Clay keeps a record of every hand that touched it, and the kiln reads it back out loud.

Clay bodies

Clay bodies are sorted by the temperature at which they mature — the point where the body has tightened as far as it usefully can without slumping. Potters name that temperature with pyrometric cones, which measure heat work (temperature plus time) rather than a thermometer reading, so the same cone can arrive at slightly different degrees in a fast or slow firing.

Earthenware

Matures around cone 06 to cone 02, roughly 1000–1140 °C. It stays porous after firing and needs a glaze to hold water. Red earthenware is plastic, forgiving, and fires to a warm brick colour on its own.

Stoneware

Matures around cone 5 to cone 10, roughly 1190–1300 °C, and vitrifies to something close to non-porous. Mid-fire stoneware at cone 6 is what most home and shared kilns already run.

Porcelain

Matures around cone 8 to cone 12. It is short — low in plasticity — so it tears at the rim, slumps when thin and remembers every bit of uneven pressure. Beautiful, and impatient.

Grog is clay that has already been fired and then crushed. Mixed back into a body it opens the structure, lets water escape more evenly, reduces warping and cracking, and gives the clay enough tooth that a tall coil wall will stand up while you build it. The trade is texture: heavily grogged clay is rough on fingertips and hard on the edge of a throwing hand. For a first bag, a medium-grogged mid-fire stoneware is the easy answer — plastic enough to pinch and coil, tough enough to survive an impatient dry, and at home at cone 6.

Hand-building basics

Three techniques cover almost everything you can make without a wheel, and all three live or die on even walls.

Pinch

Start with a ball about the size of an orange, press a thumb down to within roughly 10 mm of the bottom, then work up the wall in rings, turning the pot constantly. The turning is the whole trick: the wall thins wherever your fingers are, so it stays even only if the pressure keeps moving.

Coil

Roll ropes about as thick as a finger, score and slip each join, and press the new coil down into the one below with a thumb inside while a finger supports the outside. Smooth the inside as you go — far easier than reaching down into a finished pot.

Slab

Roll to an even thickness between two guide sticks, let the slab stiffen to soft leather-hard before assembling, and cut joins at matching angles so the faces meet flat rather than edge to face.

Whatever the method, aim for walls between 5 and 8 mm. Thinner than 5 mm and hand-built work turns fragile and dries faster at the rim than at the base; thicker than about 8 mm and the middle of the wall stays wet long after the surface looks dry, which is exactly where cracks start. Join anything to anything with score-and-slip: rough both faces with a needle or serrated rib, wet them with slip made from the same clay, press firmly, and blend the seam on at least one side. Compress every rim with a few passes of a chamois or a fingertip — an uncompressed rim keeps its loose particle alignment and is the first thing to split in the bisque.

Drying and firing

Leather-hard is the stage where clay holds its shape but still darkens a damp cloth: firm enough to carve and trim, wet enough to join. Bone-dry is lighter, and room temperature against the cheek rather than cool. It is no longer joinable — a handle attached at bone-dry will pop off in the firing.

Cracks are nearly always a drying problem wearing a firing costume. Water leaves the thin parts first, those parts shrink first, and the shrinking pulls against thick parts that have not moved yet. Even walls, slow drying under loose plastic, and a little extra cover over rims and handles solve most of it. Anything thicker than about 25 mm wants days rather than hours, and a solid lump wants to be hollowed instead.

A bisque firing to around cone 04 (roughly 1060 °C) drives off chemically bound water and burns out organic matter, leaving a pot strong enough to handle and thirsty enough to take glaze. The glaze firing for mid-fire stoneware sits around cone 6, roughly 1220 °C. Before that firing, wipe the foot and about 3 mm up the side with a damp sponge, because any glaze touching the shelf fuses to it. Kiln wash on the shelves catches the drips you miss, and wadding — small pellets of a refractory mix set under the foot — keeps a piece off the shelf when the glaze is likely to run.

Shrinkage

Glaze notes

Dipping gives the most even coat and is by far the fastest way to do a batch, but it needs enough glaze in the bucket to submerge the piece and a pair of tongs or dry fingers to hold it. Brushing works from a small jar and suits detail and overlaps, but it needs two or three coats, each one brushed at right angles to the last and left until the shine has gone before the next goes on. Either way, even thickness matters more than the count: thin patches burn through to bare clay, thick runs move downhill.

Test tiles are the only honest answer to the question of what a glaze will look like. Fire a tile of every glaze on every clay body you use, bent so it has a vertical face and a horizontal one, and dip half of it twice so single and double thickness sit side by side on the same tile. A glaze that pools handsomely at double thickness can be thin and dry at single.

Then write it down: date, clay body, glaze, how it was applied, how many coats, the cone, and roughly where in the kiln it sat. A glaze log is dull to keep and the only way you will ever repeat the good one — or explain the bad one to yourself six months later.