Bisque firings are normally done in one of the Skutt kilns because they’re quick to load, electric, and have automatic firing controllers. However, if very large pieces have to be fired, or a really large volume of smaller ware, then the Alpine is the best of the bigger kilns to use. The firing starts with an overnight candling, very similar to the way glaze firings are started, more on that below. But, when doing a bisque, there needs to be special cone packs made, a very slow start (to drive off captive moisture), and an oxidation atmosphere throughout the firing. Our target end temperature for bisque is cone 06 (1830 F).
Make a set of two cone packs with three cones each: cone 010, 08, and 06, arranged as illustrated to the right. Make them long enough before the firing so that the clay can completely dry out. The 010 cones indicate of how well the upper and lower temperatures are balanced after reaching the cherry red stage of the firing. This gives some time to make adjustments to drive up the bottom temp if needed. The cone 08’s indicate that a minimum bisque temperture has been reached. As an alternative, use cone 07’s in their place.
Prep and Start-up: In the early evening,
- Close the kiln door; but, don’t bolt it shut. Check that you can easily see both bisque cone packs through the spy holes.
- Set the damper gap to 1.5 inches. Check that both lower, side spy holes are plugged.
- Check that the kiln gas valve is shutoff for all three kilns. (Handle angle at 90 degrees to the pipe direction)
- Turn ON the room exhaust fan.
- Turn ON the main gas valve for the room.
Preheat and Candling :
- Light both pilots for a one hour preheat, watching that the pyrometer reading remains below 200F. If it exceeds 200F, the crack the door a little to drop temperature down slightly. For thick sculptural pieces or children’s ware, preheat for a little longer. Sculptural pieces fired with the Skutt User 1 Program have up to a 3 hour preheat depending on the ware’s apparent dryness and thickness.
- Next, light both burners and adjust settings to: Gas Pressure: 1/4 , and Air Speed: 10. Keep the damper gap set at 1.5 inches. Bolt the kiln door shut, and hold this setting for one more hour.
- Then, make the overnight “candle” settings; Gas Pressure: 1/2 , and Air Speed: 22.
………………(Click on photo to enlarge)
Firing Completion: The kiln slowly increases in temperature on its own overnight. Return anywhere between 8 to 9 am next morning, and the temperature should have climbed into the 1200F to 1500F range. Then,
- Increase settings to Gas:1.5 , and air 50. Watch the pyrometer and adjust gas pressure and blower speed from there to achieve a temperature increase rate of 150 to 200 degrees F per hour while staying in oxidation (That is, a blue rushing flame (noisy but not roaring), zero visible flame out the flue, and zero gas smell.) Each time the gas is increased, adjust the air also to maintain the blue rushing flame. Never allow a lazy quiet flame. Don’t allow even the faintest smell of gas !
- When the temperature reaches the 1650F level, start watching for upper and lower cone 010 movement; and start closing the damper slightly if the bottom is cooler, but not so much that flame appears. Check for flame with the room lights off.
- By the time the temperature reaches the 1800F level, both cone 06’s (or 07’s if you used them instead) should be dropping or have dropped. Shortly after that, cone 06’s will begin to dip. As long as both cone 08’s (07’s) are down, either cone 06 dropping to 3 o’Clock indicates the firing is done. Do not overfire.
- Shut off the main gas to the room, then shutdown the kiln gas, then shut off the kiln electrical switch, and then close the damper – in that order.
- Shut off the room exhaust fan.