How One Ceramic Studio Tests Glazes at Volcanic Temperatures

At the Archie Bray Foundation in Helena, Montana, resident artists fire ceramics in a wood-burning anagama kiln reaching temperatures exceeding thirteen hundred degrees Celsius, comparable to the base of a lava flow. At these temperatures, conventional glaze chemistry gives way to physics of molten rock, producing effects no laboratory furnace can predictably replicate.

The Bray's own provenance is part of the appeal. Archie Bray ran the Western Clay Manufacturing Company in Helena and set up the foundation on the brickyard in 1951; Peter Voulkos and Rudy Autio were among its first resident artists, and the beehive brick kilns still stand on the site. The kiln type is older and travelled further, the anagama reaching Japan from Korea around the fifth century, with the multi-chamber noborigama a much later refinement.

The anagama kiln, a Japanese design with fifth-century origins, is a single-chamber tunnel fired exclusively with wood. Stoking continues around the clock for five to seven days, with teams feeding wood at intervals of five to ten minutes. The labour is enormous, making each firing a communal event bonding participating artists.

At peak temperature, wood ash carried by the flame melts onto pottery surfaces creating natural ash glaze. The composition depends on wood species, kiln atmosphere, and the pot's position relative to flame. Pieces near the firebox receive heavy deposits flowing and pooling like volcanic glass.

Potters talk less about temperature than about heat work, which is time and temperature together. It is read from Orton pyrometric cones set behind the spy holes: cone ten, the usual target, bends near thirteen hundred degrees, but a slow firing arrives there cooler and a fast one hotter, and only the cone knows. Pots are set on wadding, a stiff mix of alumina and kaolin, or on seashells, so that the melting ash does not glue them to the shelf.

The kiln atmosphere fluctuates between oxidation and reduction as fuel is added and burns. This produces colours impossible in electrically fired kilns. Iron-bearing clays shift through a spectrum from red to brown to black, sometimes within a single vessel, each shift recording a moment in the firing.

Cooling is as important as firing. The anagama takes five to seven days to cool. During extended cooling, crystalline structures develop within the glaze, producing textures from rough crusts to smooth pools. The slowest-cooling areas produce the most complex and desirable surfaces.

The arithmetic explains why these kilns belong to institutions. A firing burns several cords of split wood, cut and stacked a year ahead, and needs a crew on four-hour shifts around the clock for the better part of a week. Losses are part of it, since pots stick, crack during cooling or emerge underfired at the back. Britain's equivalent is Svend Bayer, firing a large wood kiln in north Devon for decades; Jack Troy's Wood-Fired Stoneware and Porcelain remains the standard account.

To experience wood firing, attend an anagama kiln opening at a ceramic art centre. The moment the kiln door is removed and first pieces revealed, still warm, surfaces recording days of fire in geological detail, is one of the most exciting events in contemporary craft. Follow at https://www.archiebray.org