The name Biscuit firing or Bisque firing as it's sometimes known is given to the very first firing of pottery before it is glazed. Most pottery goes through a bisque firing and is then fired again to melt the glaze and fuse it to the clay body. Bisque firing pottery is the most popular type of firing and is extremely important. It transforms the object into a porous state for glazing. It allows the potter to do much more decorative work with stains, underglazes, and glazes with a greatly reduced risk of the pot being damaged. Because the bisque firing is brought to temperature much more slowly, bisquing also reduces the chances of pots cracking or exploding in the glaze firing. The slowest firing and kiln temperature increase should be done at the beginning of the process, as the most crucial point is when the chemically combined molecules of water are being removed from the clay.
01 of 07
What Kind of Kiln Do You Bisque In?
Although you can bisque fire in either electric or fuel-burning kilns, electric is preferred. The main reason for this is that the settings and temperature of an electric kiln are much easier to control. 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.
02 of 07
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.
03 of 07
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. The firing schedule for bisque firing is extremely important. For a bisque firing, there will be no soak, and the ramp (the increase in which the kiln's temperature is changing) should be very slow. The ramp rate is usually measured in degrees per hour. Make you sure you use the correct firing temperatures for the clay and glazes you have used. Every kiln is unique but 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.
04 of 07
The Overnight Warm Up
Once your pots are finished, they 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.Continue to 5 of 7 below.
05 of 07
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.) Make sure you wedge your clay thoroughly when making your piece, so you don't have any air bubbles in the clay that could crack in the firing process.
06 of 07
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.
07 of 07
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. Make sure the pieces are completely cooled before removing them from the kiln.