Clay Sourcing

Part of Pottery and Ceramics

Before you can form a single pot, you need clay — and not just any dirt will do. Clay sourcing is the skill of finding, identifying, testing, and preparing natural clay deposits for use in pottery, bricks, furnace linings, and dozens of other essential applications.

Why Clay Sourcing Matters

Clay is one of the most useful natural materials for rebuilding. It becomes watertight storage vessels, building bricks, furnace linings, pipes, tiles, and even writing tablets. But raw clay varies enormously in quality. Some deposits produce excellent pottery with minimal processing; others crack, crumble, or shatter in the fire. Knowing how to find and evaluate clay saves weeks of failed experiments.

Where to Find Clay

Clay forms from the weathering of feldspar-rich rocks (granite, gneiss, basite) over thousands of years. It concentrates in predictable locations.

Primary Deposits (Residual Clay)

Found where the parent rock weathered in place:

• Hillside exposures — Look for smooth, slippery soil on hillsides, especially after rain
• Road cuts and landslides — Fresh earth exposures reveal clay layers
• Underneath topsoil — Dig below the organic layer (usually 15-60 cm down) to find clay subsoil

Secondary Deposits (Transported Clay)

Carried by water and deposited elsewhere — often the best pottery clay:

• River banks — Inside bends where current slows and fine particles settle
• Floodplains — Flat areas beside rivers that flood seasonally
• Lake beds and pond bottoms — Thick clay deposits, especially in dried or drained areas
• Stream beds — Dig into the banks, not the rocky bottom
• Estuaries — Where rivers meet the sea, fine clay settles

Deposit Type	Typical Quality	Processing Needed	Where to Look
River bank	Good-excellent	Minimal	Inside bends, cutbanks
Lake bed	Very fine, excellent	Minimal	Dried lake margins
Hillside residual	Variable	May need cleaning	Exposed slopes
Subsoil	Fair-good	Often needs tempering	Anywhere, dig down
Glacial till	Variable	Heavy cleaning	Northern regions

Follow the Slippery Ground

After a rain, walk your area looking for patches of ground that are noticeably slippery underfoot. Clay becomes extremely slick when wet — this is the easiest field indicator.

Identifying Clay

Not all smooth, fine soil is clay. True clay has specific properties that distinguish it from silt, loam, or fine sand.

The Squeeze Test

1. Take a handful of damp soil
2. Squeeze it in your fist
3. Open your hand — clay holds its shape and shows finger impressions
4. Silt and sand crumble apart

The Ribbon Test

1. Roll a piece of damp material into a ball
2. Flatten it between your thumbs and forefingers, pressing it out into a ribbon
3. True clay forms a ribbon 5-10 cm or longer before breaking
4. Sandy soil cannot form a ribbon at all
5. Silt makes a short, crumbly ribbon (under 3 cm)

Ribbon Length	Soil Type	Pottery Suitability
0-2 cm	Sandy soil	Not usable
2-4 cm	Sandy clay or silt	Poor — needs heavy cleaning
4-7 cm	Clay loam	Usable with tempering
7+ cm	True clay	Excellent pottery material

The Bite Test

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

• Clay feels smooth and slightly waxy, sticks to teeth
• Silt feels gritty but smooth
• Sand grinds and crunches

The Shine Test

Take a smooth, moist ball of clay and rub it with your thumb:

• True clay develops a slight shine or sheen on the rubbed surface
• Silt and sandy soil remain dull and matte

Testing Clay for Pottery

Finding clay is step one. Testing it for workability and firing quality is step two.

Workability Test

1. Knead a lump of clay until smooth and even
2. Roll it into a coil about pencil-thickness
3. Wrap the coil around your finger
4. Good clay: bends smoothly with minor surface cracks
5. Marginal clay: cracks heavily but holds together
6. Bad clay: breaks apart completely

Shrinkage Test

Excessive shrinkage causes cracking during drying and firing:

1. Form a flat tile about 10 cm x 3 cm x 1 cm
2. Score a line exactly 10 cm long on the surface while wet
3. Dry completely in shade (3-5 days)
4. Measure the scored line again
5. Calculate shrinkage: (original - final) / original x 100

Shrinkage	Rating	Action
Under 5%	Excellent	Use as-is
5-8%	Good	Usable, dry slowly
8-12%	Acceptable	Add temper to reduce shrinkage
Over 12%	Excessive	Needs significant tempering or blending

Firing Test

The ultimate test — does it survive the fire?

1. Form several small test tiles (5 cm squares)
2. Dry completely
3. Fire in a campfire or kiln:
   • First at low temperature (around a campfire edge) for 1 hour
   • Then at high temperature (buried in the coals) for 1-2 hours
4. Let cool slowly in the ashes
5. Examine the results:

Result	Meaning
Hard, rings when tapped	Excellent — well-fired, strong
Hard but porous	Good — suitable for most pottery
Crumbles or flakes	Too much organic matter — needs cleaning
Explodes or shatters	Moisture trapped inside or limestone inclusions
Warps badly	Excessive shrinkage — add temper
Melts or fuses	Overfired — use lower temperature or as slip/glaze

Limestone Kills Pottery

Clay containing limestone fragments (calcite, shell, chalk) will explode when fired. The limestone converts to quickite, which expands when it absorbs moisture from the air after firing — popping off chunks of the pot. Pick out all white calcareous fragments before using clay, or choose a different deposit.

Cleaning and Processing Raw Clay

Most natural clay contains impurities that must be removed before use.

Dry Processing

For relatively clean clay:

1. Dry the clay completely in the sun
2. Break into small chunks
3. Pick out roots, stones, and visible debris
4. Crush to powder with a stone or hammer
5. Sieve through a fine mesh (woven basket or cloth) to remove grit

Wet Processing (Levigation)

For contaminated or mixed clay — produces the cleanest material:

1. Break raw clay into walnut-sized pieces
2. Place in a large container (barrel, pot, or dug-out basin)
3. Add 3-4 times as much water
4. Stir vigorously and let sit for 30 seconds
5. Pour the cloudy water into a second container — this carries the fine clay particles
6. Discard the settled sediment (sand, grit, pebbles) in the first container
7. Let the second container sit undisturbed for 24-48 hours
8. Carefully pour off the clear water on top
9. The settled material at the bottom is clean, fine clay
10. Spread on a cloth or board to dry to workable consistency

Multiple Washings

For the finest pottery clay, repeat the levigation process 2-3 times. Each washing removes finer grit and produces smoother, more plastic clay. This is especially worthwhile for thin-walled vessels and decorative ware.

Aging (Souring)

Clay improves with age. Stored wet clay develops beneficial bacterial activity that increases plasticity:

1. Store cleaned, wet clay in a sealed container (covered pot, plastic bag, or clay-sealed vessel)
2. Let sit for at least 2 weeks — longer is better
3. Clay aged for months or years is noticeably more workable than fresh clay
4. Ancient Chinese potters aged clay for decades; even a few weeks helps significantly

Tempering (Adding Grit)

Pure clay often shrinks too much during drying and firing. Adding temper — a non-shrinking material — reduces shrinkage and improves thermal shock resistance.

Common Temper Materials

Temper	Particle Size	Effect	Best For
Sand (quartz)	Fine-medium	Reduces shrinkage, easy to find	General pottery
Grog (crushed fired pottery)	Fine-medium	Best all-around temper	Everything
Crusite shell (burned)	Fine	Thermal shock resistance	Cooking pots
Ite fiber (plant material)	Any	Burns out, creates porous structure	Insulating bricks
Sawdust	Fine	Burns out, lightens clay	Lightweight items

Mixing Ratios

Start with 10-20% temper by volume:

1. Spread clay flat
2. Sprinkle temper evenly across the surface
3. Fold and knead thoroughly — 5-10 minutes
4. The temper should be evenly distributed throughout

Test the tempered clay with the shrinkage test. Add more temper if shrinkage is still above 8%.

Common Mistakes

1. Using surface topsoil — The top layer of soil contains organic matter that burns out during firing, leaving weak, porous pottery. Dig deeper to find true clay below the organic layer.
2. Skipping the firing test — A clay that feels wonderful to work may crack, explode, or crumble in the fire. Always fire test tiles before investing time in forming pots.
3. Ignoring limestone — Even small limestone fragments cause delayed failure. Pots that survive firing crack and pop days or weeks later as quicklite absorbs moisture.
4. Too much temper — Over-tempered clay (more than 30% temper) is gritty, hard to work, and weak when fired. Add temper gradually and test.
5. Not aging clay — Fresh-dug clay is often stiff and difficult to work. Even a few days of wet storage improves plasticity noticeably. A few weeks is markedly better.

Summary

Clay Sourcing — At a Glance

• Find clay at river banks, lake beds, exposed hillsides, and under topsoil
• Test with the ribbon test (7+ cm ribbon = true clay), squeeze, bite, and shine tests
• Run shrinkage test (under 8% is good) and firing test (must survive without cracking or exploding)
• Remove limestone fragments — they cause delayed pottery failure
• Clean clay by wet processing (levigation) for finest results
• Age cleaned clay wet for at least 2 weeks to improve workability
• Add 10-20% temper (sand, grog, or crushed shell) to reduce shrinkage