Before you apply plaster, conduct this simple test to find out if your wall substrate is suitable for clay: Spray, flick or paint water onto the wall. If the wall absorbs it in about five minutes, your walls have adequate absorption for clay plaster. If the wall absorbs the water very quickly (in less than one minute), the wall will require a natural primer to prevent the plaster from drying too quickly. Surfaces must be slightly rough so plaster can grip the wall. Prepare flat, smooth surfaces with a sanded primer.
1. Use a spade or shovel to access the ground’s sub-soil layer (generally 20 inches under the topsoil).
2. Take at least three soil samples over one area to assess clay levels. Mix the earth with a little water and observe how it behaves: Is it sticky? Can you make it into a cylinder without it breaking? If so, clay is present.
3. If the soil appears to have high clay content, excavate the amount you need, by hand or with a front loader.
4. Mix clay and water to make a slurry: Fill roughly 1⁄3 of a container with water, then add clay soil until the container is 3⁄4 full. To help break down the clay, stir the soaked material with a spade or using a whisk attachment on a drill. Let the mixture sit for at least 24 hours.
5. Strain soaked clay through a large sieve—1⁄4-inch mesh for a base coat plaster, 1⁄8-inch mesh for a topcoat plaster. (This removes larger sand particles so you have a smooth clay to work with.)
6. Combine clay with well-graded (particle sizes from .15 to 4 mm) sand. Play with different combinations to achieve the texture you need. To add more texture to base coats or decorative topcoats, mix in thicker natural fibers such as chopped straw or hemp. Add water to produce a mousse-like consistency and apply to the wall. Create several test samples to find the best mix for aesthetics and functionality. Ideal plasters often consist of 10 to 25 percent clay and 75 to 90 percent well-graded aggregate.
7. Wet walls before application unless you’ve applied a primer. Do not wet walls if you use primer.