Kiln Firing Chart
Firing converts ceramic work from fragile greenware into a hard, permanent material. As the temperature in the kiln rises, the clay passes through a fixed sequence of physical and chemical changes — and most clay and glaze faults trace back to one of these stages. The chart maps temperature, approximate pyrometric cone, the incandescent color of the kiln interior, and what happens at each stage, from 100 to 1400 °C.
Mullite (an aluminosilicate crystal) and cristobalite (a crystalline form of silica) develop as the clay begins to convert to glass. Particles melt together, and the ware shrinks as it becomes denser. Soaking — holding the top temperature — increases the amount of fused material and the chemical action between fluxes and the more refractory materials.
Clay particles begin to cement themselves together into a hard but still porous material: bisque.
Carbonaceous materials — impurities in the clay along with paper, wax and binders — burn out. The kiln needs ample air through this stage: above about 800 °C the sintering surface starts to seal, trapping unburned carbon and sulfides, which later cause bloating and black coring.
Quartz crystals snap from their alpha (α) to their beta (β) structure with a small, abrupt change in volume, concentrated in a window of roughly 50 °C; the inversion reverses on cooling. Temperature changes must be slow through this point — on the way up and the way down — or the stress will crack the work.
Chemically combined water is driven off as steam (dehydroxylation). Past this stage the change is permanent — fired clay can never be slaked back to plastic clay.
Cristobalite — a crystalline silica that can form in clay bodies fired above about 1100 °C — contracts suddenly at 220 °C on the way down. Cooling too fast through this point makes the ware crack (dunting).
Water boils and converts to steam. Trapped water can make the ware explode, so keep the kiln below 100 °C until all of it has evaporated.
Cone values are approximate Orton equivalents for each temperature band. Cones measure heat work — the combined effect of time and temperature — so a cone's true bending point depends on the heating rate. °F values are exact conversions of the °C benchmarks.
Incandescence names the visible glow of the kiln interior, judged in low ambient light.