Soda Firing - Kiln Log plus notes

2-18-10 Soda Firing

NOTES

The kiln was stacked rather densely. 5-6" posts through out the botton and center. A one-inch gap between the shelf columns was maintained. We staggered the height of adjacent shelves with a height differential of at least two three inches. The bag wall was stacked with two large planters, lip to lip, and two large pieces side by side. Replication of this without said large pots could be accomplished by increasing the height of the bag wall approximately 18 inches. Much of the work was unglazed. Some pieces had underglaze decoration. A few pieces were glazed with Shino and a few with Woo Blue II.

The experiment for this firing was the use of a sandblaster to introduce dry soda ash as cone 8 fell. The sandblaster functioned well. Improvement can be made to the container for the soda ash and the hose that draws the material into the gun. The hose tended to move to the side of the bucket and suck air. Perhaps some sort of bell housing at the end of the hose would keep the opening towards the bottom of the bucket.

The soda ash was introduced by sliding the 3' nozzle into the kiln and firing the gun. We varied the depth of the charge by sliding the nozzle back and forth, further into the kiln. In hindsight, this was most likely not productive enough to counter the cost of the effect. By Spraying deep into the firebox, we blasted the far wall with enough pressure and accumulation to create a basketball-sized crater approximately 1/2" deep. An aura of soda residue nearly twice the diameter and not nearly as dramatically eroded was evident surrounding the crater, which I believe is the result of firing soda from far away. This suggests that spraying from far away not only disperses the wear on the firebox, but also allows the hot air to attack and volatize more of the soda ash, more quickly.

We were short on time, so our initial charge was 5lbs. We sprayed and closed the damper to 1/2 for 10 mins to allow the soda ash to circulate and accumulate. After 10 minutes, we opened the damper to full and cleared the ware chamber for 3 minutes before introducing another 3lbs of soda ash. Again we closed the damper for 10 minutes to allow the soda ash to work. After 10 more minutes we cleared the chamber for 3 minutes and sprayed a final 2lbs. At that point the damper was left at full open and we continued to attempt to maintain heat for the last 30 minutes of the firing.

LOG

7am
A small fire was started in the firebox to begin heating the kiln and also to build a coal bed. All ports are closed with the exception of the bottom left mouse hole. The difference between the bottom and the top of the kiln is fluctuating between 100 and 150 degrees - hot on top. The damper is closed to 1/4 at 8am in an attempt to hold a bit of the heat and perhaps even out the ware chamber.

9am
stoking begins at the stoke port. The bottom left mouse hole is closed to 1/2. All other mouse holes are OPENed to 1/2. At 9:30 the propane/PRIMARY ports are opened to 1/2.

10am
The coal bed is lower than we would like. Having so much air from the mouse holes has caused the coals to burn out and/or cool. We noticed a substantial decrease in temperature gain between 10am and 10:30am. From 7am to 10am, we had gained just over 1000 degrees. From 10 to 10:30 the increase was a mere 79 degrees. PRIMARY ports are closed to 1/4 on the left and 1/2 on the right. Mouse holes are closed with the exception of the center hole, which is left at 1/2. At 10:50 the damper is opened to full open, and the center mouse hole is closed. Temperature gain picks up almost immediately. Wood consumption doubles.
By 11am the ware is starting to show some color. PRIMARY air is opened to full.

12:30pm
Damper closed to 1/4 to try to even out the ware chamber. Cone O8 is falling on top and Cone O14 is down on the bottom.

1pm
BioDiesel - the center burner is lit. ((1 pencil lead thickness of fuel // slow drip of water)) Within 20 minutes the right burner was ready to light. ((1 pencil lead of fuel // slow drip of water)) 1:35pm 3rd burner is lit. PRIMARY air is closed to 1/2 on the left.

We made the mistake of slowing everything down during the lighting of the fuel. Stoking should continue as before to maintain and increase temperature gain. During the 35 minutes that it took for us to light the burners, the temperature gained 9 degrees. Yes - 9.
Beware of the flames shooting out of the burner ports when the stoke hole is opened.

1:45pm
Fuel mixture is 1 - 1.5 pencil leads thickness with a fast drip. The increase in water prevented the fuel from creating large amounts of smoke, although temperature gain was modest. Damper is opened to full.

2pm
The ware chamber is mostly even heat. Cone O4 showing soft on the top and the bottom.

3pm
Somehow we caused the kiln to stall. We didn't change anything except crew. Notes indicate a steady stoking pattern of approximately a silver dollar thickness of wood every 10 seconds based on smoke. Coal stirring was happening every 30 -40 minutes, but probably should have happened every 15 -20. Mouse holes are opened to 1/2. Temperature remains constant until 7pm. No gain.

7pm
During the previous four hours, we had noticed that the temperature rose during lags in stoking, for instance while I filled the bio-diesel container, nobody stoked, and the temperature rose nearly 80 degrees during the 5 minutes that I was on the ladder. Frustration was building at this point and with time running short. I opted to shut everything down for a few minutes to collect my thoughts. Burners were shut down. No wood was added. Temperature was 1911 degrees; 40 degrees cooler than at 3pm.

We had been trading off the stoking and it really wasn't consistent for a couple of hours. People were coming and going on the kiln yard and a lot of conversation was happening. When asked if the coal bed had been stirred, we would look at each other for the answer, because we really weren't in sync.

Just after 7, I asked that nobody discuss the pyrometer, or tell me the temperature. I asked one of my teammates to chop wood and the other to take readings and notes on the half hour. I took over the stoking for the remainder of the firing. I began by focusing on the fire itself. When It was bright, I added wood; first to the stoke hole to pre-heat. The door to the hole was left open during the pre-heating. I would throw the whole bundle of wood into the firebox and immediately stir the coals then rearrange the wood in the firebox and load up the stoke hole again. When the fire was clean on the inside of the chamber, I shoved the preheated wood in again and immediately stirred then rearranged. The time between stokes was probably less than 5 minutes but that time was filled with the task of stirring the coals and the firebox to allow as much air circulation as possible. AGAIN I was looking for a bright flame in the firebox and not lingering smoke. By 7:30 I was ready to turn the burners back on, but I did so with trepidation. Rather than turn all three on, I turned on the two outside burners and at a very low setting. ((1 pencil lead thickness // drip)) The burners were left at that setting for the remainder of the firing.

8pm
By 8pm the flames coming from the top of the chimney were righteously active. Cone 5 was softening on top and Cone 8 was down on the bottom.

At 8:30pm Cone 10 was soft on the bottom, but Cone 8 was still standing on top. At this point we began adding soda ash as outlined above.

Once soda introduction began, the rhythm changed. Soda was added at 15 minute intervals while rings were pulled every 10 minutes (since we had more rings than we had time this made sense and occupied the crew)

9:45pm
Cone 11 was completely down on the bottom and Cone 12 was looking dangerous, so we opted to stop the firing. Burners were shut down and all ports were closed. We pulled the last ring and cleaned up the yard. The damper was closed all the way, but when flames began exiting the ware chamber and licking the roof of the shelter I decided to open it as much as was necessary to prevent a catastrophy.

Conclusions

The results of the firing were close to what we expected when we shut down the kiln. The top of the kiln was cooler, while the bottom was hot. Ash build-up on the lower pots was beautiful but ran off the pots and fused many of the wads to the glassy flow of soot, soda and ash. In the center of the ware chamber, the pots were beautiful. Glazes melted, a healthy dose of soda ash was evident and color had developed. The pieces at the top of the kiln were cold and dry. The directionality of the soda and the heat was evident as some pots were juicy on one side and very dry on the other.
In future firings, I believe it would beneficial to add soda ash in smaller increments. The initial 5lb burst was an unfortunate necessity due to lack of time, and the amount of directional accumulation is, at least partially, the result of that. Additionally, if using the sandblaster, it is completely unnecessary to insert the nozzle any deeper than the spray port. As I stated before, I believe that if the nozzle is just barely inserted into the firebox, the soda ash will combust more thoroughly and spread more evenly.