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.
Overnight warm up at very low heatTwo 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.
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. 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. 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. 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.
