
- Use thermal gloves to handle hot items and IR and UV protective glasses to look in peep holes.
- If your kiln is not working properly, do not try to use it. Contact the customer help line for that make and model.
- Make sure the exhaust fan is operating and the kiln room door is closed during a firing.
- Do not allow students in a kiln room while it is firing.
- Never leave a kiln unattended. You must be present (in the building/area) while it is firing. Do not set a kiln to operate overnight. Best is to start a firing in the morning before school, so that it begins its cooldown process before you leave in the afternoon.
- Every kiln has a circuit breaker switch. You can flip it off during long school breaks (summer & winter). When you turn it on again, it's normal for the kiln to make a beeping sound.
Kiln Maintenance
- Before you load your kiln, check to see that it is clean and free of debris. If you have any clay dust or bits in the sides of your kiln (on or around the metal coils) vacuum them away as best you can before loading and firing. It's good to vacuum out your kiln at least once a week, it will add to the life of your kiln and coils (which cost hundreds to replace).
- Be sure never to touch the coils with either your hands or the vacuum. Your skin produces oils that are harmful to the coils.
- Kiln shelves and stilts cost hundreds of dollars - be careful not to break them.
- You can also scrape your shelves periodically - if you see any spots or paint.
- If your shelves start flaking off their white color, they will need a "rewashing". Kiln shelves are coated in a special kind of paint called kiln wash - it's a white powder. You mix it with water and apply it to the shelves, letting it dry fully. If the coat is thin, add a second.
- Kilns have a thermocouple. It's a temperature measuring device. Never touch it or bump into it.
NOTE: Be sure to wear a mask before cleaning your kiln. You don't want to breathe in clay dust!
Loading a Kiln
- Plan to fully load a kiln before firing. It's more energy and time efficient and yields better results.
- Only use specially made kiln shelves and stilts to load a kiln. Make sure they are top coated with kiln wash to prevent sticking.
- NOTE: Do not apply kiln wash to the inside walls of your kiln!!!
- Place your first shelf on very short stilts, holding it slightly above the floor of the kiln. This allows airflow under the shelf and a more even heating. Do not place ceramic ware directly on the floor of any kiln.
- Fill shelves one at a time, from bottom to top, making sure to leave room for more stilts that will hold up the next shelf. Measure the height of each ceramic ware, making sure it is shorter than the stilts you will be using.
- Stilts can be stacked (carefully!) for additional height.
- Greenware may touch and be stacked for bisque firing - it won't stick together.
- Glazeware should not touch other pieces, stilts, or kiln walls - no glaze should be on the bottom of your work, or it will stick to the shelf permanently!
Operating a Kiln with Kiln Sitters
- Start with all element control knobs and timer dial turned OFF.
- Load the kiln, making sure to insert the appropriate pyrometric cone while you are still able to access the kiln sitter on the inside of the kiln.
- To set the cone, lift the drop-switch (the hinged metal rectangle) and hold the end of the kiln-sitter lever down to hold the drop-switch in place. This will create a space to position your pyrometric cone inside.
- Release the lever and drop-switch. The properly positioned cone should hold everything in place.
- Close the kiln.
- Adjust the timer dial to a maximum number of hours for firing.
- Depress the button located in the hole of the drop-switch.
- Turn element control knobs up according to the desired schedule.
- When the cone melts, the rod holding it in place will fall, releasing the drop-switch and shutting off the kiln. If the timer runs out before the cone melts, the kiln will still turn off.
Kiln Operation & Settings for Kilns with Electronic Controls (Paragon & Sentry)
- Set power switch ON
- Set exhaust vent ON.
- Press #4 to select RAMP/HOLD program.
- Select user/program:
Kiln Operation & Settings for Kilns with Electronic Controls (Genesis)
When bisque firing, make sure to set it to a low-fire temperature. FCPS recommends the following program for bisque firing:
Stage Ramp* Temp Hold
1 200 175°F 1 hour
2 300 212°F 1 hour
3 400 1200°F 10 minutes
4 9999(full) 1850°F 10 minutes
*The ramp rate is the speed at which a kiln increases in temperature, the higher the ramp rate, the faster it will heat.
This program should take about 7+ hours to reach highest temperature, before beginning the cooldown stage.
Candling: If you are worried that some work might still be green (not quite bone dry) there is a way to slow the process further, and this is called "candling". Basically you add more time to stages 1 and 2 (like an extra hour each?).
Unloading a Kiln
Once the kiln has fully cooled, open it, and take out the vessels, going top to bottom, carefully removing shelves as you go. Place the vessels on shelves.
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