Purifying clay from the ground is a straightforward water-based process: you dissolve raw soil into a thin slurry, strain out debris, then let gravity separate the fine clay particles from sand and silt. The whole process takes a few days of mostly passive waiting, and the only tools you need are buckets, water, a sieve, and patience.
Finding and Testing Raw Clay
Clay deposits show up in riverbanks, road cuts, construction sites, and anywhere erosion has exposed subsoil. Look for soil that feels slick and sticky when wet and holds together when you squeeze it. A quick field test: roll a moist pinch between your fingers. If it forms a smooth ribbon without crumbling, there’s a meaningful amount of clay in it.
To find out exactly how much clay you’re working with, run a jar test at home. Fill a straight-sided glass jar one-third full with your soil sample, add water until nearly full, then drop in a teaspoon of dishwashing soap (which helps separate the particles). Shake vigorously and set it down. Sand settles to the bottom within one minute. Silt settles over the next several hours. After 24 hours, the remaining layer on top is clay. Mark each layer and measure the proportions. Soil with at least 20 to 30 percent clay is worth processing. Anything less, and you’ll do a lot of work for very little usable material.
Preparing the Soil
Regardless of whether your clay comes out of the ground wet or dry, it all needs to end up as a thin liquid slurry. That’s the key to removing every bit of foreign matter: roots, pebbles, organic debris, and coarse mineral particles.
If the soil is already moist, break it into small chunks and drop them into a bucket of water. If it’s dry (which actually makes this easier), pound or crush it into a powder first. Dried clay breaks down much faster when you add water because the tiny particles separate more readily. Once you’ve got your crushed soil, add it to a bucket and cover it generously with water. Stir it into a thin, uniform slurry with the consistency of heavy cream. Let it soak for several hours or overnight, stirring occasionally to break up stubborn clumps.
Straining Out Debris
Once your slurry is fully dissolved, pour it through a mesh screen into a second clean bucket. A common kitchen sieve works for the first pass, catching roots, stones, and large organic matter. For finer results, follow up with a tighter mesh, such as a window screen or a piece of fabric stretched over the bucket opening. The goal is to let only water and suspended clay particles pass through while trapping everything else. You may need to stir the slurry on the screen to help it flow, and you can rinse remaining material with extra water to extract as much clay as possible before discarding the solids.
Separating Clay From Sand and Silt
This is where particle physics does the work for you. Sand, silt, and clay particles are different sizes, and they settle out of water at very different rates. Sand drops to the bottom of a container within about 2 minutes. Silt takes several hours. Clay particles are so fine that they stay suspended for a full 24 hours or longer.
You can use this to your advantage with a simple decanting process. Pour your strained slurry into a tall container and wait 2 to 5 minutes. Any sand in the mixture will have already settled to the bottom. Carefully pour the still-cloudy liquid off the top into a new container, leaving the sandy sediment behind. That cloudy liquid is your clay and silt still in suspension.
Now let this second container sit undisturbed for 24 hours. During this time, silt gradually sinks while the finest clay particles remain suspended in the water above. You can pour off or siphon the top portion (which is mostly water with suspended clay) into yet another container if you want the purest possible clay. Or, if you’re less concerned about removing every trace of silt, simply let the entire container settle for a day or two until you see clear water sitting above a layer of sediment. That sediment is your purified clay.
Removing Excess Water
After settling, you’ll have a thick layer of wet clay at the bottom of your container with a pool of water on top. Carefully ladle, siphon, or sponge off the standing water. You’ll need to do this several times over the course of a few days as more water continues to separate from the clay.
Once you’ve removed as much water as possible by hand, spread the wet clay on a plaster bat, concrete slab, or a canvas-covered board. Plaster works especially well because it actively wicks moisture from the clay. In warm, dry weather, you can spread it on a clean wooden board outside. Flip and turn the clay periodically so it dries evenly. You’re aiming for a workable consistency, firm enough to hold its shape but soft enough to bend without cracking. If it dries too much, you can always rewet it and start the drying process again.
Testing Your Purified Clay
Wild clay behaves differently from commercial clay, so before committing to a project, run a few simple tests. Roll out several small tiles about 5mm thick. Score a line exactly 10 centimeters long on each one. Let them air dry completely, then measure the line again to check drying shrinkage. Fire one tile at a low temperature and another at a higher temperature, then measure again. Total shrinkage (drying plus firing) for most natural clays ranges from 8 to 15 percent. To plan for this in finished work, divide your desired size by 100 percent minus the shrinkage percentage. For example, if your clay shrinks 10 percent and you want a 10 cm piece, you’d need to start at about 11.1 cm.
Check the fired tiles for cracks, warping, and color. Some wild clays fire beautifully on their own. Others are too “short” (crumbly) and need a plasticizer like ball clay mixed in, or too “fat” (sticky and prone to cracking) and benefit from the addition of fine sand or grog. A clay that warps badly or bloats during firing may contain too muchite organic material, which means you may need to process it again or settle it more carefully to get a cleaner result.
Storing Processed Clay
Once your clay reaches a good working consistency, wrap it tightly in plastic and store it in a sealed container. Wild clay actually improves with age. Letting it sit for weeks or months (a process potters call aging) allows bacteria to break down remaining organic particles and improves plasticity. If you’ve processed more than you can use immediately, you can also dry it completely and store it as powder indefinitely. When you’re ready to use it, just add water and slake it back into a slurry, repeating the settling and drying steps as needed.