Clay Preparation

Part of Pottery and Ceramics

Processing raw clay into a workable, consistent body ready for forming and firing.

Why This Matters

Raw clay dug from the earth is almost never ready to use directly. It contains rocks, roots, sand, organic matter, and inconsistent moisture. Attempting to form vessels from unprepared clay results in cracking, warping, explosions during firing, and weak finished pieces. Every successful pottery tradition throughout history developed reliable clay preparation methods — they are as essential as the kiln itself.

In a rebuilding scenario, clay preparation is the bottleneck between having a clay source and having usable pottery. A community might find excellent clay deposits but still produce nothing but broken shards if the processing steps are skipped or rushed. The investment of time in preparation pays off exponentially in reduced failure rates during forming and firing.

Proper clay preparation also lets you work with marginal clay sources. Pure, ready-to-use potter’s clay is geologically uncommon. But clay that’s too sandy, too silty, or too contaminated can often be processed into a serviceable body through the techniques in this article — dramatically expanding the range of usable deposits in your area.

Removing Contaminants

Picking and Screening

The first step is removing visible debris from your raw clay:

1. Break dry clay into small chunks — fist-sized or smaller. If the clay is already wet, let it dry partially first. Dry clay is easier to process.
2. Pick out visible rocks, roots, and organic matter by hand
3. Crush the chunks with a hammer, mallet, or flat stone on a hard surface. Aim for pea-sized fragments or smaller.
4. Screen through progressively finer meshes:

Screen Size	Removes	Use Case
6 mm (1/4 inch)	Rocks, large roots, gravel	First pass — removes everything obviously wrong
2 mm (window screen)	Sand, small stones, fiber	Standard pottery body — good for most vessels
1 mm or finer	Fine sand, tiny inclusions	Smooth, refined body for thin-walled work

Making Screens

Stretch woven fabric, basket-weave strips, or scavenged wire mesh over a wooden frame. Even a loosely woven cloth over a bucket works for a first pass. The finer the weave, the smoother the final clay.

Slaking (Water Dissolution)

Slaking dissolves dry clay in water, allowing contaminants to be separated by density:

1. Fill a large container (bucket, barrel, trough) halfway with water
2. Add dry clay chunks gradually — don’t dump all at once or they’ll form a solid mass
3. Let sit for 24-48 hours without stirring. The clay will slowly dissolve and settle.
4. After soaking, stir vigorously to create a smooth slurry (called “slip”)
5. Screen the slip through fine mesh to catch remaining particles
6. Let the screened slip settle for several hours
7. Heavy contaminants (sand, gravel) sink to the bottom first; lighter clay particles stay suspended longer

Levigation (Settling Separation)

This advanced technique separates clay particles by size using water:

1. Mix screened clay thoroughly with a large volume of water — aim for a thin, milky consistency
2. Pour the mixture into a tall container and let it settle undisturbed
3. After 30-60 seconds, the coarsest particles (sand, silt) settle to the bottom
4. Carefully pour or siphon the still-cloudy water into a second container, leaving the settled sediment behind
5. Let the second container settle for 12-24 hours
6. The fine clay particles settle slowly, leaving clear water on top
7. Siphon or pour off the clear water
8. The remaining material is purified, fine-particle clay

Don't Discard the Sand

The sand and coarse particles from levigation aren’t waste. They’re useful as temper (grog) for other clay bodies, or for making refractory materials and mortar.

Adjusting Clay Properties

Raw clay is rarely perfect for pottery in its natural state. You’ll typically need to adjust one or more properties.

Adding Temper (Opening Materials)

Pure fine clay shrinks dramatically during drying and firing, leading to cracking. Temper (also called “opener” or “grog”) reduces shrinkage and improves workability:

Temper Material	Effect	Best For
Crushed fired pottery (grog)	Reduces shrinkage, improves thermal shock resistance	General pottery, cooking vessels
Sand (quartz)	Reduces shrinkage, adds sparkle	Storage vessels, thick-walled pieces
Crusite shell	Improves thermal shock resistance	Cooking pots specifically
Chopped grass/straw	Burns out during firing, leaving pores that reduce thermal stress	Low-fire cooking vessels
Crushed volcanic rock	Lightweight filler, improves insulation	Large storage vessels

Typical ratios: 10-25% temper by volume, mixed into the clay body. Start at 15% and test.

Mixing Clay Bodies

If you have access to multiple clay sources, blending them can create a superior body:

• Mix a fatty (plastic) clay with a lean (sandy) clay for balanced workability and reduced cracking
• Add kaolin (white-firing clay) to red earthenware for lighter color and improved strength
• Combine stoneware-grade clay with lower-firing clay to adjust the maturing temperature

Always test blends with small batches before committing to large-scale preparation.

Moisture Adjustment

Too Wet (Sticky, Won’t Hold Shape):

1. Spread clay thinly on a plaster slab, canvas, or wooden board
2. Plaster absorbs water fastest; wood and canvas work but take longer
3. Flip and re-spread periodically for even drying
4. Test every 15-30 minutes until the clay holds a thumbprint without sticking

Too Dry (Crumbly, Cracks When Bent):

1. Poke holes throughout the clay mass with a stick
2. Spray or drizzle water into the holes
3. Wrap tightly in cloth or hide and let sit 24 hours for moisture to equalize
4. Knead (wedge) thoroughly to distribute moisture

Wedging and Aging

Wedging Methods

Wedging is the final preparation step — it homogenizes the clay, removes air bubbles, and aligns clay particles for better workability.

Spiral (Ram’s Head) Wedging:

1. Place a manageable lump (1-2 kg) on a sturdy surface at waist height
2. Push the heel of your dominant hand down and forward at 45 degrees
3. Rock the clay back with your other hand, rotating slightly
4. Repeat rhythmically 50-100 times
5. The clay develops a characteristic spiral pattern in cross-section

Cut-and-Slam:

1. Cut the clay in half with a wire
2. Inspect for air bubbles (small holes in the cross-section)
3. Slam one half onto the other
4. Rotate 90 degrees and repeat
5. Continue until no bubbles are visible — typically 20-30 cycles

Wedging Surface

A thick plaster slab is ideal — it absorbs slight excess moisture during wedging. Canvas-covered wood or stone also works. Avoid smooth surfaces where clay slides instead of compressing.

Aging (Souring)

Clay improves dramatically with age. Wrap prepared clay tightly in damp cloth, hide, or store in sealed containers (clay-lined pits, sealed pots) and let it rest:

Aging Time	Effect
1-3 days	Moisture equalizes throughout the mass
1-2 weeks	Bacterial action begins; plasticity improves noticeably
1-3 months	Significant improvement in workability and green strength
6+ months	Maximum benefit — clay becomes highly plastic and forgiving

The bacterial colonies that develop during aging produce organic acids and polysaccharides that lubricate clay particles, making the body more plastic and easier to work. Ancient Chinese potters aged clay for years or even decades. While you needn’t wait that long, even a week of aging transforms marginal clay into pleasant material.

Testing Your Prepared Clay

Before committing to a production run, test your prepared clay body:

Quick Tests

1. Plasticity test: Roll a coil about 1 cm thick and wrap it around your finger. Good clay bends without cracking. If it cracks badly, add more plastic clay or age longer.
2. Shrinkage test: Form a flat tile exactly 10 cm long. Mark the measurement on the wet tile. After drying and after firing, measure again. Shrinkage over 12% total means add more temper.
3. Drying test: Make several small pieces of varying thickness. Dry slowly. If cracks appear, the clay needs more temper or slower drying.
4. Firing test: Fire test tiles at your kiln’s temperature. Check for warping, cracking, bloating, or excessive porosity.

Recording Results

Keep notes on each clay body recipe and its test results. In a rebuilding context, this knowledge is critical — you may need to reproduce a successful recipe from a new clay source, or adjust proportions when your supply changes. Scratch notes into a fired tile if paper isn’t available — a permanent record that survives fire, water, and time.

Scaling Up Clay Preparation

For community-level production, individual hand processing is too slow. Scale-up methods:

1. Clay pit soaking: Dig a large pit, line with stones, fill with raw clay and water. Let soak and settle for weeks. Harvest processed clay from the middle layer.
2. Treading: Spread moistened clay on a clean floor and walk on it repeatedly. This is how large quantities were traditionally wedged in many cultures.
3. Pugmill: A hand-cranked or animal-powered auger that mixes and extrudes clay. Complex to build but transforms throughput. A wooden barrel with a central shaft fitted with angled wooden paddles, turned by a crank handle, forces clay through a narrow exit, mixing and de-airing it.
4. Dedicated drying beds: Plaster-lined or sand-lined shallow troughs for reducing moisture from slip to workable clay. Rotate stock — while one batch dries, another soaks.

The goal is a pipeline: raw clay goes in one end, and consistent, tested, ready-to-use clay comes out the other. This pipeline is what separates occasional pot-making from reliable pottery production that a community can depend on for its water storage, cooking, and food preservation needs.