A kiln is a controlled, insulated box that reaches over a thousand degrees and, crucially, comes down again on a schedule. Both halves matter: the temperature turns clay into ceramic, and the controlled cooling is what determines glaze surface, crystal growth and whether the ware survives at all.
What actually happens inside
Below about 600 degrees chemically combined water leaves the clay, which is why the early part of a firing is slow. Above that the body begins to sinter, and by stoneware temperature the particles have fused and the body is dense and largely non-porous. Glaze, fired separately or in the same schedule, melts into glass over that surface. None of this is reversible and none of it happens in an oven.
Bisque and glaze, the two firings
Most work is fired twice. The bisque firing turns fragile greenware into a porous, handleable body that takes glaze evenly. The glaze firing melts the glaze and matures the body. Single firing is possible and is harder, because raw glaze on unfired clay is fragile and the two processes want different schedules.
The sizes that exist
Test kilns of a few litres that plug into a normal socket; small plug-in kilns that will fire a shelf of mugs; studio kilns needing their own circuit; and larger kilns needing three-phase supply. The electrical requirement rather than the chamber volume is what most often decides which you can have, which is why it is the first question on every kiln page here.
Running one safely
A kiln makes a room hot, gives off fumes during firing, and must not be in a cupboard, a bedroom or against combustible material. It needs clearance, ventilation and a floor that can take the weight. None of that is unusual, and all of it is easier to arrange before the kiln arrives than afterwards.
Questions people ask about kilns
What is a kiln for pottery?
An insulated chamber that heats work above a thousand degrees on a controlled schedule and cools it under control, turning clay into ceramic and melting glaze into glass.
Why fire twice?
Bisque firing makes fragile greenware handleable and porous so it takes glaze evenly; the glaze firing melts the glaze and matures the body.
Can a home oven fire pottery?
No. Domestic ovens reach a fraction of the temperature needed, and nothing about the chemistry happens below it.