Collecting Clay from Riverbanks — From Soil to Ceramic Paste

Clay is a hydrated aluminum silicate (Al2Si2O5(OH)4 for kaolinite) formed by the decomposition of feldspar and other minerals through weathering. Clay particles are smaller than 0.002 mm in diameter. When moist, clay is plastic and moldable. When fired above 600 degrees Celsius, it becomes permanently hard.

Look for clay in riverbanks, road cuts, construction excavations, and eroded slopes. Clay typically appears as a compact, smooth layer of uniform color (red, yellow, gray, or white) beneath the organic topsoil. In Brazil, red clays rich in iron oxide are the most common.

Take a handful of the material, moisten it slightly, and try to form a ball the size of an orange. If the material holds its shape without cracking and you can smooth the surface with your thumb, it is clay with good plasticity. If it crumbles, it is too sandy for ceramics.

Roll the moist clay into a cylinder (snake) 1 cm in diameter and 15 cm in length. Lift by one end: if it holds at least 5 cm without breaking, it has sufficient plasticity for pottery. If it breaks immediately, it needs more fine clay or bentonite as an additive.

With a shovel or hoe, remove the top layer of organic soil (10-20 cm). The clean clay is below. Dig clay blocks and place them in buckets. Collect at least 10-15 kg to have enough material after cleaning. Avoid mixing layers of different colors.

Place the dry fragments in a large bucket and cover with water. Ratio: 1 part dry clay to 2 parts water. Let soak for 24-48 hours. The clay dissolves slowly, forming a liquid paste called slip. Stir well with a stick.

Pass the liquid slip through a 60 mesh (0.25 mm) sieve to remove stones, roots, coarse sand and organic debris. The fine clay passes through the sieve with the water. The material retained in the sieve is discarded. Repeat if necessary with a finer sieve (80 mesh).

Let the sieved slip rest in a bucket for 24 hours. The clay settles to the bottom. The clean water stays on top. Siphon or tilt the bucket to remove the clear water without disturbing the clay sediment. Repeat until the water comes out relatively clean.

Pour the settled clay onto a plaster bat or absorbent cloth. The plaster absorbs excess water quickly. If you don't have plaster, spread it on a cotton cloth in the sun. Turn the clay every few hours. The clay is ready when it has the consistency of modeling clay.

Form the ready clay into 2-3 kg blocks. Wrap each block tightly in plastic film, eliminating all air. Store in a cool, shaded place. Well-wrapped clay maintains moisture for months. Before using, knead for 2-3 minutes to rehomogenize.