Topic: Ceramic Kilns

In the Studio: Empirical Learning

The sound of an entire stack of pots falling over in a wood kiln at peak temperature isn’t as bad as you’d think. There’s no loud crash. Instead it’s a low, hollow sound dulled by the sticky, pyroplastic nature of clay at 2350°F (1288°C). Imagine in the next room over, there’s a big table full of soft, freshly thrown pots. Pretend those pots are covered in honey. Now place an ear against the door and listen as someone tips the table over, sending the whole sticky mess tumbling to the floor.

I heard that sound while standing right next to the kiln, a 25-foot-long anagama that leads into a catenary arch chamber we use for salt glazing. I grabbed a welding mask and opened the door to the small secondary firebox to inspect the kiln, but all I could see were thick, hazy flames. I stood up to look over the arch at Paul Herman, who had been working on the opposite side. “I can’t stoke,” he said, “there’s a shelf in the way.” I looked back into the kiln, squinting through the mask. As the atmosphere began to clear, I could make out a jumble of lines and forms where there should have been an empty space above the firebox. My stomach sank and any hope that the sound was a minor mishap went with it. Joe Winter, who had been masterfully tending the main firebox, was standing behind me now. I handed him the mask so he could see for himself what a mess we had made in there.

1 Before: Unconventional stacking with inadequate kiln furniture (see offset kiln posts, circled) may prove to be disastrous.

There wasn’t a great deal of discussion after looking in. We knew we had lost a whole stack and we knew why. About ⅔ of the way up that set of shelves, we had offset a kiln post a few inches, transferring the weight from above onto the shelf directly, rather than the column of posts below. That shelf, after being subjected to this treatment many times over, could no longer bear the strain. When it broke, everything resting on it came tumbling down. We knew this arrangement wasn’t ideal, but we’re a frugal bunch—resourceful, inventive, and improvisational—sometimes to a fault. Our bargain shelves have been collected, cut, and cobbled together over the years (1). Sometimes when you spend less, you get less-than-perfect equipment. Our unconventional stacking had worked for 36 firings. This time it didn’t.

We had to think fast. With the kiln close to our target temperature, we decided to close up the anagama section and move onto the salt chamber. Joe and I sealed up the front of the kiln while the rest of the crew moved fuel and prepared for an early finish. Paul positioned himself above the chimney to call the stokes. It was a swift, quiet commotion and we didn’t miss a beat. We finished the evening off like we usually do, with salt and celebration. After getting the last pots up to temperature and introducing 25 pounds of modern civilizations’ most taken-for-granted spice, we sealed up the kiln, filled our ceramic cups with whiskey, and ate homemade apple pie.

Stacking for Stability

  • Prior to loading your kiln, inspect each shelf for cracks, severe warping, or other damage.
  • Clean any glaze drips, loose kiln wash, and stuck-on bits of fired clay and/or kiln posts. Small jobs can be done with a hammer, chisel, and cleaning stone (or a scrap piece of brick). To avoid damaging the shelves, place them on a forgiving surface like foam or sand and direct your blows toward the center of the shelf when cleaning edges and corners with the chisel.
  • For bigger jobs, it helps to use an angle grinder with a diamond/masonry cup. I prefer to do this job prior to loading when no one is around, and always wear a respirator, eye protection, and ear plugs.
  • Apply kiln wash to the tops of the shelves and both ends of the kiln posts. Multiple thin coats generally adhere better than one thick one.
  • We generally use a four-post stacking method, although three posts can be adequate depending on where the stack is inside the kiln, the target temperature in that area, and the size and material of the shelves being used.
  • As you stack, make sure your posts and shelves are stable. Posts should be placed directly above each other and form a vertical column. Offsetting posts, even a little, increases the risk of breaking shelves. If it must be done, do it high in the stack and with little weight above.
  • Wadding each post can help compensate for warped shelves, slightly damaged or uneven posts, or wobbly posts.
  • When wadding the top of a post, put a small piece of newspaper on top of it to prevent it from sticking to the next shelf. This makes small shelf adjustments easier.
  • For the sake of stability, it makes sense to load all of the small work down low and taller work higher in the kiln. If you do this, be sure to pack the lower shelves loosely and the arch tightly to encourage even flame distribution. If you use taller posts at the bottom of a stack, consider using full or half bricks as posts to keep the stack more stable.
  • Remember that wood firing is a high-risk, high-reward operation. Mistakes will be made and minor disasters will happen. Learn from them and don’t get too discouraged.

2 After: Squished, fused-together pots in a toppled kiln stack caused by a shelf breaking under the weight of the offset posts.

3 Casey Clark’s lidded jar fused together with two of Paul Herman’s mugs. Photo: Sarah Lillegard.

Lessons Learned

Four days later when we gathered to unload, I found myself in an inexplicably good mood. It was a beautiful day, and our firing crew and friends were anxious to see the results. We watched gem after gem come out of the kiln, our best results in years. Finally we came to the toppled stack (2) and everyone took turns climbing in to see. We took photos of the squished pots and admired the grotesque beauty of the wreckage (3). As I watched everyone process the results, I realized my biggest fear was assuaged, nobody was upset. Sure, we lost some pots, but we all showed up that day ready to learn from the loss.

Every firing has its lessons. Sometimes they’re small ones that you have to look for closely to find, and sometimes they’re big ones that smack you right over the head. This time we took one over the head, and even though it hurt a little, we’re better for it. What did we do next? We made more pots, split more wood, and yeah, we bought the right size shelves.

Casey Clark is a studio potter living in the rural town of Doyle, California. He’s been firing his work in the anagama at Great Basin Pottery since 2004. Paul Herman and Joe Winter completed construction of the kiln in 2000 and have fired it twice per year since. You can find Casey’s unsquished work at www.rollingoutclay.com or on Instagram @rollingoutclay.

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