Glaze and Fire
Why a Kiln Firing Takes Two Days
Nothing in a kiln is in a hurry. Every stage of the schedule exists to let a specific physical or chemical change finish before the next one begins.
A kiln capable of reaching stoneware temperature could get there in a few hours if it were simply switched on. Firings are not run that way. A typical schedule climbs slowly, pauses, climbs again, and then cools for as long as it took to heat. The length is not caution for its own sake; each phase is waiting for something specific to finish.
Drying out the last of the water
Clay that feels bone dry still holds water inside the pore structure. If the kiln is taken above the boiling point of water before that has left, the water turns to steam inside a closed body and the piece breaks, sometimes violently enough to damage its neighbours.
The opening hours of a firing are therefore held low, often under 120 degrees, with the kiln vented so the moisture has somewhere to go. Thick pieces and anything with a heavy base get longer. This stage is entirely about patience and is the cheapest failure to avoid.
Burning out what is in the clay
Between roughly 300 and 900 degrees, the organic material that came with the clay burns away and a series of chemical changes take place in the body itself. Chemically combined water is driven out of the clay minerals, a change that is permanent and is the point at which clay stops being clay and becomes ceramic. Carbon and sulphur compounds oxidise and leave as gas.
These reactions need oxygen and they need time. A kiln pushed quickly through this range traps carbon inside the body, which shows later as a grey core in a broken section and, in the worst case, as bloating when trapped gas expands at top temperature.
Almost every serious firing defect is a gas that was still trying to leave when the glaze sealed the surface over it.
The quartz inversion
At about 573 degrees the quartz in the body abruptly changes crystal form and, in doing so, changes volume. The change is small but it is sudden, and it happens on the way up and again on the way down.
A kiln is slowed through this band in both directions. Uneven heating across a thick wall while the material is changing size is a reliable way to crack a pot, and the cooling pass is the more dangerous of the two because there is a temptation to open the kiln early.
Reaching maturity
Above 1000 degrees the work of the firing finally begins. Fluxes start to melt, the glaze goes through a sticky stage and then flows, and the body vitrifies. What matters here is not the peak number on a controller but the combination of temperature and time, because these reactions are cumulative.
Potters measure this with pyrometric cones, small angled forms made of ceramic material blended to bend at a specific heat and duration. A cone that has bent until its tip touches the shelf has recorded the total heat work the kiln delivered, which is the quantity that actually matters. A thermocouple reading alone can be misleading; a kiln that limps to temperature slowly delivers more heat work than one that races there.
Cooling, which is part of the firing
When the top temperature is reached the kiln is switched off, and the second half of the process begins. The rate at which a glaze cools determines whether it stays glossy or develops the microscopic crystals that produce a matt or crystalline surface, so the cooling curve is a working part of the recipe rather than dead time.
The body is also still moving. It passes back down through the quartz inversion, and it has to do so slowly and evenly. Opening a kiln at 300 degrees to save time introduces cold air against hot pots and can produce a fine network of cracks, or a clean split through a heavy piece, on work that had survived everything else.
Most schedules leave the kiln closed until it can be touched comfortably.
What a two-day schedule buys
Spread over that time, the failures that remain are mostly failures of glaze fit or of packing rather than failures of the firing. Pots come out sound, glazes come out where the recipe intended, and the kiln furniture survives to be used again.
It is a slow process because it is a sequence of irreversible changes, each of which has to complete. There is no stage that can be repeated if it is rushed.