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Pottery

Clay turned to stone by fire

Date
18,000 years ago
Era
Prehistory
Rebuild stage
II Settlement
Difficulty

Plate

Legend
  1. 1Clay coils
  2. 2Turntable
  3. 3Firebox
  4. 4Perforated floor
  5. 5Pots in the firing chamber
  6. 6Vent

Anatomy

The cut jar: its wall turns from raw clay to fired ceramic; the dark line inside it stands for carbon not yet burnt outPots stacked on the perforated floor; they glow once the kiln is hot enoughKiln cut through its stoke hole: firebox, pillar, perforated floor, chamber and ventStones at the foot, firewood, and the next pots drying on a plankWood burns in the firebox under the floor, fed through the stoke hole on the right. The hot gases rise through the holes in the floor, pass round the pots in the chamber and leave by the vent at the top, so the pots never sit in the fuel. A firing lasts many hours and the heat is raised slowly at first. Up to about 100 °C the last water in the pores dries out, and until about 250 °C it leaves as steam; a pot heated too fast can burst. Organic matter in the clay then chars and darkens the body. Between about 450 and 650 °C, depending on the clay, the water bound inside the clay crystals is driven off, and the clay can never be softened in water again. The kiln starts to glow a very dark red at about 525 °C. At 573 °C the quartz grains in the body grow by about 1 % in volume, one more reason to heat gently. The carbon left in the clay burns away from both surfaces of the wall toward the middle, which is why a pot fired too briefly keeps a black core. Iron that the air has not yet reached darkens it too. With enough air and time the core has gone by about 900 °C. Between about 700 and 900 °C, depending on the clay, the grains start to join where they touch and the pot becomes hard earthenware; above that the body begins to shrink as the first glass forms. Above about 982 °C the glow turns orange. The drawing only shows the glow growing stronger; the readout names its colour. The scale stops at 1,050 °C, about as hot as a wood-fired kiln of this kind gets. Every clay behaves a little differently, and the longer it is held at a temperature, the further each change goes. The control follows one first firing: moving it back returns to an earlier moment of that firing, because none of these changes undo themselves when the kiln cools.

Pottery is clay shaped while wet and then fired until it changes for good into a hard, stone-like material. Foragers in southern China were making clay pots, probably for cooking, about 20,000 years ago, long before farming.

What it is

Pottery covers every object made of clay and hardened by fire, from cooking pots and jars to lamps and roof tiles. Potters sort their wares into three groups by clay body and firing temperature: porous earthenware, dense stoneware, and white, glassy porcelain.

A pot of coarse, well-tempered clay can be heated slowly over a fire, so food can be boiled and stewed as well as roasted, and lidded jars kept in a dry place hold grain away from mice.

The plate shows how a pot is shaped and fired: built by hand from coils of clay, then baked in a simple updraft kiln.

How it works

A tall Jomon cooking pot covered with impressed cord patterns An early Japanese Jōmon pot, its surface marked by rolling twisted cord over the wet clay.

Clay is made of microscopic flat crystals, mostly silicates of aluminium. Wet, the flakes slide over each other on films of water, so the clay can be shaped and holds its shape. As it dries, the flakes pack together and the pot shrinks by several per cent. Dried clay is firm but would turn back to mud in the rain.

In the fire, between about 450 and 650 °C depending on the clay, the water bound inside the crystals is driven off and their structure breaks down; the clay can never be softened in water again. Hotter still, the grains start to fuse where they touch. Earthenware stays porous. At stoneware temperatures part of the body melts into glass that fills the pores, so the pot holds water without a glaze. Porcelain, made from a white, kaolin-rich body fired at high temperature, becomes hard and slightly translucent.

Pots stacked upside down under burning branches, straw and dung An open firing: pots are buried in fuel and fired on the ground, without a kiln.

Potters mix temper into the clay, such as sand, crushed shell or ground-up old pots. It opens the body so steam can escape, and reduces cracking as the pot dries and heats.

The first pots were fired in bonfires or shallow pits at 600 to 900 °C. A kiln holds the heat in. In the updraft kiln on the plate, the fire burns in a firebox below and the hot gases rise through holes in a clay floor, around the stacked pots and out of a vent at the top. The pots no longer sit in the burning fuel, so the heat is more even and easier to control, and fewer pots crack.

How to build

Good earthenware can be made with clay dug from the ground, a few simple tools and a bonfire. The steps below are an outline.

Find and prepare clay. Roll a thin sausage and bend it into a ring: if it does not crack, the clay is plastic enough. Soak it to a slurry, let stones settle, pour off the top and dry it until it can be kneaded. Knead in sand or crushed fired pottery, starting with about a quarter of the clay’s volume, and fire a few small test pieces to adjust the mix.

Build with coils. Flatten a disc for the base. Roll coils as thick as a finger and lay them on top one at a time, pinching and smoothing each one into the last on the inside and outside. Work on a board or turntable so you can rotate the pot. Thin and even the walls with a wooden paddle against a stone held inside.

Dry slowly. Keep the pot out of sun and wind for a week or more. When it is leather-hard, you can rub the surface with a smooth pebble to burnish it. Fire it only when it is bone dry and no longer cool to the touch.

Fire. Warm the pots beside the fire for an hour or two, turning them. Then stack them on embers, cover them with wood and dung, and build the fire up over several hours. Let everything cool slowly in the ashes before you lift the pots out.

Safety. Fire only outdoors, away from buildings: clear the ground, keep water at hand, watch the wind and stay out of the smoke. Damp clay can burst and throw sharp fragments, and hot ashes stay dangerous for hours. Dry clay dust contains fine silica that damages the lungs: wet-clean rather than sweep. Treat pots made this way as experimental and keep them away from food unless the clay and the finished ware have been tested. Never use glazes of unknown composition: lead can leach into food.

History and evolution

Fired clay is older than pots. People at Dolní Věstonice, in what is now the Czech Republic, made figurines of fired clay between about 29,000 and 25,000 BCE but made no vessels.

The oldest known pots come from Xianrendong Cave in Jiangxi province, China. In 2012 a team led by Wu Xiaohong and Ofer Bar-Yosef dated the earliest sherds, some with soot and scorch marks that suggest cooking, to about 20,000 to 19,000 years ago, at the height of the last ice age. Some researchers think the cave’s layers are too disturbed for certainty, but early pottery from elsewhere in East Asia confirms that ice-age foragers made pots. At Ōdai Yamamoto in northern Japan, sherds with charred food residue have been dated to between about 16,500 and 15,000 years ago, depending on the study. In the Near East it was the other way round: the first farming villages managed without pots for more than two thousand years.

A heavy wooden wheel-head on a short shaft turning in a stone socket, with a lump of clay An early potter’s wheel: a heavy wooden head turning on a pivot set in a stone.

The earliest known pottery kiln in the Middle East, at Yarim Tepe in northern Iraq, dates from about 6000 BCE or a little earlier. Slow turntables helped Near Eastern potters shape coiled pots by about 3500 BCE, and the fast wheel, heavy enough to keep spinning while the potter raised a pot from a lump of clay, followed in the third millennium BCE.

Chinese kilns could reach about 1,000 °C before 2000 BCE, and climbing dragon kilns built on hillsides reached 1,300 °C. That made stoneware possible and, by the Tang dynasty (618–907), white porcelain.

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