Clay and firing

Clay is fine-grained sediment, and it collects where slow water dropped it: stream and river banks, pond edges, the subsoil below dark topsoil, road and ditch cuts. It is usually grey, buff, red or yellow, sticky when wet, and hard when dry.

Most clay dug from the ground needs no more than cleaning. Whether a particular clay will make pots is a matter of three tests, and together they take a week.

1. The coil test — plasticity. Roll a coil about 1 cm thick and wrap it round a finger. Good clay bends without cracking. If it cracks at once, it is short — too sandy or silty — and needs a more plastic clay mixed in, or it will only make crude, thick-walled ware.
2. The shrinkage bar — how much it moves. Make a flat bar, and while it is soft, mark two lines exactly 100 mm apart. Dry it, measure; fire it, measure again. Clay shrinks about 4–8% drying and 8–15% in total after firing. Much more than 15% and the clay will crack as it dries — add temper.
3. The test firing. Fire a few small bars and a small pot in the actual fire or kiln you will use. If they bloat, slump, blister or melt, the clay cannot take that temperature. If they crumble when soaked in water afterwards, the firing was too cool to reach the ceramic change.

Check for lime. Drop vinegar on dried clay. If it fizzes, there is limestone or shell in it — small particles that will turn to quicklime in firing and later swell and pop chunks out of the pot. Sieve them out, or keep that clay for low firings.

1. Dry the raw clay and break it into small lumps.
2. Slake: drop it into a bucket of water and leave it overnight. Dry clay breaks down into a slurry on its own; wet lumps do not.
3. Sieve the slurry through a ~1 mm screen to remove stones, roots and lime particles.
4. Settle and dry. Let it settle, pour off the clear water, and dry the sludge — on a plaster slab, cloth over sand, or a porous fired surface — until it is plastic and no longer sticks to your hands.
5. Add temper if the shrinkage test called for it. Temper is non-plastic grit that opens the body so it dries and heats evenly:
   • Grog — crushed fired pottery, sieved. The best general temper: it has already shrunk.
   • Sand — clean, sharp, sieved fine. Common and effective.
   • Organic matter — chopped grass, straw, dung. It burns out, leaving a light, porous pot that survives thermal shock — good for cooking pots, weaker for storage.
   • Crushed shell — good for low-fired pots only. Above about 750 °C shell turns to lime and ruins the pot days later.
   Start at about one part temper to four or five parts clay and adjust from test bars.
6. Wedge: knead the clay like bread, cutting and slamming it together, for several minutes. It evens out moisture and removes air pockets, which expand in firing and spall the wall.

The oldest methods need no wheel, and for most of history most pots were made with them.

• Pinch. Push a thumb into a ball and rotate, pinching the wall thinner. Small bowls and cups.
• Coil. Roll coils as thick as a finger, lay them in rings or a spiral, and smooth each into the one below on the inside and outside. Pots of any size and shape; the main method of the ancient world.
• Slab. Roll flat sheets and join them. Boxes, tiles, flat-bottomed vessels.
• Paddle and anvil. Beat the wall with a flat paddle against a smooth stone held inside. Thins and compacts the wall, and makes large, strong, round pots quickly.
• Moulds. Press clay into or over a fired or carved form.

Keep walls even — about 6–10 mm for hand-built ware. A thick base under a thin wall dries and heats at different rates and cracks along the join. Join soft clay to soft clay. To attach a handle or a leather-hard part, score both surfaces and apply slip — clay mixed with water to a cream — then press together.

Clay shrinks as it dries, and cracks wherever one part shrinks faster than another. Rims, handles and thin edges dry first; the base, sitting on the ground, dries last.

1. Dry slowly, out of sun and wind, for the first days — under a cloth or in a shaded room.
2. Wrap rims and handles in damp cloth or plastic for the first day or two so they dry with the body.
3. Turn pots over once they are firm enough — leather-hard — so the base dries too.
4. Leather-hard — firm, cool, holds its shape — is the time to trim, burnish and join handles.
5. Bone-dry is when it is lighter in colour and no longer feels cool against your cheek. A pot that feels cool is still losing water by evaporation. It is not ready to fire.

How long: days for small ware, two weeks or more for large pots, longer in humid weather. There is no way to hurry it that does not cost pots.

Kiln temperature is read by the same colour scale as a forge — dull red about 600 °C, cherry red about 800, orange about 950, yellow about 1,050–1,100 — viewed through a spyhole in a darkened kiln or at dusk for an open firing.

Every broken pot is a record of which rule was broken. Read it before firing again.

• Silica dust — see the callout in the preparation section. The slow hazard, and the one that matters most over a working life.
• Lead glazes. Lead oxide makes a glossy glaze at low temperatures, which is why it was used everywhere for centuries. Lead leaches out of it into acidic food and drink — vinegar, fruit, wine, pickles — and causes lead poisoning, worst in children: anaemia, stunted development, brain damage. Never use a lead glaze on anything that will hold food or drink. Treat old low-fired glazed ware with suspicion for acidic foods.
• Carbon monoxide from a kiln or pit firing in any enclosed or sheltered space. See the fire guide.
• Burns and flying fragments when a pot bursts in the fire. Stand back while a firing climbs through the water-smoking stage, and wear eye protection when opening a hot kiln or pit.