Pottery

01

Finding clay, and testing it

Usable clay turns up in river banks, road cuttings, the bottom of ponds, subsoil under topsoil, and anywhere the ground goes sticky when wet and cracks when dry.

The field test is simple. Take a handful, wet it, and roll it into a rope about as thick as a pencil. If it will bend around your finger without breaking apart, there's enough clay in it to work with. If it crumbles, it's too sandy; if it's slick and greasy and cracks as it dries, it may be too fine and need sand adding.

Then clean it. The traditional method is levigation: mix the raw material with plenty of water, let the sand and grit settle out for a short while, pour off the clay-laden water into another container, and let that settle for longer before draining off the clear water on top. What's left is refined clay, and it costs only time.

Different clays fire at different temperatures and to different colours, and the only reliable way to find out what yours does is to make small test pieces and fire them. That test is worth doing before making anything you care about.

02

Preparing the clay

Preparation decides whether a pot survives, and skipping it is behind most cracked and exploded work.

Wedging is the kneading step, and it does two jobs: it makes the moisture content even throughout, and it removes air pockets. An air pocket in a wall expands in the kiln and blows the piece apart. It's physical, repetitive and not optional.

Temper is material added to the clay to open it up, usually sand, grog which is crushed fired clay, or in some traditions crushed shell or plant fibre. It reduces shrinkage, lets moisture escape during drying and firing, and makes a body far more resistant to thermal shock, which is why cooking vessels are traditionally heavily tempered.

Plasticity is what lets clay hold a shape without cracking, and it improves with age. Clay stored damp for weeks or months works noticeably better than clay used the day it was dug, which is why potters traditionally kept a supply maturing.

03

Forming: by hand and on a wheel

Every pot ever made was formed by one of a small number of methods, and the hand methods came first and need no equipment.

Pinching starts with a ball and opens it with the thumbs, and it makes small vessels quickly. Coiling builds walls from ropes of clay joined and smoothed together, and it will make anything of any size, including very large storage jars. Slab building constructs from flat sheets and suits angular forms. Moulding presses clay into or over a form and is how repeated shapes were made in quantity.

Coiling deserves particular attention because it's the workhorse. Almost all the world's traditional large vessels are coiled, it needs nothing but hands, and it gives complete control over wall thickness and profile.

The wheel is faster for round forms and is a considerable skill of its own. Centring the clay is the part that defeats beginners, and it's a matter of practice and body position and not strength. A wheel is a productivity tool, not a requirement, and plenty of excellent work has never been near one.

Whatever the method, even wall thickness matters more than almost anything else, because uneven walls dry and fire at different rates and that's what cracks them.

04

Drying without cracking

More work is lost here than at any other stage, and the cause is nearly always drying too fast or unevenly.

Clay shrinks as water leaves it. If one part dries faster than another, the two shrink at different rates and the piece tears itself apart. Thin edges, rims and handles dry fastest, which is exactly why cracks appear there.

The answer is to slow it down and even it out. Cover the piece loosely, keep it out of sun and draught, turn it so all sides get the same exposure, and wrap the parts that dry fastest. A large or complicated piece may want a week or more under cover.

Bone dry is the target before firing, and it takes longer than it looks. A pot that feels dry can still hold a surprising amount of water in its walls, and warming it gently somewhere for a period before it goes near real heat is the traditional insurance.

05

Firing

Firing is what makes the change permanent. Below a certain temperature clay dries and can be re-wetted; above it the structure changes and it can never return to clay.

The first stage is slow and matters most. Remaining moisture has to leave gently, and chemically bound water comes out at higher temperatures again. Rushing either is what makes pots explode, which is why every tradition brings the heat up slowly at the start.

Pit firing is the oldest method and needs no built structure: pots are packed with fuel in a pit and burned, reaching modest temperatures and producing earthenware that's porous but entirely usable, particularly if the surface is burnished or sealed. A simple built kiln holds heat better, reaches higher temperatures and gives more control, and a great deal can be made from earth, brick or stone.

Higher temperatures produce denser, harder, less porous ware. Earthenware fires low and stays porous unless glazed; stoneware fires higher and becomes watertight in itself. Which you can reach depends on your kiln and your fuel, and that decides what your pots can be used for.

06

Surfaces, glaze and holding water

A pot that leaks is a pot that isn't finished, and there's more than one way to close the surface.

Burnishing rubs the leather-hard surface with a smooth stone until the fine particles align and the surface becomes dense and shiny. It survives low firing, reduces porosity considerably, and needs no glaze at all. It's how a great deal of unglazed traditional ware holds liquid.

Glaze is a glass layer melted onto the surface, and it's genuinely chemistry, not decoration. It has to melt at the temperature the pot is fired to, and it has to shrink at close to the same rate as the clay beneath it, or it crazes into a network of cracks or flakes off entirely. Matching glaze to body is most of the difficulty.

Traditional sealing methods work without glaze. Fats, oils, resins, milk and beeswax have all been used to close the pores of earthenware, and they're the reason unglazed vessels could store liquid. Some of these are heat-limited, which matters for anything used for cooking.

And the food-safety point from the caution above applies here instead of anywhere else: if a piece will hold food or drink, the surface has to be one known to be safe.