Refining clay means turning raw, dirt-filled material into a smooth, workable body you can shape and fire. The basic process involves breaking the clay down in water, screening out debris, letting the fine particles settle, then drying the slip to a usable consistency. Whether you’ve dug clay from a riverbank or a hillside, the steps are the same, and the whole process can be done with household tools over a few days.
Finding and Identifying Raw Clay
Clay forms wherever rock meets water, air, or steam over long periods. The most common places to find it are eroded hillsides, creek banks, lake beds, and areas where the soil has a slick, sticky quality when wet. Road cuts and construction sites often expose clay-rich layers beneath the topsoil. The color can range from white to red to gray to nearly black, depending on the minerals present.
A quick field test: grab a handful of the material, wet it, and try to roll it into a thin coil about the width of a pencil. If you can wrap the coil around your finger without it cracking apart, you’re looking at clay with decent plasticity. If it crumbles immediately, the material is too sandy or silty to be worth refining. You can also rub a wet pinch between your fingers. Clay feels smooth and slippery, not gritty.
Breaking Down the Clay (Slaking)
Start by breaking your raw clay into small chunks, roughly walnut-sized or smaller. Spread them out and let them dry completely. Dry clay absorbs water much faster than damp clay, which speeds up the process considerably. Once dry, place the chunks in a bucket and cover them with water. The clay will begin to dissolve and soften on its own. This is called slaking.
Give it several hours or overnight. Some dense clays take longer. Once the chunks have broken down, stir the mixture vigorously until you have a uniform, thin slurry with the consistency of heavy cream. There shouldn’t be any lumps left. If stubborn chunks remain, let them soak longer or break them apart by hand.
Screening Out Debris
This is where refining actually happens. Pour your slurry through a mesh screen into a second clean bucket. The screen catches rocks, roots, pebbles, and coarse sand while letting the fine clay particles pass through in suspension.
The mesh size you choose depends on what you’re making. A 30-mesh screen removes the largest grit and is the bare minimum for functional pottery. Screening to 50 or 60 mesh produces a noticeably smoother clay that’s more pleasant to throw or hand-build with. For very fine work, you can go even finer, though the process gets slower as the mesh tightens. A common kitchen strainer works in a pinch for small batches, but a purpose-built sieve (available from ceramic supply shops) is much faster for larger amounts.
You may need to push the slurry through with your hand or a rubber spatula, especially with finer screens. Rinse the screen periodically as it clogs.
Settling and Removing Excess Water
After screening, you’ll have a bucket of thin, watery slip. Let it sit undisturbed for at least a full day. The clay particles will slowly sink to the bottom, leaving a layer of clear water on top.
If the water on top is truly clear, pour or siphon it off carefully without disturbing the settled clay beneath. A length of tubing works well for siphoning. A turkey baster or large sponge can remove the last bit of water near the clay surface. If the water still looks cloudy, leave it. That cloudiness is ultra-fine clay particles still in suspension, and you want to keep those.
You can repeat this settling and siphoning cycle if the clay is still very thin after the first round. Each time, you remove more water and the slip thickens.
Drying to a Workable Consistency
Once you’ve removed as much standing water as possible, you need to dry the slip further until it reaches a plastic, workable state. There are several ways to do this:
- Plaster slab: Pour the thick slip onto a plaster bat or a large plaster block. The plaster wicks moisture out of the clay surprisingly fast, often within hours depending on thickness and humidity.
- Fabric method: Pour the slip into a pillowcase or a cloth bag and hang it to drain. The water seeps through the fabric while the clay stays inside. This can take a day or two.
- Open-air evaporation: Spread the slip in a shallow container (a plastic tub or wooden box lined with fabric) and let it air dry, stirring occasionally. This is the slowest method but requires the least equipment.
Check the clay periodically. You’re aiming for a consistency that holds its shape when squeezed but doesn’t crack or crumble. It should feel like firm modeling clay, not sticky mud.
Adding Sand and Kneading
Wild clay often benefits from the addition of a temper material, typically sand, to reduce cracking during drying and firing. A ratio of about 20% sand to 80% clay by volume is a reliable starting point. Use clean, fine-grained sand without organic material in it.
Mix the dry sand into the clay thoroughly. For larger batches, you can pour the dry clay and sand back and forth between two buckets until the mixture looks consistent with no visible streaks of pure clay or pure sand. For smaller batches, knead by hand on a clean surface. Add water a little at a time as you knead if the mixture is too dry. Go slowly here because the clay can shift from too dry to too wet faster than you’d expect. Add a small splash, knead it in fully, then assess before adding more.
Testing Before You Commit
Wild clay is unpredictable. Before making a full project, run a few simple tests. Roll out several small bars of your refined clay, each about the length and width of a finger. Mark a line exactly 10 centimeters long on each bar while the clay is still wet. After the bars dry completely, measure that line again. The difference tells you your drying shrinkage percentage.
Fire your test bars at several different temperatures if you have access to a kiln. After firing, measure the marked line a third time to calculate fired shrinkage. You can also weigh each bar, boil it in water for a couple of hours, then weigh it again. The weight difference tells you how much water the fired clay absorbed, which indicates how mature (fully vitrified) the clay body is at that temperature. High absorption means the clay needs a higher firing temperature. Low absorption means it’s approaching its limit.
Total shrinkage from wet to fired typically falls in the range of 10 to 15 percent for most clays. To predict your final dimensions on any piece, multiply the original measurement by (1 minus the shrinkage percentage expressed as a decimal). So for a clay with 14% total shrinkage, a 10-centimeter-wide bowl will end up about 8.6 centimeters wide after firing.
Keeping Yourself Safe
The most serious health risk in clay processing is inhaling fine silica dust. Crystalline silica particles, present in virtually all clay and sand, can cause silicosis when breathed in over time. Symptoms include persistent cough, shortness of breath, chest pain, and wheezing, and the damage is permanent.
The simplest rule: keep clay wet. Wet clay produces no airborne dust. When you must handle dry clay (crushing raw chunks, mixing dry powders, cleaning up dried scraps), wear an N95 respirator or better. Never sweep dried clay dust with a dry broom. Use a damp sponge, a wet mop, or a vacuum equipped with a HEPA filter. Work outdoors or in a well-ventilated space when processing dry material, and mist the clay with a spray bottle before handling it to knock down dust before it becomes airborne.