Sourcing Clay
Part of Brick Making
Finding and extracting clay deposits in the landscape.
Why This Matters
Without clay, there are no bricks, no pottery, no waterproof linings, and no mortar for stone construction. Clay is the fundamental building material that separates permanent settlements from temporary camps. Finding a reliable clay source is one of the first priorities for any rebuilding community, and the quality of that clay determines the quality of everything built from it.
Clay deposits exist virtually everywhere, but not all clay is equal. Some clays produce excellent bricks; others crack, crumble, or melt in rain. Learning to read the landscape for clay indicators, test deposits quickly in the field, and extract clay efficiently can save weeks of failed experiments and wasted labor. A good clay source, once found, can supply a community’s construction needs for generations.
The skills involved are straightforward observation and simple physical tests — no chemistry lab required. Indigenous peoples and traditional potters worldwide have identified and used clay deposits for millennia using exactly the methods described here.
Reading the Landscape
Clay deposits reveal themselves through several visual and environmental clues. Learn to read these signs and you can survey a large area quickly.
Terrain Indicators
River banks and stream cuts. Flowing water erodes through soil layers, exposing the geology beneath. Look for exposed layers of grey, yellow, red, or blue-grey material on cut banks. Clay layers are visually distinct — they appear smooth and dense compared to sandy or gravelly layers above and below.
Erosion gullies. Where water has cut channels through hillsides, the sides of the gully often show soil stratigraphy. Clay layers resist erosion more than sandy layers, so they often protrude slightly as shelves or ledges in the gully wall.
Pond and lake beds. Standing water often indicates an impermeable clay layer beneath. Seasonal ponds that hold water well into dry season are strong indicators. The bed material, when exposed during drought, is often usable clay.
Termite mounds. In tropical and subtropical regions, termites build their mounds from subsoil material, often clay-rich. Abandoned mounds are an excellent pre-processed clay source — the termites have already separated fine clay particles from coarser material.
Landslide scars. Fresh landslides expose subsoil layers. The slip plane is often a clay layer that became saturated and lost cohesion. The exposed material above and at the slip plane may be clay-rich.
Vegetation Indicators
Certain plants thrive in clay soils and serve as biological indicators:
| Plant Type | Indication |
|---|---|
| Willows, alders | Wet clay near water table |
| Horsetail (Equisetum) | Heavy clay, poorly drained |
| Dock (Rumex) | Compacted clay soil |
| Dense, fine grass on bare slopes | Clay subsoil near surface |
| Sparse, patchy vegetation on flat ground | Heavy clay preventing root penetration |
The absence of deep-rooted trees on otherwise suitable ground often indicates a clay hardpan close to the surface. Roots cannot penetrate dense clay, so large trees are replaced by shallow-rooted grasses and shrubs.
Color Clues
Clay color tells you about composition:
- Red or orange: Iron oxide present; good for brick making; fires to deep red
- Grey or blue-grey: Reduced iron; common in waterlogged areas; fires to lighter color
- Yellow: Iron hydroxide; moderate quality; fires to tan or light red
- White or cream: Kaolin or low-iron clay; excellent for pottery, adequate for bricks
- Dark brown to black: Likely high organic content; needs processing or is unsuitable
Field Testing Deposits
When you find a potential deposit, test it before investing in excavation.
Dig a Test Pit
Dig through the topsoil (typically 150-300 mm) until you reach subsoil. Continue to at least 500 mm depth. Note:
- How deep the clay layer starts
- How thick the layer is (dig until you hit a different material)
- Whether the layer is continuous or has sandy lenses within it
- The color and consistency at different depths
A deposit less than 300 mm thick may not justify the effort of excavation. Ideally, you want a meter or more of usable clay.
Quick Identification Tests
Feel test. Take a pinch of moist material and rub it between your fingers. Clay feels smooth, slippery, and slightly greasy. Sand feels gritty. Silt feels smooth but not sticky. If you rub the material between your front teeth (a traditional potter’s test), clay feels like rubbing on glass — absolutely smooth with no grit.
Plasticity test. Moisten a lump of material and try to roll it into a coil about pencil-thickness (6-8 mm). If the coil holds together and can be bent into a ring without cracking, the clay content is high. If it cracks and falls apart, it is too sandy or silty for direct use.
Shine test. Cut a lump of moist clay with a knife. Good plastic clay shows a shiny, smooth cut surface. Sandy or silty material shows a matte, rough surface.
The Tongue Test
In truly primitive conditions, touch the material to your tongue. Pure clay sticks to the tongue and feels slightly chalky. Sand does not stick. Silt sticks very lightly. This crude test has been used by geologists and potters for centuries.
Jar Settling Test
Bring a sample back to camp for a more precise test:
- Fill a straight-sided glass jar one-third with the material
- Fill to the top with clean water
- Add a pinch of salt (helps clay settle faster)
- Cap the jar and shake vigorously for two full minutes
- Set it on a flat surface and observe
Sand settles within 30 seconds. Silt settles within 2-4 hours. Clay remains suspended for 12-24 hours before settling. After 24 hours, measure the distinct layers:
| Layer | Good Brick Clay | Too Sandy | Too Much Clay |
|---|---|---|---|
| Sand (bottom) | 50-70% | >75% | <40% |
| Silt (middle) | 10-20% | 5-15% | 10-20% |
| Clay (top) | 15-30% | <10% | >40% |
Extraction Methods
Once you have confirmed a usable deposit, extraction can begin.
Open Pit Extraction
The simplest method. Strip the topsoil and set it aside for later site restoration or garden use. Dig the clay in steps or terraces, working from one end. Key practices:
- Keep the pit floor angled so rainwater drains to one end for easy removal
- Step the walls to prevent collapse — vertical walls in clay taller than 1.5 m are dangerous
- Separate layers — if the deposit has bands of different quality, extract them separately and test each one
- Stockpile topsoil for restoration; a responsible builder leaves the landscape usable
Bank or Cliff Extraction
When clay is exposed in a river bank or hillside, you can extract horizontally rather than digging down. This is often easier because gravity helps move material. Work from the base up, undercutting the clay face so blocks fall away. Stand clear of overhangs.
Underwater Extraction
Lake and pond beds often contain excellent clay. In shallow water (under 1 m), you can reach the bottom and scoop clay directly. In deeper water, a weighted bucket on a rope, dragged along the bottom, collects material. Let extracted clay dry on shore before transport.
Processing Raw Clay
Extracted clay often contains stones, roots, and inconsistent moisture. Basic processing improves quality dramatically.
Drying and crushing. Spread extracted clay in thin layers and let it sun-dry until brittle. Crush dried clay with a heavy stone, wooden mallet, or log roller. Break it into pieces smaller than 10 mm.
Screening. Pass crushed clay through a screen (woven basket, perforated metal, or a frame with stretched cord at 5-10 mm spacing). This removes stones, root fragments, and large clay lumps that would create weak spots in finished bricks.
Slaking. For very lumpy or hard clay, soak it in water for 24-48 hours. The clay absorbs water and breaks apart into a slurry. This slurry can then be passed through a screen to remove debris, allowed to settle, and the water poured off to leave clean, uniform clay.
Blending. If your clay is too pure (cracks when dried into a test brick), blend it with sand. If too sandy, blend with clay from a different, richer source. Test blends with small bricks before committing to large production.
Claim Your Source
In a rebuilding scenario, a good clay deposit is a strategic resource. Mark it, map it, and ensure your community has uncontested access. A single deposit can supply thousands of bricks, but conflicts over resource access can derail construction projects. Establish clear agreements early.