Pottery and the first kiln
Make a fire-proof, waterproof, permanent container — and learn to control temperature.
Everything for this step
- Field guide
- Clay and firing → 7 sections
- Builds
- Updraught kiln
- Reference sheets
- None.
- Track
- Ceramics and glass, level 2 of 4 · next: Brick and tile
Why this matters
Pottery is far more important than pots. Building a kiln is your first exercise in reaching and holding a target temperature, and that skill — not the vessels — is what leads to lime, glass, and metal. Every high-temperature step above this one is a descendant of the first kiln.
The shape of it
- Find and test clay. Riverbanks, cut banks, the base of eroding slopes. Roll a coil and bend it around your finger: if it holds without cracking, it is workable. Settle a sample in water to see the sand-to-clay ratio.
- Prepare it: soak, remove stones and roots, then wedge it thoroughly to drive out air. Air pockets turn into steam and blow the pot apart in the fire.
- Temper it. Add sand, crushed fired pot (grog) or ash — roughly a fifth to a third by volume. Untempered clay shrinks unevenly and cracks. This one step separates most failures from most successes.
- Form by pinch, coil or slab. A wheel comes later and is not needed.
- Dry slowly and completely — a week or more, out of sun and wind, then warm near the fire. Any water left in the wall will flash to steam and destroy the pot.
- Pit-fire first: pots in a pit, surrounded by fuel, burnt slowly with a long pre-warming period, then covered to hold heat. Expect to lose most of the first firing and learn from the breaks.
- Then build a real kiln — an updraught chamber with a firebox below and a flue above, so flame passes the ware and heat is retained rather than radiated away. This is the structure you will later adapt for lime and charcoal.
- Glaze later: wood ash and clay slip give a simple functional glaze at high temperature; salt thrown into the kiln gives another. Avoid lead glazes on anything used for food or drink.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 · Find clay and measure how much it shrinks
Look at riverbanks, cut banks and the base of eroding slopes. Tests, in order:
- Coil test: roll a coil the thickness of a pencil and wrap it round your finger. Holds without cracking — workable. Cracks — too sandy or too short.
- Jar test: settle a sample in water overnight to see sand versus clay.
- Shrinkage bar: make a bar, scratch two marks exactly 10 cm apart, dry it, fire it, and measure again. Total shrinkage of about 5–12% is usable. Over about 15%, the clay needs more temper or it will crack in drying and firing.
Keep a record of which bank gave which clay.
2 · Clean and wedge it until no air remains
Dry the raw clay, break it up, and slake it in water until it is a slurry. Pour it through a sieve or cloth, or let stones and roots settle and pour off the clay layer. Dry it on a porous surface — plaster, brick, cloth over sand — to a stiff plastic state.
Then wedge: fold, press and turn the lump repeatedly, driving out air. Air pockets turn to steam in the fire and blow the pot apart, often taking the neighbouring pots with it.
3 · Temper it — the step that separates most failures from most successes
Add sand, crushed fired pot (grog) or crushed shell, sieved to about 0.5–2 mm, at roughly a fifth to a third by volume. Temper opens the body so water leaves evenly and the clay shrinks less, and it stops cracks running through the wall.
Cooking pots need a coarser, more open body that survives thermal shock. Shell or limestone temper must be fired below about 750–800 °C — above that it turns to quicklime, which later swells with moisture and spalls pieces off the pot.
4 · Form walls of even thickness
Pinch, coil or slab — a wheel comes later and is not needed. The single rule is even thickness: walls around 6–10 mm, and a base no thicker than the walls. A thick spot dries last and fires unevenly, and that is where it cracks.
When joining coils or slabs, score both surfaces and add slip (liquid clay), then smooth the join through on both faces. A join that is not worked through is a crack waiting for the fire. Burnish the surface with a smooth pebble when leather-hard to tighten it.
5 · Dry slowly and completely — this is not optional
Dry out of sun and wind for 1–2 weeks: a pot drying faster on one side warps and cracks. Turn pots occasionally. Once leather-hard, cover rims with cloth for a day so they do not dry ahead of the base.
The test: hold the pot against your cheek or the inside of your wrist. If it feels cool, there is still water in it — evaporation is cooling it. Bone-dry clay feels room temperature. Then warm the pots near the fire for a day before firing. Any water left in the wall flashes to steam and destroys the pot.
6 · Pit-fire first, with a long, slow warm-up
Line a shallow pit with dry fuel, set the pre-warmed pots in, and build the fire up slowly around them over 2–3 hours before putting fuel on top. The early stage — up to about 200 °C — drives out the last water and must be slow.
Bring it to red heat, about 600–900 °C, for an hour or more, then cover with earth, sherds or green material to hold heat and cool slowly overnight. Do not uncover until the pots can be touched. Expect to lose most of the first firing, and examine every break — the fracture tells you whether it was damp, air, temper or shock.
7 · Build an updraught kiln and control the heating rate
A firebox below, a chamber with a perforated floor, and a flue above — so flame passes the ware and heat is held rather than radiated. See the updraught kiln.
Heat slowly to about 200 °C — around 50–100 °C an hour — then faster to 900–1050 °C for earthenware. Read temperature by colour in a dim light: dull red about 600 °C, cherry red about 750–800 °C, orange about 900–1000 °C, yellow about 1100 °C. Hold at top temperature, then seal the kiln and cool for at least 12 hours. Do not open above about 200 °C, and pass slowly through 573 °C on the way down, where quartz in the body changes size abruptly.
8 · Test for water-tightness, and keep lead off food
Ring test: tap the fired pot — a clear ring means sound; a dull thud means a crack. Water test: fill it and stand it on dry paper overnight. Earthenware fired around 1000 °C is porous and sweats; seal the inside by burnishing before firing, or by boiling milk or flour-and-water in it, which clogs the pores.
Ash glazes need around 1200 °C and above — stoneware temperatures — to melt properly. Salt glazing gives off hydrogen chloride fumes; do it outdoors and upwind. See the clay guide.
Build it
Dimensioned drawings, bills of materials and construction procedures for the things this step asks for.