Firing is a one-way door
Unfired CLAY is just wet earth: leave a dried pot out in the rain and it slumps back into mud. Heat it past about 600°C in a KILN and something permanent happens — water locked inside the clay's crystals is driven off, and the particles begin to fuse. After that it is CERAMIC, and no amount of soaking will turn it back.
Hotter firing makes harder pots. EARTHENWARE, the reddish clay of flowerpots, is fired to around 1,000–1,150°C and stays slightly porous. Stoneware goes to about 1,250°C and becomes watertight on its own. Porcelain, fired hotter still, can be so thin that light shows through it — and it was made first in China, so English still calls fine plates china. Our ancient China page covers the civilisation that perfected it.
Older than farming
The oldest known pottery vessels, found at Xianrendong Cave in southern China, are around 20,000 years old. That surprised archaeologists, because they were made by hunter-gatherers during the last Ice Age, long before anyone in that region was farming; they may have been used for cooking shellfish or making broth. Fired clay figures are older still — a small figure found at Dolní Věstonice in the Czech Republic is about 26,000 years old.
The potter's WHEEL came much later, around 6,000 years ago, and let a skilled potter make in a few minutes a pot that would take an hour to build by hand.
Pinch, coil, throw
There are three basic ways to make a pot by hand. A PINCH pot starts as a ball of clay with a thumb pressed into it, squeezed and turned until the walls are thin. A COIL pot is built up from long sausages of clay laid in rings and then smoothed together. Slab building uses rolled-out sheets, cut and joined like cardboard.
To THROW a pot on a wheel — the word comes from an old English word meaning to twist or turn — the POTTER first has to centre the clay while it spins, which is the hardest part to learn. Joins are made with SLIP, runny clay used as glue after both surfaces are scratched. Air trapped inside can make a pot burst in the kiln, so clay is kneaded first. A HANDLE goes on once the MUG is firm but not yet dry, a stage potters call leather-hard.
Glaze is glass
A GLAZE is a coating of powdered minerals that melts in the kiln into a thin skin of glass. It makes a pot waterproof, easy to clean and shiny, and its colour usually looks nothing like it does going in — a dull grey powder can come out deep blue. Glazed pots have to be kept apart in the kiln, or they fuse to each other.
In Japan, broken pottery is sometimes mended with lacquer mixed with gold, a technique called kintsugi, so the cracks become bright lines that are part of the object's history rather than something to hide. Factories make identical pieces by pouring liquid clay into a plaster MOULD. The rest of the grid is the craft itself: the STUDIO, the FIRE, the BOWL and the VASE, and the chance to SCULPT, SHAPE, SPIN and SMOOTH.