A kiln is a thermally insulated chamber used to fire clay at temperatures high enough to vitrify the clay body and melt any applied glaze. The type of kiln determines not just the maximum temperature achievable but the firing atmosphere — whether oxygen is abundant, restricted, or absent — and that atmosphere fundamentally changes the colour, texture, and surface character of the finished piece.
Canadian studio potters work with four principal kiln types: electric, gas, wood-fired, and raku. Each has a distinct operating profile, and the choice between them is rarely about convenience alone.
Electric Kilns
Electric kilns fire in an oxidation atmosphere because no fuel is burned inside the chamber. Combustion products, carbon monoxide, and the reduction of oxygen are absent. The result is colour predictability: copper glazes fire green, cobalt fires blue, iron fires warm amber or brown. For potters who want consistent, repeatable results from batch to batch, electric kilns provide the tightest control.
Standard front-loading studio electrics in Canada operate at 208–240V and reach cone 6 (1222°C) or cone 10 (1285°C) depending on the element type and kiln size. Bartlett Genesis and similar digital controllers allow precise ramp-and-hold firing schedules to be programmed in advance — reducing the risk of thermal shock in thick-walled pieces by slowing the rate of temperature change at critical transitions.
The principal limitation of electric firing is the lack of atmospheric variation. The visual effects possible in reduction — iron-saturate glazes breaking from amber to black, copper reds, carbon trapping — are not achievable in a standard electric kiln without modification.
Gas Kilns
Gas kilns fire using natural gas or propane and produce heat through combustion rather than resistance. This combustion can be managed to control the oxygen-to-fuel ratio inside the kiln chamber. When excess fuel is present, the flame scavenges oxygen from iron and copper compounds in glazes and clay bodies, producing the characteristic colour shifts associated with reduction firing.
Reduction atmosphere converts iron from its oxidised (Fe³⁺) ferric form to its reduced (Fe²⁺) ferrous form, which produces greens and greys in iron-bearing glazes rather than the ambers of oxidation. Copper in reduction produces deep oxblood reds — a result impossible to replicate without an atmospheric shift.
Canadian studio potters firing gas typically use downdraft kilns, where exhaust exits through a flue at the base rather than the top. This produces a longer flame path and more even heat distribution across the kiln load. Updraft designs are still found in older facilities but are less common in new studio builds.
Wood-Fired Kilns
Wood firing is the most labour-intensive kiln method and the most variable in outcome. A single firing of a large anagama (tunnel kiln) or noborigama (climbing kiln) in Canada may require 24 to 72 hours of continuous stoking by a rotating team and consume one to four cords of wood. The reward is a surface quality achievable no other way: ash from the fire deposits on the ware as it travels through the kiln, melting into natural glaze effects whose distribution cannot be precisely predicted or controlled.
The flame path in a wood kiln is also more direct than in gas firing, and the movement of ash, carbon, and particulate matter creates a narrative on the surface of each piece — thicker ash deposits on the side facing the firebox, charcoal flashing on shoulders, subtle colour gradients from front to back of the stack. These effects, collectively called "flame marks" or "natural ash glaze," are valued precisely because they document the firing event.
Several Canadian pottery communities maintain communal wood kilns available by application, including groups in Nova Scotia, Ontario's Haliburton Highlands, and the Gulf Islands of British Columbia.
Raku Firing
Raku is not defined by a kiln design so much as by a firing process. Pieces are placed in a small gas kiln and brought rapidly to around 950–1000°C — much lower than stoneware temperatures. At temperature, the glowing pieces are removed from the kiln with tongs and placed in a metal container filled with combustible material (newspaper, straw, or sawdust). The container is sealed, the combustibles ignite from the heat of the clay, and oxygen is rapidly depleted, creating a localised reduction environment.
The thermal shock of removing the piece at temperature causes cracking in any unglazed areas — producing the characteristic crackle pattern associated with raku. Carbon from the combustible material is drawn into these cracks and into any unglazed clay, producing a matte black or grey surface alongside the often iridescent, metallic glaze effects.
Because raku pieces are fired at lower temperatures, the clay body is not fully vitrified and the glaze is not food-safe. Raku is primarily used for sculptural and decorative work rather than functional tableware.
Temperature Reference
- Cone 06 (~998°C): Low-fire earthenware, raku
- Cone 6 (~1222°C): Mid-fire stoneware, most studio electric firing
- Cone 10 (~1285°C): High-fire stoneware, most gas and wood firing
- Cone 13 (~1335°C): High-fire porcelain
The Orton Cone Chart, maintained by the Edward Orton Jr. Ceramic Foundation, provides the complete temperature-to-cone reference used by Canadian potters and kiln manufacturers.
Bisque and Glaze Firings
Most ceramic work goes through two firings. The first — bisque firing — takes the raw, dry clay to around cone 06 (998°C). This burns off organic material, eliminates chemical water from the clay minerals, and produces a porous, hard enough to handle but still absorbent enough to accept water-based glazes. The second — glaze firing — takes the bisqueware to the full maturation temperature of the clay body.
Some potters fire only once (single-firing or raw glazing), applying glaze to unfired greenware. This requires careful glaze application to avoid the glaze absorbing too much moisture and causing the piece to collapse, and precise kiln control to manage the simultaneous completion of clay body and glaze maturation. Single-firing is more common in industrial ceramics than in studio practice.
Related Articles
Hand-Building Techniques for Beginner Potters — constructing the clay forms that go into the kiln.
Glaze Chemistry and Surface Finishes — how glaze interacts with the kiln atmosphere.