Two Firings, Two Jobs
Most studio ceramics passes through the kiln twice. The two firings do different things.
Bisque firing converts raw clay to ceramic. It burns out organics and drives off chemical water. This leaves a porous body strong enough to handle and glaze. Its typical range is 950 to 1000 °C (950, 1000).
Glaze firing melts the glaze into glass fused to the body. It also vitrifies the clay. The temperature depends entirely on the clay body:
| Body | Typical glaze firing | Result |
|---|---|---|
| Earthenware | 1000–1100 °C | Porous body; glaze provides the seal |
| Stoneware | 1200–1300 °C | Vitrified, strong, waterproof |
| Porcelain | 1280–1400 °C | Vitrified, translucent when thin |
The first firing is usually the cooler one. Bisque only needs to make ware porous. Glaze firings must melt glass. Bisquing too high prevents the body from absorbing glaze properly.
Schedule, Cones, and Atmosphere
A kiln firing is a schedule, not a target. Three points need care:
- Below 200 °C: physical water becomes steam. Firing too fast breaks damp ware.
- Around 573 °C: quartz inversion. Silica suddenly changes volume. Rushing through 573 °C cracks work on the way up and on cooling.
- Near the top: soaking evens out heat and matures the glaze. Ceramics responds to heat work—temperature over time—not peak temperature alone.
Pyrometric cones measure actual heat work. These calibrated pillars bend at specific heat work values, unlike single-point thermocouples.
Kiln atmosphere also alters results:
- Oxidation: plentiful oxygen. Standard in electric kilns; colours stay predictable.
- Reduction: oxygen-starved. Fuel flames pull oxygen from the clay and glaze, shifting colours.
A glaze recipe only gives a colour under a stated atmosphere and temperature (between 300 °C and peak).
Common pitfall: treating a kiln like an oven with a single target temperature. Schedule, atmosphere, and heat work determine the final piece.