Firing and Kilns
Firing is the stage that makes pottery permanent. Heat drives off the remaining water and then triggers chemical and physical changes that convert a fragile dried object into hardened ceramic. The change is irreversible: a fired pot can never be returned to plastic clay by adding water. Because the transformation is one-way, firing rewards patience and a clear understanding of what the heat is doing at each point.
Two firings, two jobs
Most studio work is fired at least twice, and the two firings do different things.
- Bisque firing. The first firing hardens the dried clay enough to handle and glaze without it dissolving, while keeping it porous so it absorbs glaze. It also burns away organic matter in the body.
- Glaze firing. A later, usually hotter firing melts the applied glaze into a glassy surface and brings the clay body to its mature, dense state.
What heat does to the clay
As temperature climbs, the clay body passes through changes that have to happen in order. Chemically combined water is driven out gradually. At higher temperatures the body begins to vitrify, meaning particles partly fuse and the pot becomes harder and less porous. Pushing a body well past its maturing temperature can cause it to deform or bloat, which is why matching the firing to the clay's data sheet matters.
Firing schedules ramp temperature slowly rather than as fast as possible. Rapid heating can trap escaping water or gas inside a wall and crack it, and uneven heating creates stress between hot and cool regions. A controlled climb gives the whole piece time to change together.
Kiln types in brief
Kilns are grouped largely by their heat source. Electric kilns are common in studios and schools because they are straightforward to control. Gas and wood-fired kilns introduce a flame and its atmosphere, which can change how glazes and clay surfaces develop. The principle is shared across all of them: deliver heat in a controlled, even way and let it climb and fall on a planned schedule.
Kilns reach high temperatures and require correct ventilation and clearances. Follow the kiln manufacturer's manual and any local electrical, fire, and ventilation regulations. The guidance here is general and educational, not a substitute for those instructions.
Firing completes the cycle that begins with a soft, workable lump. To revisit how that lump becomes a vessel, see Forming Clay on the Wheel, and for the stage that must be finished before any firing begins, see Drying Ceramic Pieces.