Tips and Tools: Firing Schedules Gabriel Kline

Appears in the November 2022 issue of Ceramics Monthly.

Firing kilns, particularly atmospheric kilns, is often a community activity in which those with less experience can learn from their more experienced peers. Here’s an introduction to the theory behind the practice. 

The rate of climb in a glaze firing can be faster than bisque for the first part of the firing, as the clay no longer needs to shed water or burn out organic materials, but if your glaze hasn’t had time to dry, you will want to go slowly to avoid it flaking off or crawling. The rate of climb will be slowed toward the top of the firing in order to give the glaze materials time to mature. As the kiln heats up, glazes will release gases, bubble, and occasionally crawl before fluxing out to coat the piece in a smooth, liquid layer of glaze. Going slower at the top of a glaze firing allows time for these processes to take place. As the kiln begins to cool, the liquid glaze will “freeze” back into a solid on the surface of your piece. Glazed work should be allowed to cool to at least 250°F (121°C) (room temperature is even better). Opening the kiln early can cause cracking of the ware if cold air hits hot pots unevenly. If you are using a computerized kiln, we recommend using a medium cone-fire program for most glazes. A fast-fire program can be used in time-sensitive situations, and some glazes such as those fluxed with zinc oxide actually benefit from a fast firing.

Oxidation Firing Profiles

These oxidation profiles can be used in electric or gas kilns.

We used the program above for all the cone-05 and cone-6 glaze-fire tests in this book (Amazing Glaze: Recipes and Combinations). Note:  The formula for straight temperature conversion from Fahrenheit to Celcius is C = (F - 32) ÷ 1.8. The formula for rate rise is C = F ÷ 1.8. 

Down Fire Profile

Another program you can use, developed by Pete Pinnell, is a down fire program in which the kiln continues to fire after it has reached temperature, causing a slower cooling process. This program encourages crystal growth and can combat glaze defects such as pinholing.

Reduction Firing

It is unlikely that you will find yourself standing in front of a gas kiln with only this book to explain how to get a great reduction firing. More likely, someone who has more experience with the kiln will teach you the ins and outs of firing it: how to avoid stalling out at high temperatures, where the pockets of good reduction are, and just how far you need to push the damper in to get the kiln to reduce adequately. This oral transmission of knowledge from kiln master to kiln neophyte is an incredibly important part of learning how to fire. If you are able, ride along with this person for several firings, and then have them shadow you until you really get the hang of it. Link up with a seasoned pro to learn the ropes when firing a new kiln.

Here are some general guidelines to get you ready. Though oxidation firings can be done in an electric or a gas kiln, reduction firings (which “reduce” the available amount of oxygen in the kiln to influence the color and texture of glazes) are generally the provenance of gas-fired kilns. For reduction firing in a gas kiln, you will need to monitor both the rate of climb and the atmosphere inside the kiln. The amount of oxygen present in the kiln can be manipulated by moving the damper in or out. Very small movements of the damper, as little as ¹⁄₈ inch (3 mm), can cause noticeable differences in the amount of reduction.

At Odyssey Clayworks in Asheville, North Carolina, we fire a 24-year-old Geil kiln, beginning in oxidation with the damper open 2½ inches (6 cm) until the kiln reaches cone 012 (1582°F (861°C)). When cone 012 begins to bend, we push the damper in to deprive the kiln of oxygen by reducing airflow. The temperature generally falls with heavy reduction as some of the gas will not completely combust, which creates carbon monoxide that in turn reduces BTUs. We begin by reducing heavily for about five minutes and then start pulling the damper out until we achieve a “climbing reduction” where the temperature inside the kiln goes up approximately 60°F to 80°F (33°C to 44°C) per hour, or approximately 1 degree per minute. We continue firing this way until the kiln reaches cone 04 (1946°F (1063°C)). This generally takes four hours or so. When cone 04 bends, we open the damper back up and continue to fire the kiln until it reaches top temperature. The process is the same for mid-range firings, with reduction occurring from cone 012–04.

Turn the gas off and push the damper the whole way in to allow for a slow cooling. Whether you are oxidation glaze firing in an electric kiln or reducing in a gas kiln, the ware should be allowed to cool slowly. Only open the kiln once it’s back to room temperature. If you have to peek—and do try to resist the impulse—make sure the kiln is 250°F (121°C) or below, and do not open the kiln door until the temperature is 150°F (66°C) or below.

Excerpted with permission from Amazing Glaze: Recipes and Combinations (Quarry Books, an imprint of The Quarto Group) by Gabriel Kline. Learn more at quarto.com.