Pottery Firing

What is it?

Wet, plastic clay can be shaped by hand and then fired, irreversibly transforming it into hard, water-stable ceramic — humanity's first artificially made material.

What is it good for?

Vessels for storing water and food, cooking and baking ware, jugs, bowls, oil lamps, fermentation crocks — and the same knowledge underpins brick, roof tile, and oven daub. A porous (unglazed) earthenware jug even cools the water stored in it through evaporation.

The physics behind it

Clay consists of microscopic platelets of sheet-silicate minerals (clay minerals) that slide over one another in the presence of water — this is what gives it plasticity. The stages of firing:

• Drying: the physically bound water leaves and the vessel shrinks — this must be finished before firing.
• Around 100 °C (212 °F): any remaining moisture turns to steam; if the vessel was not bone dry or the heat-up is fast, the steam bursts it apart (the characteristic "pop" in the fire).
• Roughly 450–600 °C (840–1,110 °F): the chemically bound water also leaves the clay minerals (dehydroxylation) — this is the irreversible transformation into ceramic: from this point on, the material no longer slakes apart in water.
• 573 °C (1,063 °F): the sudden volume change of quartz grains (the quartz inversion) — a crack-prone transition on both heating and cooling.
• At higher temperatures the grains fuse together with melted bridges (sintering), and the vessel becomes ever denser and stronger.

A pit fire typically reaches 700–900 °C (1,290–1,650 °F) — yielding unglazed, porous, but durable earthenware.

History

The earliest known ceramic objects are fired clay figurines (Dolní Věstonice, roughly 26,000-30,000 years old); the oldest pottery vessels come from East Asia (roughly 15,000-20,000 years old) — pottery firing is thus older than agriculture. It became a foundational technology of every settled culture: storage, cooking, and fermentation depended on it. The potter's wheel appeared in Mesopotamia (4th millennium BCE), glazes and high-fire kilns later — but pit firing remains a working, zero-infrastructure method to this day.

Simple version

Pit firing: a bed of embers at the bottom of a shallow pit; on it, the bone-dry vessels, pre-warmed beside a fire; around and above them, layered fuel (wood, straw, dry dung). Let the fire burn through gradually, then let the whole thing cool completely undisturbed.

Advanced version

A bonfire (clamp) firing covered with potsherds or a layer of dung (more even heat, less thermal shock), then a simple updraft kiln: a firebox below, the vessels on a grate above it, and a chimney-like opening on top — higher and better-controlled temperatures. Vessels packed in saggars to shield them from direct flame licking; burnishing (polishing at the leather-hard stage) or engobe to improve water tightness; a smoky reduction phase for blackware.

Industrial version

Gas and electric kilns with programmed firing curves, monitored with pyrometric cones or thermocouples; glazed, twice-fired ware (bisque firing + glaze firing); continuously operating tunnel kilns; tightly controlled sintering of technical ceramics.

Building your own

1. Sourcing and cleaning the clay: soak clayey soil, then levigate it (gravel settles out; the fine clay slip can be poured off and thickened), and let it rest.
2. Tempering: mix sharp sand or crushed pottery into fat clay (clay that is too plastic and shrinks heavily).
3. Shaping: pinch pots, coil building (stacked and smoothed clay coils) — with even, not-too-thick walls, kneading out any air pockets.
4. Drying: slowly, in a spot away from drafts and sun, until bone dry (a bone-dry vessel no longer feels cold to the touch) — this can take 1-2 weeks.
5. Pre-warming: warm the vessels near the fire, turning them.
6. Firing: place on the ember bed, layer the fuel, heat up slowly, let it burn through completely, then cool undisturbed (best left until the next day).

Common mistakes

• A vessel that is not bone dry goes into the fire → steam explosion, it shatters
• Heating or cooling too fast (thermal shock, quartz inversion) → cracks
• Fat clay without temper → cracking during drying and firing
• Uneven wall thickness or trapped air pockets → stress, breakage
• Pulling vessels out hot, or cold wind/rain hitting the hot pit
• Impatient drying in the sun or beside a stove → the surface shrinks before the core does, and it cracks

How to measure

• Clay quality: the knead/coil test (a coil bent around a finger doesn't crack), and firing a test tile before risking the real vessels
• Drying shrinkage: scribe a known distance on a wet test tile and re-measure it when dry
• Temperature in the fire: glow color is a rough indicator (a dull red glow from about 600 °C / 1,110 °F, bright orange around 900 °C / 1,650 °F); pyrometric cones are more accurate
• Finished vessel: the ring test (well-fired pottery rings when tapped; poorly fired ware gives a dull thud), and a water-seepage test

Videos

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Downloadable PDF

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