Firing and Kiln Operation
Firing turns clay and glaze into ceramic. The exam tests schedules, kiln types, heat work and the faults caused by firing too fast or reading the wrong instrument.
Temperature stages
- 100–200 °C, water smoking - free and adsorbed water leaves. Go slowly.
- 500–600 °C, quartz inversion - quartz changes crystal form at 573 °C with a volume change; keep below about 80–100 °C/h or expect dunting.
- 300–700 °C, carbon burnout - needs oxygen, so keep the kiln in oxidation.
- 900 °C and above - sintering and vitrification; the body densifies and absorption falls.
- Typical: bisque 950–1000 °C, earthenware glaze 1020–1100 °C, stoneware 1200–1280 °C.
Heat work and cones
Pyrometric cones measure heat work - temperature plus time. A slow firing bends a cone at a lower indicated temperature than a fast one. Cone 06 is roughly 1000 °C, cone 04 about 1060 °C, cone 6 about 1220 °C, cone 10 about 1300 °C at standard rates; treat these as approximate.
Kiln types
Updraught and downdraught fuel kilns, electric (resistance elements, clean oxidation), gas, wood and raku. Electric kilns cannot easily reduce; gas and wood can.
Worked example
A kiln reaches 1000 °C in 5 hours instead of 9. The faster firing has done less heat work, so a cone 04 pack may not bend even though the thermocouple reads 1060 °C. The fix is to fire to the cone, not to a number, and to log both time and temperature.
Traps
- Opening the kiln above about 200 °C - thermal shock cracks the ware.
- Reducing too early, trapping carbon in the body (black coring).
- Leaving a kiln firing unattended with no alarm.
- Treating a thermocouple reading as if it were a cone.