Almost every piece of glazed pottery is fired twice. The first firing — the bisque — hardens raw clay into durable, porous ceramic. The second — the glaze firing — melts the glaze into a glassy surface. Knowing what each firing does, and why bisque comes first, is the foundation of reliable results.
Open KilnLog — log both firings free →Bisque firing takes bone-dry greenware and converts it permanently into ceramic. As the kiln climbs, chemically bound water and organic matter burn off, and around 573 °C (1063 °F) the clay passes through quartz inversion. By the end the piece is hard, easy to handle, and still porous enough to soak up glaze.
Most studios bisque to cone 04 (about 1060 °C / 1940 °F) or cone 06 (around 1000 °C / 1830 °F). A slightly hotter bisque (04) burns out more carbon and sulfur, which helps prevent glaze defects later. Bisque is almost always an oxidation firing with good air flow.
The glaze firing is usually hotter and fires to the maturing temperature of your clay body and glaze — commonly cone 6 (about 1222 °C / 2232 °F) for mid-range stoneware, or cone 10 (around 1285 °C / 2345 °F) for high-fire work. Earthenware glazes mature lower, near cone 04–06. At peak heat the glaze materials melt and fuse to the clay, then cool into a hard, often vitreous surface.
You can single-fire (glaze raw greenware and fire once), but bisquing first solves real problems:
Because the two firings do different jobs, it pays to track them separately: the cone and ramp you bisqued at, and the cone, schedule and atmosphere of the glaze firing. Over time you'll see exactly which combinations give clean glazes and which cause pinholing — and you'll be able to repeat the good ones.
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