Clay Selection

Part of Brick Making

Choosing the right clay types and sources for producing quality bricks.

Why This Matters

Not all clay makes good bricks. Some clays crack during drying, some crumble after firing, and some contain contite that cause bricks to disintegrate within months. Selecting the right clay source before you invest days of labor in digging, preparing, forming, and firing saves enormous amounts of wasted effort.

Good brick clay is abundant in most environments — riverbanks, hillside cuts, flood plains, and subsoil layers all yield usable material. The challenge is not finding clay but identifying which deposits will perform well. A 30-minute field evaluation of a clay source tells you more about its brick-making potential than days of trial and error.

Understanding what makes clay suitable for bricks also lets you blend different clays to compensate for individual weaknesses. A too-plastic clay mixed with a sandy clay produces better bricks than either alone. This knowledge turns marginal clay sources into productive ones.

Where to Find Brick Clay

Natural Deposits

Clay accumulates wherever water slows down and drops fine particles. The most reliable sources:

Riverbanks and flood plains: Alluvial clay deposited by flooding is often well-sorted and relatively pure. Look along the inside of river bends where water velocity drops. Dig below the topsoil layer (typically 20-40 cm down) to reach clay relatively free of organic matter.

Hillside exposures: Road cuts, stream-cut banks, and eroded hillsides expose clay layers. Look for smooth, shiny surfaces where water has polished exposed clay. The color is often distinctive — reds, yellows, grays, and blues all indicate clay presence.

Subsoil layers: In many areas, clay sits 0.5-2 meters below the surface under topsoil and subsoil. Dig test holes in areas where surface water pools after rain — standing water often indicates an impermeable clay layer below.

Pond and lake beds: The bottom of seasonal ponds and dried lake beds often contains thick clay deposits. These clays tend to be very fine-grained and plastic, sometimes requiring sand addition for brick making.

Signs of Clay Presence

Indicator	What It Means
Standing water after rain	Impermeable clay layer below
Cracked, polygonal surface pattern when dry	High clay content in surface soil
Slippery when wet, hard when dry	Clay-rich soil
Smooth, shiny cut surfaces	Clay exposed by erosion
Specific plants (willows, rushes, cattails)	Wet clay soil underneath
Termite mounds	Insects bring clay to surface from depth

What Makes Good Brick Clay

Essential Properties

Plasticity: The clay must be plastic enough to form into bricks and hold their shape. Pure sand has zero plasticity. Pure clay minerals have maximum plasticity. Brick clay needs moderate plasticity — enough to fill a mold cleanly but not so much that it shrinks excessively during drying.

Shrinkage tolerance: Clay shrinks as it loses water during drying and again during firing. Total shrinkage above 10% causes excessive cracking. The ideal range is 5-8% total shrinkage.

Firing behavior: Good brick clay contains enough flux minerals (iron oxides, calcium, alkali metals) to partially vitrify at achievable kiln temperatures (800-1,050 C). Clays that require temperatures above 1,100 C to harden are impractical for primitive firing.

Freedom from contamination: Lime nodules (calcium carbonate lumps) are the most dangerous contamination. They calcine during firing and then absorb moisture from the air, expanding and shattering the brick from within. This can happen weeks after firing — a perfectly good-looking brick suddenly cracks apart.

Color as a Guide

Clay color indicates mineral content, which affects firing behavior:

Raw Clay Color	Mineral Content	Fired Color	Brick Quality
Red/orange	High iron oxide	Red/brown	Excellent — iron acts as flux
Yellow/tan	Moderate iron	Buff/cream	Good — fires at moderate temps
Gray/blue	Organic matter, reduced iron	Red (oxidized) or gray	Good if organics burn out
White	Low iron, high alumina	White/cream	Needs high temps — harder to fire
Black	High organic content	Depends	Must test — organics may cause problems

The Iron Advantage

Iron-rich (red) clays are the easiest to fire into good bricks. The iron oxide acts as a natural flux, helping the clay vitrify at lower temperatures. If you have a choice between a red clay and a white clay source, choose the red for brick making.

Evaluating a Clay Source

Quick Field Assessment

Before investing time in extraction, evaluate the source:

1. Volume: Is there enough clay for your needs? A single-story building requires 5,000-8,000 bricks. At roughly 2.5 kg of clay per standard brick, that is 12-20 tonnes of raw clay. Ensure the deposit is large enough.

2. Accessibility: Can you dig and transport the clay with available labor? A source 500 meters from your brick-forming site is practical. Five kilometers away is not, unless you have animal transport.

3. Depth of overburden: How much topsoil and unsuitable material must you remove to reach the clay? More than 1 meter of overburden makes extraction expensive in labor terms.

4. Consistency: Dig in several spots across the deposit. Is the clay similar throughout, or does it vary dramatically? Consistent deposits are easier to work with.

5. Water access: Clay preparation requires large quantities of water. A source near a stream or pond saves considerable transport effort.

Laboratory-Free Quality Assessment

You do not need a laboratory to evaluate clay quality. The following field tests, covered in detail in the Clay Testing article, give you reliable information:

• Bite test: Gritty between teeth = sandy. Smooth = high clay content. Slightly gritty with a smooth finish = ideal brick clay.
• Ribbon test: Roll clay into a cylinder and flatten. Good brick clay forms a ribbon 5-8 cm long before breaking.
• Shrinkage bars: Form small bars, measure before and after drying. Under 8% shrinkage is acceptable.
• Trial bricks: Nothing replaces making 10-20 test bricks and firing them. This 3-day investment can save weeks of wasted effort.

Blending Clays

Single clay sources rarely have perfect properties. Blending compensates for individual deficiencies:

Common Blending Scenarios

Too plastic (sticky, high shrinkage): Blend with sandy clay or add sharp sand at 20-30% by volume. This reduces plasticity and shrinkage while maintaining strength.

Too sandy (crumbly, won’t hold shape): Blend with a more plastic clay. Even small additions (10-20%) of high-plasticity clay can transform a sandy soil into workable brick clay.

Too high firing temperature: Add iron-rich clay or wood ash as a flux. This lowers the temperature needed for vitrification.

Contains lime nodules: If the deposit is otherwise excellent, screen the clay through a coarse mesh (5 mm) after soaking to remove lime nodules. This is labor-intensive but worth it for a good clay source.

Blending Procedure

1. Prepare each clay separately — soak, break down
2. Measure components by volume using consistent containers
3. Mix thoroughly using the pit or foot method
4. Make 5-10 test bricks from the blend
5. Dry and fire the test bricks
6. Evaluate results and adjust the ratio if needed

Document Your Blend

When you find a good blend ratio, record it precisely. “About this much of the red clay” is not helpful six months later. Use standardized containers (a specific bucket, a basket) and count scoops.

Sustainable Extraction

If your community will make bricks for years, plan your clay extraction to avoid creating hazards or destroying useful land:

• Dig systematically from one side of a deposit, moving across rather than digging random holes
• Backfill exhausted pits with topsoil and rubble — unfilled clay pits fill with stagnant water and breed mosquitoes
• Leave at least 2 meters between your pit edge and any structures, roads, or property boundaries
• If digging near a riverbank, leave a buffer of undisturbed soil to prevent bank collapse
• Rotate between multiple smaller pits rather than creating one enormous excavation