Studio and Soda Kiln
After moving to North Carolina in 2018, I set up a workshop and studio in our large, walk-out basement. From the time I started grad school in 1985 until retirement from Tennessee Tech University in 2018, my studio has been within the clay facility at the university. As a teacher, that has its advantages, but also means frequent interruptions amidst the chaotic demands of an academic teaching position.
During the same period of time I never had a proper workshop, whereas before my academic career I had wonderful workshops ever since I was a child. I’ve been a tinkerer all my life, and at one time worked as a mechanic and welder for the City of Arcata, California. A well-equipped workshop is very important to me. While at the Appalachian Center for Craft, I had the run of the wood and metals studios and didn’t suffer for lack of a proper workshop of my own. Now, in retirement, I have a workshop once again.
I fully appreciate the luxury of having all the studio time I want with few interruptions. I had that was 35 years ago in California, but it was a necessity, not a luxury. I was a solo production potter in my studio, Railroad Stoneware in Blue Lake, California, doing my share to support the family. I am adjusting to this new reality. Click here to see a short video tour of my studio and workshop.
Below is a gallery of images of the studio and work-in-progress, a gallery of the first two firings of the soda kiln, and finally a gallery showing construction of the soda kiln, followed by information about the kiln design.
My Studio
We live on five acres in northern Chatham County outside Chapel Hill, and the house is on the south slope of Collins Mountain. My workshop and studio are in the large walk-out basement. When we purchased the house, two basement rooms were finished and became my workshop. The third and largest space had a dirt floor covered by a plastic vapor barrier. We had a slab poured in that space, and then windows, insulation, and drywall installed. The readymix truck was able to back up close to the basement entrance, but the studio is at the opposite end of the house. Randy the concrete guy rented a Georgia Buggy, essentially a motorized oversized wheelbarrow. The concrete work required two readymix trucks, and Randy moved all of that concrete the length of the house one Georgia-Buggy-full at a time.
The most recent images are at the top, and they descend in reverse chronological order. Click on thumbnails to see a larger image. Once you’ve enlarged the image, scrolling works best with the arrow keys on your keyboard, or on a phone or tablet by tapping either edge of the enlarged image.
The Soda Firings
I’ve built and fired a half-dozen soda kilns before this one, and every brand new kiln involves a certain amount of experimentation and guesswork during the first few firings. In the initial firing I experienced a serious cold spot in the bottom of the kiln. Cone-7 was flat at the top front and rear. Cone-5 was bending at the bottom rear but not at the front. Most of the wares on the bottom shelf were underfired. Fortunately, I fit way more than half the work in the first firing, and thus had plenty of room for refires in the next one.
I decided that the uneven temperatures were due to three factors. First, charging soda generally stalls the kiln, so I turned the pressure up from 5 PSI to 7.5 PSI as I began charging, and I think that pushed the temperature in the upper part of the kiln. Second, I was worried about fitting all the wares in two firings, and thus packed the first firing too tightly, interfering with circulation of heat and vapors. Third, I modeled the bag wall after the ones I built in the soda kilns at the Appalachian Center for Craft, and after the first firing decided that it was too solid and too tall. This kiln is virtually identical to the soda kilns I built at the Craft Center, but a half brick larger in width and depth to accommodate the 12.5 x 25″ Advancer kiln shelves. The bagwall was four bricks high and built solid on the bottom course with no spaces. Also, our standard firing protocol for the soda kilns at the Craft Center suggested turning up the gas pressure as high as 10 PSI during high-firing. 7.5 PSI seemed modest in comparison. I had been going on previous experience, but like I said, every kiln is different, for a host of reasons.
Before the second firing I rebuilt the bagwall, with more openings at the bottom level and only three courses of bricks overall. Even with all the refires, the second firing was stacked looser than the first. In the second firing I never turned the pressure above 5 PSI. It took a little longer, but it may be the most even soda firing I have done. The cone-08 body-reduction cones were touching in the top spyholes as they were bending in the bottoms, which is unusually even that early in the firing. By the time I reached midrange, Cone-6 went down top and bottom, front and rear, all at the same time. I am lucky to have achieved such excellent results on the second firing of this kiln. I’ve followed the same protocol since then.
I’ve thought carefully about what was different about those ACC kilns, besides the slightly smaller interior dimensions. In those kilns we were using two 12 x 24″ shelves side-by-side in a space 27″ square, giving only 1.5″ clearance between shelves and the kiln walls. That’s not much room for circulation of the heat and soda vapors, and thus we needed higher gas pressure at the burners during midrange and highfiring. In my new kiln, I am using two 12.5 x 25″ shelves side-by-side in a space 31.5″ square, giving slightly more than 3″ clearance all the way around between shelves and kiln wall.
Also, those ACC kilns required a very tall chimney in order to clear the overhead roof. We control draft with the damper, but with a tall chimney acting as a convection tower, what gets past the damper is traveling at high velocity. That makes the exit flue at floor level act like a vacuum cleaner, sucking aggressively. Thus we needed a taller bagwall that was solid at the lower level in order to achieve even temperatures. That said, we always did struggle a bit with uneven temperatures in those kilns.
The chimney on my new kiln is approximately twice the height of the ware chamber, which is plenty adequate for good combustion and control, but with far less velocity at the flue opening. Thus, the difference in bagwall design.
I start charging the soda when cone-6 is mostly down. I dissolved six pounds of soda ash (sodium carbonate) in two gallons of hot water, accelerating dissolution with a drill impeller-mixer. Charging is done with a Hudson #90183 Constructo 3-gallon sprayer with a poly tank and a brass spray wand. If you soda fire and like to insert the wand in the charging ports when charging, make sure you are spraying as you insert it, and continue spraying until you remove it, or the tip will melt off. As of spring 2022 I’ve fired the kiln eight times with no damage to the brass wand. At the Appalachian Center for Craft despite stern warnings, my students almost invariably melted the tip on the first use. When that happened, we cut off the melted end and hammered the new end flat, and it still worked.
For each charge, I sprayed approximately thirty seconds each in the front and rear charging ports. I periodically pull a draw-ring to gauge the soda deposition, stopping when I am happy with the results. I do not keep track of the number of charges, but I believe it is usually seven or eight charges in each of the two charging ports. After each firing I measure the remaining volume of soda solution to determine how much soda I charged, and it averages about three pounds per firing. At the Appalachian Center for Craft, I got good results charging about a pound and a half of soda, but that was in a well-seasoned kiln.
I was happy with most of the work from the first three firings, but much of it seemed slightly underfired. I communicated with other soda-fire potters, and after some discussion came to the conclusion that cone readings in a soda kiln are affected by soda deposition, and the cones tend to fall at least one cone lower in temperature. So, in the earlier firings, I thought I was firing to a high cone-6, but was actually a high cone=5. I ordered a box of cone-8 cones, and now I fire to cone-8 touching. That would be pushing Starworks Star White-6 clay towards over-vitrification, so now I am using a claybody combining three parts Star White-6 white to one part Okeewemee-10, blended in my pugmill. I am very happy with the glaze results and the more-aggressive soda-fired effects.
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Construction of the Soda Kiln
The process of building the new soda kiln began in earnest in late winter of 2019. My sister Kazi was a concert violist, but in retirement has become a potter. She lives in Southern California, and was planning a visit with her daughter in Philadelphia at the end of March. She suggested extending the visit in order to come down and help build the kiln. She could only be here for a long weekend, and during her visit I wanted to tackle the portion of construction that would provide valuable experience for Kazi. Before she arrived I prepared the kiln site, built the cinderblock foundation and base, and laid a layer of fiberglass-reinforced cement-board and three layers of hardbrick for the kiln floor. Kazi and I worked diligently during her visit, and as you’ll see below, we accomplished a lot in three days.
The propane company was slow to install and fill the tank and complete the hookup to the kiln. They finally finished all of that in early July, right before I left for my summer workshop tour out west. Thus, I didn’t get to fire the kiln until this fall. Scroll down below the images for more information about the kiln.
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This kiln is a 30-cubic-foot sprung-arch propane-fired soda kiln with about 24 cubic feet of stacking space. Most of what you see is pretty standard hardbrick construction, with a few notable exceptions. I always do a single-thickness brick sprung arch and then add a 4″ rigid insulating layer atop the arch. If you have a source of scrap IFB, that makes a good insulating layer, laid with mortar. I have done that when scrap IFB were available, but otherwise I mix a homemade insulating castable refractory. To get insulating properties, there must be permanent voids in the castable. That can be achieved by incorporating sawdust or crushed walnut shells that burn out when the kiln is fired, or with a refractory material that creates voids. The disadvantage of organic insulating fillers is that they produce volumes of smoke for many firings, and it can take years to burn away. Two inexpensive and readily available insulating refractory materials are vermiculite (expanded mica), and Perlite (expanded silica). Both can be purchased from garden/landscaping suppliers. Vermiculite is traditionally popular for this use, but I find that it disintegrates and collapses too much in the mixing process, losing much of its insulating properties. I prefer Perlite, available in bulk four-cubic-foot bags from garden and landscape suppliers.
I’ve always known that you can add a small portion of Portland cement to homemade insulating castable to achieve a weatherproof layer, but until this kiln it never occurred to me that the addition of cement also causes the insulating layer to air-set hard overnight, minimizing any subsequent shrinkage cracking during drying and firing. Without the Portland cement, the castable shrinks significantly as it dries, leaving gaping cracks that must be filled with more of the same insulating mixture.
Recipe for Homemade Castable for Insulating Layer Above Arch
This is not a hotface castable. It is only appropriate for use outside a layer of brick. The recipe is by volume, not by weight. I used a two-quart plastic measuring cup to measure out the parts of each material.
- 6 parts Perlite
- 2 parts fireclay
- 1 part cheap builder’s sand
- 1 part Portland cement.
In the past I’ve used a regular portable cement mixer to blend insulation castable ingredients, and that requires that you add enough water to make the castable move around in the mixer as the barrel rotates. This time a neighbor loaned me a proper mortar mixer, which works far better. A mortar mixer has stationary hopper with rotating paddles mounted on a horizontal shaft, and will produce a castable mix with far less water content than would be possible when using a conventional cement mixer. Less water means less shrinkage, and it means that the castable will stay in place without flowing. If you plan to mix any castable recipe for a kiln, I strongly recommend that you borrow or rent a proper mortar mixer. Most equipment-rental yards have them. Put a few gallons of water in the bottom of the mortar mixer and start adding materials, cycling through all of them. For example, add 6 measuring cups of Perlite, 2 measuring cups of fireclay, 1 measuring cup of sand, and 1 measuring cup of Portland cement. If the mix is still quite fluid, add another round of dry materials.
Add more water as needed as you keep adding dry materials, always adding all of them in the correct proportions. Add barely enough water to encourage proper mixing and puddling. If in doubt, take out a handful and toss it back and forth between your hands. If it holds its shape but becomes wet and sticky, it contains enough water. If it will not hold its shape and sags, it contains too much water.
Stop adding materials when the mixer is filled to the rotating shaft, and dump it into a wheelbarrow. At the kiln site, transfer the mix to a bucket and lift it onto the kiln a bucket-full at a time, gently patting it in place with your hand. When mixed with a mortar mixer, the castable will not flow, so it will stay where you put it. Keep mixing castable and packing it in place until you have a layer at least 4″ thick completely covering the arch. You can gently pat and rub the surface to get a decent finish, but there’s no reason to try to make it smooth. Let the texture of sand and Perlite show on the surface.
The insulating layer cured very hard in about 36 hours, and a month later no cracks had appeared. After the first two firings, a few very fine cracks appeared, but they in no way negatively affect the performance or durability of the insulating layer.
Burner System
The burner system has three GACO MR-100 venturi burners and one Ransome B-1 venturi as a pilot burner. The propane comes through an adjustable high-pressure regulator and a Johnson’s Controls high-pressure Baso safety shutoff valve, with the thermocouple tip mounted in the pilot flame, which is in turn mounted directly beneath the center burner. There’s more information about the burner system in the photo captions, but you can also read my PDF handout Soda Kiln Burner System.
Kiln Foundation and Base
I built my very first kiln in 1974. I was on a tight budget, and laid the floor bricks on a leveled bed of sand. That worked fine, but all other kilns I had built before my retirement were built on concrete slabs. Once a slab is cast, demolition and removal is extremely difficult. In the location of my new soda kiln, I did not want a permanent concrete slab. The site needed to be leveled, so I built a rock embankment along the low edges and had 5 cubic yards of gravel delivered. I got 1″ crushed rock with fines, which is what they use under concrete slabs because it compacts easily and shifts/settles very little over time. I hauled all of the gravel, cinderblocks, and hardbrick over to the kiln site a wheelbarrow at a time. After the gravel was graded level (with a rake), I watered it down generously with a sprinkler and let it settle for a month.
Atop the gravel I laid a poly tarp trimmed to size, and atop the tarp a bed of cheap play sand. I leveled and tamped the sand bed and laid solid 4″ cinderblocks to create a slab slightly larger than the kiln footprint. Atop that I laid standard cinderblocks with the holes facing upwards to elevate the kiln for easier access. On top of that I laid a single thickness of 1/2″ fiberglass-reinforced cement-board cut to fit, and then three layers of hardbrick, giving 7 1/2″ of brick in the kiln floor.
Hardbrick Versus IFB in a Soda or Salt Kiln
There has been a lot of discussion recently about whether to use hardbrick or IFB on the hotface of a salt or soda kiln. IFB offers significant fuel savings, but there’s a lot to consider. An unlined IFB hotface will deteriorate quickly in the caustic sodium vapor atmosphere, especially on a salt kiln. Also, an IFB lining means that the kiln cools faster, which doesn’t allow much of the crystal growth that gives richness and variety to many glazes. A saturating application of a refractory coating can significantly retard degradation of an IFB hotface, but such coatings are expensive, and a saturating coating reduces the insulating value of the IFB, partially defeating the whole purpose. IFB are approximately twice as expensive as hardbrick. I paid $1.65 apiece for my hqardbrick, so choosing IFB would have cost an additional $2000. To get good glaze results, I’d have to fire-down (keep the burners on during the cooling cycle to slow down cooling). And likely, I’d have to rebuild the kiln within five to ten years. One must consider the environmental costs of manufacturing those several thousand brick. Even considering the additional fuel consumption in a hardbrick kiln, I could not justify using IFB.
I ordered my hardbrick from Larkin Refractory Solutions in Georgia. When you buy firebrick from Larkin, you deal with nice people who sell reliable products for excellent prices, but the bricks are often very slightly irregular. The alternative is to pay a whole lot more to Harbison-Walker International and get bricks that are more uniform. If you buy from Larkin, get a 4.5″ diamond cup wheel for your angle grinder. As you are building the walls and the surface comes out unacceptably off-level, put on your dust mask and safety goggles and grind down the high spots. It happens fast with a diamond cup wheel. We did that quite a few times while building this kiln.
For general information on soda kiln construction, check out my PDF handout Building a Cross-Draft Soda Kiln. As is recommended in that handout, the chimney is separate from the kiln rather than integrated into the kiln wall. The latter design takes a lot less brick, but if the kiln needs to be rebuilt, you have to tear down the chimney as well. A separate chimney will outlast quite a few kiln incarnations.
I did several things different in my current design. The burner ports are one brick (2 1/2″) off the kiln floor. As soda slag accumulates on the firebox floor, as it eventually always will, it cannot flow into the burner ports. Also, in the past, the charging ports on all my soda kilns have been just one brick high (2 1/2″) through the full 9″ thickness of the kiln wall, which gives a narrow spray angle. When charging the kiln, you want to fan the spray of aqueous soda ash solution up and down the kiln wall and across the floor. For better access, some designs incorporate a charging port two bricks high (5″). For this kiln, the charging port is two bricks high on the inside layer of bricks, and one brick high on the outside. That gives a broad spray angle while minimizing blow-back whenever charging with the damper closed down, which retains the soda vapors in the kiln longer and gives more of an all-over soda effect on the wares.
Below are PDF plans for this kiln. The plans were drawn in MS-Paint, and thus are a bit of a compromise at best. The scale is pretty accurate, and they show approximate placement of wall bricks in header and stretcher courses, but no exact details. In building nay brick kiln, you must constantly be making adjustments to avoid vertical alignment of seams whenever possible. If you use these plans to build a kiln, please give me credit for them, and consult the how-to handouts below. Note that the plans included in the “Building a Crossdraft Soda Kiln” handout are for a 20-cubic-foot soda kiln, while the plans listed separately below are for my current 30-cubic-foot soda kiln. The handout on “Soda Kiln Burner System” is for a soda kiln from 20 to 30 cubic feet.
Building a Crossdraft Soda Kiln
Pitelka Soda Kiln – Front Cross-Section
Pitelka Soda Kiln – Side View with Burners
Pitelka Soda Kiln – Side View with Chimney
Pitelka Soda Kiln – Top View at Floor Level
Please contact me if you have any questions about this kiln.