Clay Testing

Part of Pottery and Ceramics

Systematic methods for evaluating whether a clay deposit is suitable for pottery and determining its working properties.

Why This Matters

Not all clay is pottery clay. The earth is full of clay-like materials that look promising but crack, crumble, melt, or explode when you try to make vessels from them. Without testing, you might spend days digging, processing, forming, and firing — only to open the kiln to a pile of broken shards. Systematic testing prevents this waste and identifies the best deposits in your area before you commit labor to full-scale production.

In a rebuilding scenario, your survival community needs reliable pottery for water storage, cooking, and food preservation. You can’t afford trial-and-error at scale. A few hours of testing a clay sample will tell you whether it’s worth building your entire pottery operation around that deposit or whether you should keep looking.

Testing also reveals what adjustments a clay needs. A deposit that fails as-is might become excellent pottery clay with the addition of temper, blending with another clay, or firing at a different temperature. Without testing, you’d never know what modifications would work — with testing, you can engineer a successful clay body from imperfect raw materials.

Field Identification Tests

These tests can be performed on-site when evaluating a potential clay deposit, requiring no equipment beyond your hands.

Visual Identification

Clay deposits appear as:

• Smooth, fine-grained soil that feels slippery when wet
• Exposed cuts in riverbanks, road cuts, hillsides, or erosion gullies
• Layers distinct from surrounding soil — often gray, red, yellow, or white bands
• Areas where water pools after rain (clay is impermeable)

Color	Typical Composition	Notes
Red/orange	Iron-rich earthenware	Most common; fires red-brown at low temperatures
Gray/blue	Reduced iron clay	Often found in waterlogged areas; may fire buff or red
White/cream	Kaolin or low-iron clay	Higher firing temperature; rarer and more valuable
Yellow/tan	Iron + limestone mix	Watch for limestone — can cause lime popping
Dark brown/black	High organic content	Needs processing; organics burn out during firing

The Bite Test

Place a small piece of the material between your front teeth and bite gently:

• Clay: Feels smooth, slightly gritty, no crunch. Slides between teeth.
• Silt: Feels gritty, like very fine sandpaper. Some crunch.
• Sand: Distinctly gritty, crunches, individual grains felt clearly.

You want material that tests as clay or clay-dominant. High silt or sand content means the material will need significant processing or blending.

The Ribbon Test

1. Moisten a small handful of the material until it’s pliable (like firm dough)
2. Roll it into a ball about 3 cm across
3. Flatten the ball between your thumb and forefinger, squeezing it into a thin ribbon
4. Let the ribbon extend over your finger and see how long it gets before breaking

Ribbon Length	Clay Content	Suitability
Less than 3 cm	Low — mostly silt or sand	Poor; needs heavy blending with plastic clay
3-5 cm	Moderate	Usable with temper addition; may be good as-is for some forms
5-8 cm	Good	Suitable for most pottery; may need temper to reduce shrinkage
Over 8 cm	Very high	Excellent plasticity but likely high shrinkage; add temper

The Worm Test

Roll a piece of moistened clay into a coil about pencil thickness (6-7 mm). Slowly bend it into a circle:

• Cracks immediately: Too lean (sandy) for pottery without modification
• Cracks at a gentle curve: Marginal; needs blending or aging
• Forms a circle without cracking: Good plasticity; suitable for pottery
• Wraps around a pencil without cracking: Excellent plasticity

Workshop Tests

These require basic tools and a few days but give much more reliable results than field tests alone.

Shrinkage Test

Shrinkage determines how much your pieces will change size during drying and firing — critical for planning vessel sizes and predicting cracking risk.

1. Form a flat tile approximately 12 cm long, 3 cm wide, and 1 cm thick
2. While still wet, use a sharp stick or knife to scribe two parallel lines exactly 10.0 cm apart on the surface
3. Measure and record: “Wet length: 10.0 cm”
4. Allow the tile to dry slowly (3-5 days in shade)
5. Measure the distance between the lines: “Dry length: ____ cm”
6. Fire the tile in your kiln
7. Measure again: “Fired length: ____ cm”

Calculate shrinkage:

• Drying shrinkage = (10.0 - dry length) / 10.0 x 100%
• Total shrinkage = (10.0 - fired length) / 10.0 x 100%

Total Shrinkage	Assessment
Under 8%	Low shrinkage — may be too lean (sandy), but easy to dry
8-12%	Ideal range for most pottery
12-15%	Workable but higher cracking risk; add temper
Over 15%	Problematic — needs significant temper or blending

Absorption Test

This measures how porous the fired clay is — important for vessels that must hold water.

1. Fire a test tile completely
2. Weigh the fired tile (dry weight)
3. Submerge the tile in water for 24 hours
4. Remove, wipe surface dry with a cloth, weigh again (wet weight)
5. Absorption = (wet weight - dry weight) / dry weight x 100%

Absorption	Fired State	Use
Under 2%	Vitrified (glass-like)	Stoneware; waterproof without glaze
2-8%	Semi-vitrified	Good for glazed ware; holds glaze well
8-15%	Porous earthenware	Needs glaze for waterproofing; fine for cooking pots
Over 15%	Very porous	Suitable for evaporative coolers but not water storage

No Scale Available?

Without a scale, use a balance beam with known counterweights (equal stones). Or simply observe: does water bead on the surface (low absorption) or soak in immediately (high absorption)?

Firing Temperature Test

Different clays mature (reach maximum strength and minimum porosity) at different temperatures. Testing across a range reveals your clay’s sweet spot.

1. Make 5-6 identical test tiles from the same clay batch
2. Fire them at progressively higher temperatures:
   • Tile 1: Low fire (~700-800°C) — dull red heat
   • Tile 2: Medium-low (~850-950°C) — bright cherry red
   • Tile 3: Medium (~950-1050°C) — orange-red
   • Tile 4: Medium-high (~1050-1150°C) — yellow-orange
   • Tile 5: High (~1150-1250°C) — yellow-white
3. Compare the tiles for:
   • Color change (darkening then lightening indicates maturation)
   • Hardness (scratch test with a knife — harder is more mature)
   • Sound (tap with a knuckle — dull thud means underfired, clear ring means mature)
   • Absorption (water drop test on surface)
   • Warping or bloating (signs of overfiring)

Bloating and Melting

If a tile bloats (swells up with internal bubbles) or begins to slump and deform, you’ve exceeded that clay’s maximum safe temperature. Back off by at least 50-100°C for production firing.

Thermal Shock Test

Critical for cooking vessels that will be placed directly on fire:

1. Fire a small test bowl or cup completely
2. Heat it gradually over a fire until it’s too hot to touch
3. Immediately plunge it into cold water
4. If it survives without cracking, repeat the cycle 3-5 times
5. Clay bodies with crushed shell or grog temper perform best

Problem Diagnosis

Common Issues and Solutions

Problem in Test	Likely Cause	Solution
Cracking during drying	Too plastic (fatty); insufficient temper	Add 15-25% grog or sand
Cracking during firing	Residual moisture; too rapid temperature rise	Dry longer; fire slower
Warping	Uneven wall thickness or uneven drying	Improve forming technique
Lime popping (small chunks flaking off after firing)	Limestone particles in clay	Screen through finer mesh; or soak fired ware in water for a week to slake all lime before use
Weak, crumbly after firing	Underfired	Increase temperature or firing duration
Bloating	Overfired	Reduce temperature
Too porous	Underfired or naturally refractory clay	Fire hotter, or blend with lower-firing clay

The Lime Test

Limestone contamination is a serious problem. Limestone particles convert to quickite during firing, then absorb moisture from the air and expand, popping out chunks of the vessel wall days or weeks after firing.

Detection:

1. Drip a few drops of vinegar (or any acid) onto raw clay
2. If it fizzes/bubbles, calcium carbonate (limestone) is present
3. Alternatively, look for white specks in the raw clay — these may be limestone

Prevention:

• Screen clay through a fine (1 mm or less) mesh to remove limestone particles
• Slake and levitate the clay to separate heavy limestone fragments
• If unavoidable, fire to higher temperatures where limestone begins to flux rather than causing spalling

Recording and Comparing Results

Create a standardized test record for every clay source you evaluate. Scratch the results into a fired tile — this creates a permanent, fireproof record.

Essential data to record:

1. Source location (distance and direction from settlement)
2. Deposit size (estimated volume available)
3. Field test results (ribbon length, worm test)
4. Shrinkage (drying and total)
5. Optimal firing temperature (based on tile tests)
6. Absorption at optimal temperature
7. Thermal shock resistance (pass/fail)
8. Color at optimal temperature
9. Modifications needed (temper type and amount, blending partner)

When you’ve tested multiple deposits, compare systematically. The best clay source isn’t always the one with the best raw properties — it’s the one that balances quality, quantity, accessibility, and ease of processing. A moderate clay source a short walk from your settlement may be more valuable than a superior deposit a day’s journey away.

Testing is an investment that pays for itself many times over. A few days of systematic evaluation saves months of wasted effort and gives your pottery operation a foundation of reliable, predictable materials.