Drying and Firing

Clay must dry before a first firing. Water leaves the piece in stages; firing then drives off remaining water chemically and densifies the clay. Exact cone numbers belong to your clay and kiln manufacturer — this page only repeats ranges published by museums and college ceramics programs.

Moisture stages

Colorado State University Extension lists common stages: slip (liquid clay), wet workable clay, soft leather-hard, leather-hard, stiff leather-hard, dry, and bone dry. Bone-dry clay has no cool dampness left and is ready for bisque or for underglaze on greenware when that is the method. Dry slowly under loose plastic at first so thick and thin parts shrink together; uneven drying causes cracks.

Bisque firing

Bisque (biscuit) is a first firing that hardens clay enough to handle for glazing. Bellevue College’s firing chart notes that normal or soft bisque for high-firing stoneware and porcelain often falls around cone 08 to 04, while some low-fire white and terra cotta bodies are bisqued higher (the chart lists cone 02 to 1 for those cases). Always use the schedule for your clay.

Glaze and clay maturity ranges (general)

USC Pacific Asia Museum places earthenware roughly 650–1000°C, stoneware roughly 1150–1300°C, and porcelain above about 1300°C. Bellevue College’s cone chart shows mid-range stoneware and porcelain often maturing around cones 4–6, and high-fire work near cone 10, with the warning that low-fire clays will melt if taken into stoneware ranges. Cone temperature equivalents depend on heating rate; the Bellevue chart states its numbers assume about 270°F (150°C) per hour with a large cone.

Kilns

Electric kilns commonly fire in oxidation. Fuel kilns can be run for reduction atmospheres that change clay and glaze color. This site does not publish firing programs. Read kiln manuals, use kiln wash and stilts as directed, and never fire closed hollow forms without a vent hole.

Sources

• Colorado State University Extension, Ceramics Unit 1 (PDF)
• Bellevue College, Firing Temperatures & Cone Equivalents (PDF)
• USC Pacific Asia Museum, Kinds of Ceramics