What Kind of Kiln Do You Bisque In?
Photo © 2009 Beth E Peterson
Although you can bisque fire in either electric or fuel-burning kilns, electric is preferred. The main reason for this is that an electric kiln is much easier to control at the very low, preheating settings. Fuel-fired kilns, such as those using natural gas or propane, tend to rise in temperature much faster due to the amount of fuel that must come through the fuel nozzle in order for the burner to remain lit.
What Temperature Should a Bisque Firing Go To?
Generally, bisque firing is done between cone 08 and cone 04, no matter what the maturation temperature of the clay and of the glazes that will be used later.
By cone 08, the ware is sintered and has become a ceramic material. At the same time, the clay body still is quite porous and absorbent enough for easy glazing. It does remain more fragile, however, and extra care will need to be taken when handling this bisqueware.
Bisque firing can be done up to cone 04. While this makes the bisqued pots a bit less fragile, it can increase glazing time and may adversely effect glaze adhesion, as the pot's fabric has tightened and become less porous and absorbent. The higher the temperature, the less porous the ware becomes.
The Firing Ramp and Firing Schedule
The terms "firing schedule" and "firing ramp" are strongly related. Both refer to the rate at which the firing is done, including the heating, soaking period (if there is one), and cooling. For a bisque firing, there will be no soak, and the ramp (increase in temperature) should be very slow. Generally, the firing schedule should be similar to the following:
- Overnight warm up at very low heat
- Two hours at low heat (an increase in temperature of no more than 200°F per hour)
- Two hours at medium heat (an increase in temperature of no more than 300°F per hour)
- High heat (an increase in temperature of 300 to 400°F per hour) until the required temperature has been reached.
The Overnight Warm Up
Pots should be bone dry before being loaded into the kiln.
For an electric kiln, follow the manufacturer's instructions. If there is no controller, only the bottom-most element should be on, with the lid open slightly and the spyholes open.
For a fuel-fired kiln, begin with the pilots lit and the door and spyholes closed. For an updraft, close the damper completely; for a downdraft leave the damper just slightly open. If your kiln doesn't have pilot lights, light only one burner and bring it to its lowest sustainable rate. Make certain all dampers and spyholes are open, as well as having the door open about two inches. Make certain the burner stays lit. Continually monitor the kiln temperature and burners.
The Low Ramp
After the warm up is completed, close any open doors and dampers and increase the heating energy. For an electric kiln with switches, turn all switches to low. For an electric kiln with a programmable controller, follow the manufacturer's instructions. For a fuel-fired kiln, bring all the burners to a low setting.
If your ware is thick-walled, increase the low ramp time to four or six hours, depending on the thickness of the clay. If you begin to hear any noise from the kiln such as popping sounds, lower the heat-energy immediately. The ramp is too steep and your ware is at peril. (This is most likely to occur in fuel-fired kilns.)
The Medium and High Ramps
After the low ramp, bring the kiln to a medium heating setting for two hours. Again, if your ware has exceptionally thick walls, you may want to increase the medium ramp to four or six hours.
At the end of the medium ramp, the interior of the kiln should be at red heat. At this point, you can bring the heat sources to their highest setting. For the average electric kiln, bisque temperature will generally be reached three to eight hours after the kiln goes on high. The controller or kiln sitter should automatically shut the kiln down.
For a fuel-fired kiln, check the cone packs every half hour. Once the first cone begins to tip, check every fifteen minutes. When the target cone has bent to a 90° angle, shut the kiln off.
Cool the Kiln
After the kiln has reached temperature, make sure all heat sources are off. Close any openings and leave the kiln to cool at its own rate. Generally, expect your kiln to cool for as long as it was heating (minus the overnight warm up). As a good rule of thumb, if you fire the kiln one day, let it cool overnight and unload it the next.When you think the kiln has cooled enough, crack open the door. If any heat comes out, place a piece of paper in the opening. If it lights, the kiln is still too hot to open. If the paper does not light, but you hear pinging sounds, the kiln is still too hot to open. In either case, close the door immediately and allow the kiln to cool for several more hours.