Clay Types
Part of Brick Making
Different clay varieties, their mineral composition, and how each performs for construction.
Why This Matters
Clay is not a single material. It is a family of minerals with widely varying properties. Two clay deposits 100 meters apart can behave completely differently during forming, drying, and firing. Understanding the major clay types helps you predict behavior, troubleshoot problems, and choose the right clay for each application.
In a rebuilding scenario, you work with whatever clay is locally available. Knowing the properties of different types lets you compensate for weaknesses — adding sand to overly plastic clay, blending clays for better firing behavior, or recognizing that a particular deposit is better suited for pottery than for bricks. This knowledge transforms “trial and error” into informed decision-making.
The clay mineral groups described here are identified by their behavior rather than by laboratory analysis. You do not need a microscope to tell kaolinite from montmorillonite — their handling properties are distinctly different.
Major Clay Mineral Groups
Kaolinite (China Clay)
How to recognize it: White or cream-colored when dry. Relatively non-sticky when wet. Low plasticity — it feels grainy and does not stretch easily. Dries quickly without cracking. Found in decomposed granite and feldspar deposits, often in hilly or mountainous areas.
Properties:
| Characteristic | Value |
|---|---|
| Plasticity | Low to moderate |
| Shrinkage | Low (3-6%) |
| Firing temperature | High (1,200-1,400 C) |
| Color when fired | White to cream |
| Water absorption | Low when fully vitrified |
Brick making assessment: Kaolinite alone makes poor bricks for primitive firing because it requires temperatures above what a clamp fire or simple kiln typically achieves. However, blending kaolinite with iron-rich clays or adding flux materials (wood ash, ground feldspar) can lower the firing temperature to a workable range. If kaolinite is your only available clay, expect higher fuel consumption and longer firing times.
Best uses: Refractory bricks (heat-resistant linings for kilns, chimneys, and fireplaces). Its high-temperature resistance is an advantage when that is specifically what you need.
Illite (Common Clay)
How to recognize it: The most common clay type worldwide. Typically gray, green-gray, or yellow-brown. Moderate plasticity — it feels smooth and workable but not excessively sticky. Forms decent ribbons in the plasticity test (5-8 cm).
Properties:
| Characteristic | Value |
|---|---|
| Plasticity | Moderate |
| Shrinkage | Moderate (5-8%) |
| Firing temperature | Moderate (900-1,100 C) |
| Color when fired | Buff, tan, or light red |
| Water absorption | Moderate |
Brick making assessment: Illite is the ideal brick-making clay. It has enough plasticity to fill molds cleanly, moderate shrinkage that is manageable with reasonable tempering, and fires to a good hard brick at temperatures achievable in a clamp fire or simple kiln. Most of the world’s brick-making traditions are based on illite-rich clays.
Best uses: General construction bricks, pavers, tiles. The workhorse of brick making.
Montmorillonite (Bentonite/Smectite)
How to recognize it: Extremely sticky and slippery when wet — almost soap-like. Swells dramatically when water is added. Very high plasticity — ribbons can extend 12-15 cm or more. Cracks badly when drying. Often found in volcanic ash deposits and marine sediments. Can be gray, green, brown, or white.
Properties:
| Characteristic | Value |
|---|---|
| Plasticity | Very high |
| Shrinkage | Very high (10-20%) |
| Firing temperature | Variable (800-1,000 C) |
| Color when fired | Variable |
| Water absorption | Very high (unfired) |
Brick making assessment: Montmorillonite alone is nearly impossible to use for bricks. The extreme shrinkage causes catastrophic cracking during drying. However, small additions of montmorillonite (5-15%) to lean, sandy clays dramatically improve plasticity and workability. Think of it as a clay supplement rather than a primary material.
Identification Warning
If your clay swells to twice its volume when soaked and becomes impossibly sticky, you are dealing with montmorillonite. Do not attempt to make bricks from it without heavy sand or grog addition (30-50% by volume).
Best uses: As an additive to improve lean clays. Also excellent for lining ponds (its swelling seals water in) and as a drilling mud substitute.
Chlorite Clay
How to recognize it: Green or dark green-gray. Moderate plasticity. Often found in metamorphic rock areas. Feels slightly soapy or waxy when wet.
Properties:
| Characteristic | Value |
|---|---|
| Plasticity | Low to moderate |
| Shrinkage | Low to moderate (4-7%) |
| Firing temperature | Moderate (900-1,100 C) |
| Color when fired | Dark red to brown |
| Water absorption | Moderate |
Brick making assessment: Chlorite clays make acceptable bricks but are less common than illite. Their green color can be misleading — they fire to red or brown like any iron-bearing clay. Workable for bricks with standard preparation.
Mixed and Natural Blends
In practice, most clay deposits contain mixtures of clay minerals plus non-clay components. These natural blends often make better bricks than pure clay minerals.
Alluvial Clay (River Clay)
Deposited by flowing water, alluvial clay is typically a mixture of illite and fine silt with some sand. It is pre-blended by nature — the water sorting process removes very coarse material while retaining a mix of particle sizes that often produces excellent brick clay with minimal preparation.
Typical properties: Moderate plasticity, 5-7% shrinkage, fires at 850-1,000 C. Often gray or brown from organic content.
Advantage: Already partially tempered by natural silt and sand content. May need little or no additional sand.
Glacial Clay (Till Clay)
Left behind by glaciers, these clays are often mixed with a wide range of particle sizes from clay to gravel. Quality is highly variable — some glacial deposits are excellent brick clay while others are too contaminated with stones.
Typical properties: Variable plasticity, may contain lime nodules (test carefully). Color ranges from gray to brown.
Advantage: Often available in enormous quantities. Disadvantage: Inconsistent quality, may need extensive screening.
Laterite (Tropical Clay)
Found in tropical and subtropical regions, laterite is clay heavily enriched with iron and aluminum oxides. It is often red, orange, or yellow.
Typical properties: Moderate to low plasticity, low shrinkage, fires at relatively low temperatures due to high iron content. Self-tempering — the iron oxides act as natural grog.
Advantage: Excellent for bricks. Some laterite deposits can be cut into blocks directly and air-dried into usable building material without firing at all (laterite blocks).
Choosing Clay by Application
| Application | Best Clay Type | Key Property Needed |
|---|---|---|
| Common bricks | Illite, alluvial | Moderate plasticity, fires at achievable temps |
| Load-bearing bricks | Illite with grog temper | High fired strength |
| Kiln lining | Kaolinite | High temperature resistance |
| Roof tiles | Illite, low-shrinkage blend | Low warping during firing |
| Floor tiles | Laterite, iron-rich illite | Hardness, wear resistance |
| Water pipes | Dense-firing illite | Low water absorption |
| Mortar clay | Any plastic clay | Workability, not fired |
| Plaster/render | Montmorillonite blend | Adhesion, smooth finish |
Improving What You Have
Since you cannot choose which clay minerals exist near your settlement, here are strategies for making the best of each type:
If your clay is too plastic (montmorillonite-rich): Add sharp sand (20-40% by volume), crushed fired brick, or fine gravel. Mix thoroughly. Test with shrinkage bars before committing to production.
If your clay is too lean (kaolinite-rich or sandy): Add a small amount of plastic clay from another source. Even 10-15% montmorillonite-rich clay transforms a lean mix. Alternatively, ball-mill or foot-mix the clay more intensely to develop whatever plasticity exists.
If your clay fires too soft (kaolinite without flux): Add wood ash (5-10% by volume) as a flux to lower the vitrification temperature. Iron-rich soil additions also serve as flux. Alternatively, fire longer and hotter — build a better kiln.
If your clay contains lime: Screen meticulously through a 3 mm mesh after soaking. Crush any remaining lumps during preparation. There is no chemical fix — physical removal is the only reliable solution.
The Best Brick Clay Is Usually Boring
The ideal brick clay is not the most plastic, not the most colorful, and not the most unusual deposit. It is the moderate, well-balanced, slightly gritty clay that feels like bread dough when wet and dries without drama. Seek the unremarkable.