Dunting
Dunt: a heavy blow, a thunk, a knock. In ceramics dunting refers to cracks in pottery that occur in the kiln. Dunt is a heavy word, solid and round and a little onomatopoetic, forbidding. Close to the ground. When pots crack in the kiln usually it is at the end of the firing — either around a thousand degrees (fahrenheit), or just below five hundred. Usually they come apart into fragments with sharp edges, the brand-new glass of their glazes suddenly shattered into bits you have to carefully pick off the kiln shelf.
Why would this happen? I’ve never thought much about dunting, and over the years I’ve been a potter I’ve always just used the word crack, which can refer to what happens to clay at any stage of being formed into something — S cracks, ring cracks, radial cracks…I usually just shrug, discard, and make another piece. You can wonder or you can just…whatever.
Now though, this early summer, the first few hot days of the year, is a time to be more specific. To wonder. To use the right term. In ceramics, as in psychology, a crack of any kind is a split — a separation — an opening, and the point of it is to relieve tension. A pottery form is thrown, or constructed, or sculpted from a solid block of wet clay — any way you go about it, just like an individual human being born and starting to grow up, there are many points along the way when the whole is put at risk and the possibility of staying together as a unitary thing, a personality, is put in doubt.
Pieces crack when the wet clay you used to make them starts to dry because it shrinks and uneven pieces shrink unevenly. They crack when being heated in the kiln due to changes in their clay’s chemical structure…kind of a ceramic adolescence. Finally, and sometimes months after the fact, they crack due to incompatibility between the clay and the glaze if the one by nature is shrinking and compressing & the other refusing to shrink.
Over this last weekend I learned to say dunt. Dunting. True, a crack can be the most important and beautiful part of a piece — it can enliven, energize, separate, open — always though it is a last resort, a way to resolve the unresolvable. You put emptiness where before there was substance. On Saturday I thought to take a day off before the long hours I had scheduled Sunday & Monday for glazing & firing the kiln I’d recently acquired for my ‘private’ studio (don’t worry the kiln down on Lena Street is working great). The Taos Box was running about a thousand cubic feet per second, not really big but good. 10am to meet? Yes, I texted. I’ll be there!
In retrospect everything is a sign you should have seen, the downstream wind, the easy lines through the class IV drops…almost too easy. At the takeout I pulled my drytop over my head and felt that release like a stretched rubber band leaving your hands — the seal that keeps the water out had just torn, two paddling seasons of sunscreen had degraded the latex and let it tear. I drove home. I got to work. Charging the battery on the electric drill, mixing glazes, dipping pieces in buckets, scraping kiln shelves — I had a new full set of these, the old mullite kind of kiln shelf, thick and massive, each like an old gas-guzzling Ford. Beginning to stack new unfired work into the kiln. Reading the little tags I keep on a rail like a short order cook, the special orders: eight plates with shino glaze, better make twelve to be sure, thirty little cups from a local clay I dig in Abiquiu. Make that shino glaze thick — make it white. No glaze on the bottoms of the plates please. I read my notes & followed along, midnight now and the kiln nearly complete. Light the pilot ring. Rest. Sleep for a few hours.
Turn the gas on low. The fiddly thermocouple — the previous owners of this kiln had stripped off a lot of the finer sensors & valves, replacing them with generic heating appliance gear, including a twelve-dollar thermocouple that didn’t belong — as the heat increased it would loosen, I knew this, it would slip out of place & I couldn’t sleep. Twenty-four hours later I was watching the kiln approach its final temperature, it should have reached that hours ago, what is happening that the temperature won’t climb? Two, three, four in the morning. It was good I had gone paddling two days before. My neoprene gear still hung on the line, dry long ago & having had a whole day to bleach in the sun. Insufficient pressure to the burners? The bottom shelf too low in front of the flue? Finally I was within a hundred degrees of completion & after a semi-doze before dawn I went out to check, hearing a suspicious silence. Sure enough the thermocouple had slipped. It lost contact with its flame, and sent a distress signal to the gas valve and the gas valve, the wise superego of the kiln, closed. No more gas. The delicate chemistry of the molten clays and glazes is suspended — six weeks’ worth work may or may not make it to sunup.
I put on gloves and knelt down with the vice grips, extending my arm into the planetary glow of the underside of the two-thousand degree kiln, and tried to readjust. I pushed the red pilot button, same way you do on a space heater in an apartment rental. The burners relit. I watched the temperature climb back through all the twenty-one hundreds as the sun rose on my second night awake. Cone nine finally dropped and I gave up. I pushed the damper shut all the way, knowing this’ll make the kiln cool very slowly…whatever. This was supposed to be a cone ten firing…I had stopped caring.
In the end, the firing lasted twenty-nine hours. Here is the more technical section of this little writeup, I include it partly just for my own archive. I taught a day of classes minus two nights’ sleep on a Tuesday, and eagerly returned home to the new-old kiln. By now I had, like a boat or an old car, a name for it. I glanced at the temperature, cool enough to peek in? Five hundred? Four-fifty? That is cool enough. Right? I gingerly undid the clasps on the big metal door and swung it a little. With a flashlight I took a peek at the long-suffering wares in the chamber. Some looked alright! Open a little farther…hear that pinging and dinging of the cooling-down pots like a van pulling a trailer when you stop for gas.
Wednesday morning, and when I could swing the door wide I saw that the plates — all twelve of them to a one — were cracked. It was like someone had come through with a great pair of scissors and snipped them all exactly in the middle. They were dunted. Nothing else was - some pots looked good, some not good, only the plates though had been snipped. I consulted The Potter’s Dictionary of Materials and Techniques and began jotting notes:
The main causes of dunting during the cool-down phase of the firing are the two silica inversions which take place at 1063F and 439F, the level-headed authors recorded. All pots have stresses within them and most pots are strong enough to contain them the book said evenly. The few pots that crack do so because a number of factors come together: excessive stress, poor design, and perhaps imprudent firing. Geez, I thought. Isn’t that a little severe? Was the thermocouple my fault? I closed the book and felt like I had just left a therapist’s office, chastened, wiser. Here are the things that went wrong:
When a stoneware firing goes on too long, free silica in the claybody begins to form into cristobalite crystals. These begin to occur as the temperature passes two thousand degrees, and most potters move quickly from this to the final setpoint, and quickly back down. Cristobalite crystals fold up & shrink dramatically at four thirty-nine degrees, the ‘feared’ cristobalite inversion during which pots contract suddenly and if the foot & the rim can’t do this at the same time — they dunt.
Those thick old mullite shelves — they are a heat sink, and keep the foot of the pot from cooling in the cooling kiln, while the rim, perhaps of a wide flat plate, is already passing four thirty-nine into the safe zone. To really be safe, the foot and rim should pass through the inversion together.
Prudent potters keep the kiln door closed from five hundred degrees down to where they can handle the newly-glazed pieces without gloves.
Anytime you have thick glaze on one side of a big flat piece, and no glaze at all on the opposite side — you risk that excessive tension that is only resolved by cracking.
I slunk back to the Lena Street studio that morning, Wednesday, head hanging low. When you have a bad firing, one of our resident potters asked, after listening patiently as I indulged in a little story-telling about my difficult times, does that make you never want to fire the kiln again — or does that make you want to fire it again as soon as possible?
I raised my head & brightened a little. I didn’t have to think. I am grateful for the crew of resident potters & for our students! A lot of these questions you encounter as a potter are difficult & even though thousands of years of potters have encountered these same ones — many times they are daunting. Dunting. It is good to be able to talk them through, and here was one question that was easy.
I knew the answer right away.
- Theo Helmstadter
June 2021
