Clay Preparation

Part of Brick Making

Processing raw clay for brick making through soaking, mixing, tempering, and de-airing.

Why This Matters

Raw clay dug from the ground is almost never ready to form into bricks. It contains stones, roots, pockets of different soil types, and inconsistent moisture. Forming bricks from unprepared clay produces weak, cracked, and dimensionally inconsistent results — bricks that crumble under load or shatter during firing.

Clay preparation is the single most important step in brick making. Well-prepared clay produces bricks that are uniform in size, dry without cracking, and fire to consistent hardness. The difference between a 50% and 90% success rate in a firing often comes down to how thoroughly the clay was processed before forming.

Every brick-making tradition worldwide developed specific preparation techniques suited to local clays. The underlying principles are universal: break down the clay structure, remove contaminants, achieve uniform moisture, and add temper to control shrinkage. These steps require no tools more sophisticated than a pit, water, and human labor.

Weathering and Soaking

Weathering (Passive Preparation)

The easiest preparation method requires only time. Dig raw clay and spread it in thin layers (10-15 cm) on open ground. Leave it exposed to weather for weeks or months. Freeze-thaw cycles break down clay lumps. Rain washes out soluble salts. Sun drying creates cracks that further break the structure.

Weathered clay is dramatically easier to work than freshly dug clay. If you have the luxury of time, weather your clay for at least one full season before use. Dig in autumn, use in spring.

Soaking (Active Preparation)

When time is short, soaking accelerates the process:

1. Break raw clay into fist-sized lumps
2. Place in a pit, barrel, or any watertight container
3. Cover with water — use 1.5 times the volume of water to clay
4. Let soak for 24-72 hours, stirring occasionally
5. The clay absorbs water and breaks down into a thick slurry

The Slaking Test

Properly soaked clay should squish easily between your fingers with no hard lumps. If you find hard chunks after 48 hours of soaking, the clay has highite content and needs longer soaking or physical processing.

Levigation (Washing)

For clays contaminated with stones, sand, or organic material, levigation separates pure clay from impurities:

1. Mix clay with excess water to create a thin, milky slurry
2. Pour through a screen (woven basket or cloth) into a settling tank
3. Let stand for 24-48 hours — sand and stones settle first, fine clay particles last
4. Carefully pour off the top layer of pure clay slip
5. Let the clay slip dry to working consistency in shallow trays or on plaster

Levigation produces the purest clay but is labor-intensive. Reserve it for fine work like pottery or facing bricks. Common bricks rarely need this level of purification.

Mixing and Tempering

Why Temper Matters

Pure clay shrinks 8-15% as it dries and fires. This shrinkage causes cracks, warping, and breakage. Temper — non-plastic material mixed into the clay — reduces shrinkage by providing a rigid skeleton within the clay body.

Common Temper Materials

Material	Ratio (temper:clay)	Effect	Best For
Sand (sharp/angular)	1:3 to 1:4	Reduces shrinkage, improves drainage	General brick making
Grog (crushed brick)	1:4 to 1:5	Reduces shrinkage, improves firing	Refractory bricks
Straw/grass (chopped)	Small handful per brick	Binds green brick, burns out in firing	Sun-dried or low-fire bricks
Rice husks/chaff	1:6 to 1:8	Lightens brick, improves insulation	Lightweight construction
Wood ash	1:8 to 1:10	Flux — lowers firing temperature	When fuel is limited

Mixing Methods

Foot mixing (pugmill substitute): Spread tempered clay on a clean floor in a layer 15-20 cm thick. Walk through it repeatedly, folding and turning with a shovel between passes. Continue for 30-60 minutes until the mixture is completely uniform with no visible streaks of different material.

Pit mixing: Dig a shallow pit (30 cm deep, 2-3 m across). Add clay and temper in alternating layers. Flood with water. Let soak overnight. The next day, wade in and mix with feet and hands. This is the most common traditional method worldwide.

Animal mixing: In many cultures, oxen or horses walked in circles through clay pits. The animals’ weight provided superior mixing force. A single ox can prepare enough clay for 500-1,000 bricks per day.

Consistency Check

Cut a ball of mixed clay in half with a wire or string. The cross-section should show uniform color and texture with no visible pockets of sand or unmixed clay. If you see streaks, mix longer.

De-Airing

Trapped air bubbles in clay create weak points. When fired, air pockets expand and can cause the brick to crack or explode. De-airing removes these bubbles.

Wedging (Manual De-Airing)

The most practical method without machinery:

1. Take a lump of prepared clay (2-3 kg)
2. Slam it forcefully onto a hard surface
3. Fold it in half
4. Slam again
5. Repeat 30-50 times, rotating the clay quarter-turn each fold

This ram’s-head wedging technique compresses the clay and forces out trapped air. You can hear the difference — freshly mixed clay makes a hollow thud when slammed. Well-wedged clay makes a solid slap.

Wire-Cut Test

After wedging, cut the clay with a wire. The cross-section should be smooth and uniform with no visible air holes. A single pinhole-sized void means more wedging is needed.

Aging (Souring)

After mixing and initial de-airing, storing clay in sealed conditions (covered pit, wrapped in wet cloth) for 1-4 weeks dramatically improves workability. Bacterial action breaks down organic matter in the clay and produces organic acids that improve plasticity. This process is called souring or aging.

Aged clay is easier to form, holds shape better, and produces stronger green bricks. Chinese and Japanese potters traditionally aged clay for years. For brick making, even 1-2 weeks of aging makes a noticeable difference.

Achieving Correct Moisture Content

The Goldilocks Problem

Too wet: bricks slump, stick to molds, take forever to dry, and crack from excessive shrinkage. Too dry: clay does not fill molds completely, bricks crumble and have poor structural integrity.

Testing Moisture Content

Ball drop test: Form a ball of clay the size of a tennis ball. Drop it from waist height onto a hard surface. If it splatters — too wet. If it shatters — too dry. If it flattens to a thick disk with minor edge cracking — correct.

Ribbon test: Roll a piece of clay into a ribbon 1 cm thick. Drape it over your finger. If it bends smoothly without cracking, it may be slightly wet but workable. If it cracks immediately, add water. If it droops and stretches, let it dry.

Hand squeeze test: Squeeze a handful of clay firmly. Open your hand. The clay should hold the shape of your fingers with clean edges and no water squeezing out. If water appears between your fingers, spread the clay on a board to dry.

Adjusting Moisture

• Too wet: Spread on a dry surface (wood, concrete, plaster) in thin layers. Turn every few hours. Alternatively, mix in dry temper material.
• Too dry: Poke holes in the clay mass, fill with water, cover with wet cloth, and wait 24 hours. Then remix thoroughly.

Production Workflow

For efficient brick making, prepare clay in batches ahead of forming:

1. Day 1-2: Dig and soak clay
2. Day 2-3: Levitate if needed, begin tempering
3. Day 3-4: Mix thoroughly (foot or pit method)
4. Day 4-5: Wedge, de-air, test moisture
5. Day 5+: Form bricks immediately or store covered for aging

A two-person team can prepare enough clay for 200-300 bricks per day. Having clay preparation running one cycle ahead of brick forming ensures the forming team never waits for material. This pipeline approach doubles overall production compared to preparing and forming in the same batch.