Dig your hands deep down into the earth. Get beneath the dark layer of topsoil and into the rich subsoil, where the earth gets dense and sticky. Here you find clay, a plentiful raw material humans have been using throughout history to build and decorate structures around the world. Rudimentary clay plasters waterproofed our ancestors’ primordial structures. Refined versions covered the walls of well-to-do 19th-century European homes. Rich red clay was baked into bricks to form the shape of the humble adobes that dot the southwestern United States. Today we can take advantage of this healthy, sustainable, user-friendly material to warm up the walls of our homes.
“Clay is one of the most abundant raw materials available,” says Adam Weismann, coauthor of Using Natural Finishes. “It’s right under your feet—right now. You just have to know how to use it in a living space.” When mixed with water, natural fibers and aggregates, clay acts as a binding agent, adhering the malleable mix to walls. It can be manipulated into varied shapes and styles—clean lines for modern and traditional tastes or undulating curves for an organic feel. If you can get the materials from your own land (or from a friend’s), the cost is minimal, and sourcing locally conserves the energy that would otherwise be used for shipping. “It’s also extremely rewarding and visually pleasing when the building materials that create a structure are literally a part of their natural surroundings,” Weismann says.
Making your own clay plaster out of materials available onsite is an inexpensive, local option, but it’s not easy. Digging clay and creating the proper mix for use on walls can be a labor- and time-intensive endeavor. If you want to give it a go, see our recipe and instructions at the end of this article. If you’re in the market for clay walls that require less effort, purchasing a ready-made mix from a manufacturer such as American Clay is an easier route. Based in New Mexico, American Clay sources 72 percent of its materials from within 500 miles of its manufacturing site, and its pigments are made of naturally occurring oxides and minerals from the United States. Most of its plasters are shipped as 50-pound bags of powder, which you hand-mix with water and a packet of one of the company’s 42 colored pigments. At about $1 to $1.35 per square foot, it costs more than the DIY version but provides consistent, colorful results. Want the look of clay with even less work? Consider roll-on clay paints (see Resources), which offer the warmth of clay but aren’t as thick and textured as clay plaster.
When made without synthetic additives, natural clay plasters and paints are entirely nontoxic, making them ideal for people with chemical sensitivities, pregnant women, young children and anyone who wants a healthy home. Clay plaster walls’ thick thermal mass means they also help regulate temperature and humidity. “Clay is breathable,” Weismann says. “It can safely absorb and hold moisture vapor when humidity levels are high and then release it back into the atmosphere as the air becomes dry.” This provides a comfortable living space and helps prevent condensation and mold. In fact, museums and galleries have begun using clay plaster to help protect artwork and artifacts.
Clay is a durable, user-friendly material that can be applied over most interior wall surfaces including new-construction drywall, painted drywall and conventional plaster. Most people apply several layers of plaster with a trowel to achieve thick, cozy walls. (Make sure to allow time for the clay to dry between layers.) Some manufacturers also recommend using a base coat of primer to help the plaster adhere to walls. If you’re using clay paints, which are thick and provide excellent coverage, you can usually roll them directly onto walls without a primer. It’s wise to do a test panel in an inconspicuous spot before you begin the job. Be cautious using clay finishes in areas where walls come in direct and prolonged contact with water. If you are using clay in the kitchen or bath, consider tiling the areas around sinks, showers and tubs and/or coating walls with a protective wax sealant.
Clay plaster is an excellent material for beginners because it is endlessly forgiving of mistakes. “Unlike other binders used for plasters, such as lime and cement, clay does not undergo a chemical transformation as it dries,” Weismann says. “This means it can be indefinitely rewetted and reworked.” The slow drying time allows you to work at your own pace, and it’s hard to make mistakes—clay is by nature an imperfect material, and its texture variations are part of its appeal. It is also incredibly durable, typically lasting the lifetime of the structure. Because you can put it on directly over cracked plaster or drywall, clay plaster can restore walls in older homes at a fraction of the cost of replacing the original finish. In new homes, it imparts timeless character.
DIY Clay Plaster
Clay varies vastly from region to region. When making your own plaster from locally sourced materials, be prepared to test and feel your way to a good mix through trial and error. There isn’t one correct formula, but every good recipe begins with clay-rich subsoil, found about 20 inches under the topsoil. A good sample will stick together and hold its shape when moistened and molded into a ball. Always make test batches for each layer of plaster.
1 part clay-rich subsoil
3 to 4 parts sand
Just enough water (too much causes slumping and cracking)
½ part fiber such as chopped straw
(¼ inch or less is best for detail work and thin plasters)
1. Mix all ingredients in a wheelbarrow or bucket using your hands, a shovel or power tools. Make the plaster a bit wetter than peanut butter, and adjust as you go. Remember, it’s easier to add water than to take it away.
2. Remove coarse materials by sifting plaster through a window screen or 1⁄4-inch hardware cloth. The finer your materials, the finer your plaster.
3. Smear the mud on by hand or with masonry trowels. You can apply up to about 1⁄2-inch thickness before you’ll start having trouble with cracks and slumping.
4. For more thickness, apply multiple coats, allowing each coat to dry then lightly misting the first coat before applying the second. Add longer fiber to give it a textured surface to “grab” onto.
5. Use circular strokes to smooth the surface. Try different effects—from deep finger and hand prints to subtle strokes of your palm. Wood tools give a softer finish, metal a harder polish. A tool held perpendicular to the mud will make a ragged line, but at a low angle, it will make a clean line. Scratch or carve in a design using forks, knives or spoons—experiment! Adjust the recipe “to taste.”
Recipe adapted with permission from Mother Earth News.
Clay plasters and paints
The Canelo Project
The Natural Plaster Book
by Cedar Rose Guelberth and Dan Chiras
Using Natural Finishes
by Adam Weismann and Katy Bryce