When most people refer to “dirt,” they are talking about common soil, which is a complex mixture of organic material and mineral particles like sand, silt, and clay. Clay is the finest mineral component within soil, composed of microscopic, plate-like particles of hydrous aluminum silicates. These tiny particles, typically less than two micrometers in diameter, give wet clay its characteristic stickiness and plasticity. Transforming ordinary soil into usable pottery clay is essentially a purification process, separating these fine clay particles from the coarser sand and silt components. This refinement prepares the material for forming and firing.
Identifying Clay-Rich Soil
The first step in making pottery clay is determining if the soil source contains enough of the right mineral content to be worth processing. A simple field test involves wetting a handful of subsoil, avoiding the organic-rich topsoil, and kneading it until it is uniformly moist but not dripping wet. If the sample feels distinctly sticky and will hold its shape without crumbling, it suggests a good clay content.
A more specific test is the “ribbon test,” where you attempt to roll the wet soil into a long, thin coil between your thumb and forefinger. A soil that forms a ribbon longer than two inches before breaking from its own weight indicates a high percentage of clay particles. Observing the landscape can also help, as clay-rich deposits often retain water, leading to standing pools or a cracked, dry surface after a rain event.
Separating Clay from Sediment (Levigation)
Once a promising soil is found, the separation process, known as levigation, mechanically isolates the microscopic clay particles based on their size and weight. This technique involves mixing the clay-rich soil with a large volume of water to create a slurry. The water suspends all the mineral components, but the heavier and larger particles, such as gravel and sand, quickly sink to the bottom of the container.
After a short settling period, often just a few minutes, the water above the bottom layer remains cloudy, holding the finer silt and the ultra-fine clay particles in what is called a clay slip. The next step is to carefully pour, or siphon, this cloudy suspension into a separate container, leaving the heavy, quick-settling sand behind. This transferred slip still contains silt, which is heavier than clay but lighter than sand.
To remove the silt, the slip is allowed to settle for a longer period, typically several hours or overnight, depending on the volume. Silt particles will sink to form a layer above the sand, while the finest clay particles remain suspended in the water due to their minimal mass. The purified clay slip is then carefully poured off the top of the settled silt layer. This process may be repeated to achieve the highest possible purity.
Preparing the Clay for Modeling
After the levigation process yields a clean clay slip, the next stage is to remove the excess water until the material reaches a workable, plastic state. One effective method is to pour the liquid slip onto an absorbent surface, such as a large plaster slab or a piece of untreated canvas or burlap. The absorbent material slowly draws moisture out of the slip through capillary action, causing the clay to thicken.
This dehydration process is complete when the clay develops a dough-like consistency, firm enough to hold its shape but still soft enough to manipulate. The final step before the clay is ready for modeling is “wedging,” a specialized form of kneading. Wedging homogenizes the clay’s moisture content, ensuring a uniform consistency throughout the block.
The mechanical action of wedging forces out trapped air bubbles from the clay body. Removing air pockets is crucial because they can expand and cause pottery to explode during firing. Techniques like the ram’s head spiral wedge or the cut-and-slam method are used to align the clay particles and create a smooth, dense, and pliable material, making it ready for shaping into a ceramic object.