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First Firing of the Cobb Mtn Art & Ecology Wood/Gas Soda Kiln Designed by HP Bloomer & Scott Parady

Back in October I designed a soda kiln for Cobb Mtn Art & Ecology Project with Scott Parady. We had a limited amount of space and needed to tie into the stack that already existed for the train kiln, as well as keep the kiln as versatile as possible. Initially Scott wanted to have the kiln exclusively gas fired, but we eventually decided to make it a hybrid that could do both gas and wood but did not need one exclusively over the other.  We have fired the kiln twice so far and are still making adjustments to the bag wall in order to create a more even distribution of heat from top to bottom. The design was initially to be a catenary arch, but as I got into drawing up the plans we realized that the space would be greatly limited by having a completely catenary arch. This is why the flue side of the arch is a barrel arch and the firebox side is a catenary. Choosing to do this was quite an adventure for me, especially since we were building completely with straight bricks and did not have any arch bricks.

I decided on a cross draft and down draft hybrid based on personal experiences I have had with both types of kilns. The cross draft style of kiln gives great directional effects but frequently is inefficient in fuel use and has cold spots. Having cold spots in a kiln means that there are points where the flame is not heating efficiently. To resolve this you either have to over-fire the rest of the work or sustain a peak temperature for longer in order to let the heat “soak” in these areas which can also be equivalent to over-firing the rest of the kiln. By dropping the flue below the ware chamber the flame is drawn down and through the wares, and so the flame heats the ware chamber more efficiently.   Since we have no natural gas lines in this area and have to run off of propane, which is more expensive, it seemed to make sense to have a more efficient type of kiln that gave the option of being woodfired or gas fired.

Overall this is proving to be a great little kiln that can be filled fairly quickly. Firings have also proven to be less expensive than I had initially thought they might be. My previous experiences with propane to cone 10 have cost $180-200 per firing. We started the first firing without the gas and fired up to bisque temperature solely with wood. This didn’t seem to use too much wood and took us about 16 hours to get to ~2075° at the top of the arch. We turned on the gas around midnight and took it up to 2400° in three hours.  The last firing I shared with Scott before our Studio Open House and Sale in November was only propane fueled and used only $148 in propane. I think that we can speed the firing up when firing only bisqued work. I am gradually opening up the bag wall so that the kiln fires more evenly top to bottom, and am waiting for the shelves that this kiln was designed for. It was designed to use the standard 12″ x 24″ silicon carbide shelf instead of the shelves that we are currently using.

Fun things we did with this kiln that were new to me:

  • King brick as grate system. Used 13.5″ King brick as the grate instead of steel bars. We did this because we already had the brick and did not want to have to buy steel bars which would more quickly erode when exposed to the soda in the kiln.
  • Complex arch. This was done to give more usable loading space for work.
  • No arch brick. The arch was also built, as mentioned above, with no arch brick. This means that all bricks are straight and not keyed. All adjustments to the curvature were done with mortar.
  • Doglegged flue. The flue is at a 45 degrees to the kiln and the stack. This was because we needed to move the kiln over so that it would not block a spy on the train kiln and to give us more space in the kiln area.
  • Burner Manifold. We included a manifold between the propane line and the burners. We did this to equalize pressure so all burners received a equal amount of fuel.
  • Excavated firebox. Because we were tying in to a preexisting kiln our flue level was already at a fixed position. This meant that we had to excavate the firebox to have it be at the appropriate height. That is why we have a bricked-in clean-out pit in front of the kiln. Normally potters get around this by having the kiln elevated on cinder blocks and building individual chimneys for each kiln but this requires a great number of bricks and space. Tying two kilns together can limit firings to one at a time but can save a lot of space an bricks.

If I were to build this kiln again:

  • If I were to build this design again with more time and resources I would include more primary airs for the coal bed and firebox.
  • I would take more time to build a more complex floor that further recessed a channel for the flue into the floor.
  • I would like to make the firebox a little wider with a deeper recess for the coal bed.
  • I would give more space for a shelf to slide in behind the kiln and close it off. This is not something I want to allocate a pristine new shelf to, and older shelves have a tendency have a curve or accumulation on them that need to be accommodated with a slightly wider slot.
  • I like catenary arches for larger kilns and even more so when the arch is perpendicular to the stack. If I were to build this kiln again I would go with a sprung arch or a barrel arch to accommodate more room for work, the firebox, and the stoke hole. Building these types of kilns requires more steel for a frame to brace the arch.
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