We used 50 pounds of American Clay’s dried Loma clay and a jar of pigment ($90 at a local “green” building-supply store). That covered about 100 square feet with two coats, after using a sanded primer ($45 for a gallon from American Clay) to ensure that the plaster adhered.
Tools and materials
• Paintbrush or roller
• Clay and pigment powder (mixed or separate)
• 5-gal. bucket with airtight lid
• Heavy-duty electric drill with a plaster-mixing blade
• Pool-plastering trowel made of stainless steel
• Hawk (a plasterer’s tray)
• Pump-pressurized garden sprayer or mister bottle
• Tile sponge
1. Make sure wall is smooth and clean. Stir primer, apply with a paintbrush to seal wall, and let dry.
2. Pour half the bag of clay and half the jar of pigment (or clay-and-pigment mix) into the bucket, then slowly add water as you mix with the drill fitted with a plaster-mixing blade. Do not let mixture become soupy; it should be like grainy pudding that doesn’t slide off the trowel. Pour in remainder of clay and pigment, add more water, and mix. Let stand at least 30 minutes.
3. Load a scoop of plaster onto the hawk and transfer about a cup to the trowel. Hold the trowel at a slight angle and, starting at the wall’s bottom, apply plaster in long, sweeping arcs. Work back and forth across the wall, spreading plaster in a thin coat. Allow the surface to remain slightly rough, to help the next coat bind to this one.
4. Cover remaining plaster mix with the bucket lid and let the first coat dry completely, or else the second coat may crack. Expect the color to lighten as water evaporates.
5. Before applying second coat, lightly mist wall with pump sprayer. Apply the second coat thinly and fairly smoothly.
6. Let coat dry until it is still damp, but slight pressure from your thumb does not leave a thumbprint. Lightly mist wall. For a smooth or matte finish, compress and smooth the surface with repeated passes of the trowel. For a sandpaper-like surface, take a nearly dry tile sponge and smooth wall in circular patterns. You can store any leftover clay plaster in the airtight bucket.