Fired clay and a pit kiln

Pottery is the oldest craft in this domain and the one with the lowest material cost: clay is free, under a great deal of Britain, and a wood fire in a hole in the ground will reach the temperature that turns it permanently into ceramic.

It is also the craft with the highest failure rate for beginners, and almost all of that failure comes from one cause, which is drying.

Finding and testing clay

Look where water sits and the ground stays sticky. Riverbanks, stream cuttings, ditches, the spoil from any excavation, the bottom of a pond, and the subsoil under the topsoil in much of southern and midland England.

The ribbon test, which is the whole assessment:

1. Take a lump, wet it, and work it until it is smooth and plastic.
2. Roll it into a sausage the thickness of a finger.
3. Wrap it round your finger. Good clay bends right round without cracking. Poor clay cracks and crumbles.
4. Roll a coil and hold it out horizontally. Clay with enough plasticity holds its own weight briefly.

Then fire a test piece before you commit to a batch. Make three small tiles, dry them thoroughly, put them in an ordinary fire, and see what happens. Clay varies enormously between two spots a hundred metres apart, and one test firing tells you more than any amount of examining.

Cleaning and preparing it

Raw dug clay is full of stones, roots and grit, and any of them will crack a pot.

Levigation, which is the thorough method:

1. Dry the raw clay and break it up small.
2. Slake it in a bucket of water until it collapses into a slurry.
3. Stir hard, then let it stand for a minute. Sand and stones sink fast; clay stays suspended.
4. Pour off the cloudy water into a second container, leaving the grit behind.
5. Let the second container settle for a day, then pour off the clear water on top.
6. Dry the remaining sludge on a cloth or a plaster slab until it is workable.

Wedging is the last step: knead the clay like dough, repeatedly, to drive out air bubbles and make the moisture even. An air bubble in the wall of a pot expands in the fire and blows a hole in it, so this is not optional.

Temper

Temper is non-clay material mixed in to open the structure so water can escape during drying and firing, and so the pot can expand and contract without cracking.

• Sand or crushed grit, the commonest.
• Grog, which is crushed already-fired pottery, and is the best because it behaves identically to the pot around it.
• Crushed shell, or fine ash.
• Chopped straw or dung, which burns out and leaves a light porous body. Traditional, effective, and it makes a weaker pot.

Roughly a quarter to a third temper by volume for a cooking or storage vessel. Too little and it cracks; too much and it will not hold together.

Temper is essential for anything going near heat. An untempered pot in a fire cracks almost immediately.

Building a pot

No wheel needed, and a wheel is not the useful part of this craft.

Pinching, for small pots: a ball of clay, thumb pressed into the middle, walls squeezed out evenly between thumb and fingers while turning.

Coiling, for anything larger:

1. Roll coils of even thickness, about a finger thick.
2. Build up in rings, one on top of the last.
3. Weld each coil to the one below, inside and out, by smearing the clay downwards with a thumb. This is the step that decides whether the pot survives, and skipping it leaves a line the pot will split along.
4. Scrape and smooth the surface with a rib, a shell or a smooth stone.
5. Keep the wall an even thickness. Uneven walls dry unevenly and crack.

Burnishing, rubbing the leather-hard surface with a smooth pebble, compacts it and gives a shine, and makes a low-fired pot noticeably less porous.

Keep it under damp cloth and a bag between sessions so it does not dry unevenly.

Drying, which is where batches are lost

The pit firing

1. Dig a pit roughly half a metre deep and wide enough for the pots with space around each.
2. Line the bottom with a bed of dry sawdust, wood shavings or straw.
3. Set the preheated pots in, not touching each other or the sides, and pack the bedding around and inside them.
4. Build fuel over the top: small wood first, then larger, in a mound.
5. Light the top, not the bottom. Burning downwards raises the temperature slowly, which is exactly what you want.
6. Let it burn down over several hours, adding fuel to maintain heat rather than to increase it suddenly.
7. Cover with ash or earth once it has burned down, to hold the heat and slow the cooling.
8. Leave it overnight. Do not open it to look. Opening a hot pit lets cold air in and cracks the pots at the last moment.
9. Remove them cold.

Expect losses. A third of an early batch failing is normal. Losses fall sharply once you learn how long your clay needs to dry.

What you actually get

Low-fired earthenware. A pit or bonfire firing reaches somewhere around 600 to 900 degrees, which is enough to change the clay permanently into ceramic and not enough to vitrify it.

That means it is porous. It will hold dry goods indefinitely, it will hold water for a while and seep, and it will not survive being knocked about the way a shop-bought bowl will.

It is genuinely useful for: dry storage, fermenting and pickling crocks, water jars that sweat and cool by evaporation, lamps, crucibles, plant pots, weights and beads.

It is not a substitute for a saucepan, and a pot that has been fired once and used over a fire will eventually crack.

Making it hold water

• Burnishing before firing compacts the surface and reduces porosity noticeably. Cheapest and safest.
• A slip of very fine clay brushed on before firing does the same.
• Sealing after firing with beeswax, pine pitch, or animal fat rubbed into a warm pot. Traditional, effective, and it means the pot cannot then be used over direct heat.
• Milk firing: soaking a fired pot in milk and reheating it gently deposits protein in the pores. It works and it is not permanent.
• Do not attempt lead or unknown glazes on anything for food or drink.

A porous water jar is a feature, not a fault. Evaporation through the wall cools the contents, which is why unglazed water jars are used in hot climates. See heat exhaustion and heatstroke.