Today, as promised, Bruce goes into detail about the firing schedule he uses with this kiln. Plus he explains how he gets excellent results by adding soda and salt into the mix. – Jennifer Harnetty, editor.
Gas and Wood
Our kiln is fired with propane gas using a model S-22 atmospheric Venturi burner, made by the Hauok Manufacturing Company in Lebanon, Pennsylvania. A flexible hose allows the burner to be used for our conversion kiln as well as for our main gas kiln. The converted kiln is fired with gas pressure from one pound to slightly over four pounds.
The gas burner is augmented by the inclusion of about 100 running feet of 1×2-inch pine (#2 pine). The wood is sawed into 1-foot lengths and soaked in a super-saturated solution of 1 pound of soda bicarbonate (baking soda) and one pound of sodium carbonate (soda ash) for about five days. By slowly adding these two chemicals to boiling water, it is easy to create a super-saturated solution. The wood is then dried in the sun. When done properly, a thin white residue will be visible on the surface of the boards. This technique allows the soda to be introduced very slowly and to naturally follow the direction of the flame and ash.
Also, 8 ounces (total) of fully saturated salt water is sprayed onto the last 25 lengths of wood, to be stoked after Cone 9 is bending. This helps prevent totally dry areas on the pots.
9 A.M. Open damper fully, turn gas to 1 pound of pressure and open primary air flange to produce a short, oxidizing flame.
10 A.M. The first blush of color is seen in the kiln atmosphere. The damper is still fully open and the gas is turned to 2.5 pounds of pressure. Primary air is opened slightly to continue with fully oxidizing flame.
11:30 A.M. Cone 08 is down. The damper is closed slightly. The gas pressure is turned up to about 4 pounds. Primary air is cut back slightly. No blowback of flame at the burner port is visible at any point in the firing cycle. (We have had our best results when we forego a flame-deflecting brick inside of the burner port.) A distinctly orange flame about 6 inches long is consistently visible at the damper during this stage of clay body reduction. There is also slight backpressure at the bottom stoke hole and a lazy, hazy flame is traveling throughout the kiln. No smoke is visible.
12:30 P.M. The damper is almost fully open. Gas pressure is turned up to slightly less than 5 pounds. Primary air is opened a bit to create a neutral atmosphere. A short flame is visible at the damper (light orange with a slight greenish tinge). At this point, wood is added, one piece at a time, through the stoke hole. This activity immediately creates a reducing atmosphere and the stoke hole is quickly plugged with a piece of soft brick after each addition of wood. Pine burns with a long, somewhat lazy flame and its effects are immediately apparent in the kiln atmosphere. The kiln is allowed to clear completely to neutral before the next stoking. Cone 2 is reached at about 1 P.M
1:30 P.M. Cone 8 is just starting to bend. From this point on, we make virtually no adjustments to the gas pressure, the damper setting or the amount of primary air. We are simply firing with a neutral atmosphere (as far as the gas is concerned), and are falling into a pattern of alternating neutral and reducing atmospheres similar to those that one would achieve in a normal wood-fired kiln. Stoking of the soda-soaked (but dried) pine is increased to two pieces at a time. A moderate orange flame appears at the damper after each stoking. After letting the kiln atmosphere clear for about five minutes, we repeat the process.
2:30–4:30 P.M. All settings remain the same, except stoking increases to the rate of three pieces of pine about every five to seven minutes. This creates moderately heavy reduction with no smoke but with a strong orange flame about 12 inches high at the damper. The kiln clears to neutral and then is stoked again. Cone 9 is down by 3 P.M. After 3 P.M., the same stoking frequency is maintained, but 8 ounces of fully saturated salt water is sprayed, using a plant mister, directly onto the pine just before stoking. This is repeated with each stoke cycle using only a small volume of the solution each time. The water vapor and the burning wood help distribute the salt along the path of the flame. Due to the high vapor pressure of salt, it migrates throughout the kiln very well. At 4:30, Cone 10 is down and the main propane tank is turned off, the line to the burner is bled and the kiln is closed.
The temperature climb slows quite a bit during the last two hours. It is very easy to get to Cone 8, but harder to get to Cones 9 and 10. This is partially due to the increase in stoking and the accompanying reduction. It is also the result of firing in a kiln that was originally rated for Cone 6–8. We are, therefore, pushing the insulating properties of the refractory bricks and are losing a lot of heat through the walls and lid. We estimate that, with proper care and religious kiln washing, this kiln should be able to withstand about 40 firings. Luckily, we have a few more abandoned kilns waiting for conversion and several offers of future donations. For people who love wood firing, but don’t have ready access to a wood kiln, this project is a wonderful adventure.
Bruce Bowers offers kiln conversion workshops. To learn more about the workshops and see more images of his work, please visit www.bowerswoodfireandpics.com.