The biggest misunderstanding about the wood-fired Anagama tunnel kiln, according to Swanville potter Jody Johnstone, is that the appearance of the final product is magically unpredictable.

“It’s very tempting, and a lot of articles say you surrender it all to the kiln gods,” she said, making a gesture that conveyed humor and a certain amount of exasperation.

A week earlier, Johnstone and three fellow potters finished firing their work in the only kiln of its kind in the state of Maine — one that Johnstone herself built 15 years ago and has fired twice a year since.

The 24-foot long kiln takes a week to load, with each piece placed to give a specific effect. The firing process takes eight days, during which the fires must be tended 24 hours a day with the temperature carefully monitored on a digital pyrometer.

The temptation to romanticize the process is understandable. Wood-firing creates ash that gets distributed throughout the kiln during the firing process, accumulating on the shoulders of pots and vases and dripping down the sides. The clay discolors in places, glazes crackle, pots stacked in certain parts of the kiln become buried in ash, and while the results have an irreproducible organic appearance, Johnstone said she basically knows what to expect.

Prior to moving to Maine, Johnstone apprenticed in Bizen, Japan, with National Living Treasure Jun Isezaki, who told her that if you don’t know what you’re going to get, you don’t know what you’re doing. The process is, after all, well tested. Anagama kilns have been used in Korea and Japan for more than 1,000 years.

The exterior of Johnstone’s kiln, built into a hillside near her house, resembles a sort of Neolithic submarine, partially submerged. Square holes, used for “side stoking” the fire, appear along the length of the kiln, into which several wires disappear: the pyrometer probe — its digital display attached to one of the rafters of the roof above the kiln — and when the kiln is cool, utility lamps to illuminate the dark interior.

The structure of the kiln, visible from the inside, is an elegant mortarless vault of red brick, accessed through an arched doorway at one end. The floor starts out high enough for a child to stand and shrinks by a series of steps leading toward the chimney at the far end.

Johnstone fires her wares twice each year, in the spring and fall, inviting three other potters to fire their work in exchange for helping with the firing process. All three of her helpers this year — Betsy Levine, David Orser and Ellen Sedgwick — had helped with past firings.

The kiln can hold upward of 800 pieces, of which Johnstone said roughly two-thirds are hers. During the firing, the crew members take shifts stoking the fire, day and night. Over the course of the week, they push the internal temperature from what Johnstone described as a “campfire” — tended for the first two days to dry out the kiln — to a white-hot 2350 degrees Fahrenheit, the temperature at which the clay vitrifies.

In the final stages the crew feeds wood into the side-stoking holes at a rate of 15 logs every 15 minutes, to bring the rear of the kiln up to the temperature of the front, all while keeping a watchful eye on the pyrometer.

The kiln must cool for a week, after which the crew removes the pots, sometimes from under ash that has accumulated to a depth of more than a foot. Pots that emerge from the ash bear a certain blackened look. Those that are near the firebox at the front of the kiln have a more dramatic appearance than those in the back, which tend to have a more subtle finish.

When the fire is at its hottest, a 10-foot flame blasts from the top of the 15-foot-high chimney at the rear of the kiln and the attendants must wear special glasses to avoid damaging their vision when looking into the kiln through a small opening above the firebox.

Direct exposure to the flame gives the firebox side of the pot a different appearance than the chimney side. Johnstone held up a medium-sized vase upon which ash had coated the top and dripped down either side. The pattern of the ash seemed arbitrary, but she said the uncoated area between the two drips occurred because a smaller piece — possibly a cup — was between the vase and the flame, causing a kind of shadow. In this way, it becomes possible to “paint” with the arrangement of the pieces on the shelves, she said.

Johnstone’s pottery bears the appearance of work produced in the Bizen region of Japan where she studied, though many of the designs are her own. Typically, she includes a few experiments in each firing — a new clay body, a slightly different shape — but much of the work remains similar from season to season, something Johnstone credits to the fact that she makes her living from her work.

During the firing process, even less is left to chance.

Johnstone offers her wares at her Kiln Opening Show and Sale, June 19-20, 9 a.m. to 4 p.m. 135 Webster Road, Swanville.