It doesn't take much to fall in love with a wall covered in natural plaster. One touch will do it. The surface is smooth as marble but warmer, more welcoming.
"It breathes," says natural-home builder Jeff Rottler. "It creates an interior environment that is much more comfortable."
Rottler is one of the directors of Tierra y Cal, a nonprofit based in San Miguel de Allende, Mexico, that uses compressed earth blocks for building, "green" kilns for firing bricks and clay-based plasters and paints to cover walls. Besides working with local builders and the Guanajuato state government to cut air pollution from kilns, Tierra y Cal also hosts workshops teaching its methods to groups such as Engineers Without Borders.
One primary goal is to reduce the amount of cement used in construction. Cement has obvious benefits and belongs in the building process, says Rottler, but it is increasingly relied on unnecessarily, dramatically raising the carbon footprint of construction. "There's cement in everything now — floors, foundations, walls, plaster, roofs," he says. "We're trying to build with materials we have locally."
In central Mexico the local recipe for a natural plaster is simple: clay, lime, borax and a slurry of cut-up nopal cactus paddles. The lime acts as a stabilizer that makes the clay resistant to moisture, the borax retards molds and the cactus works as a binder, a natural latex.
"It's five parts of water to one part of nopal, cut into 1.5-inch square pieces," Rottler says. "We put it in a barrel and let it sit to ferment for a day or two, depending on the weather. The chunks loosen up and let off mucilage. We put that [nopal slurry] in the mix and it makes it more viscous. It's easier to work with and dries slower. The lime will be more water-resistant if it has a slow cure."
Back where he's from in Colorado, just about every interior plaster used in homes is cement-based stucco. Even in Mexico, stucco is common, and when Rottler comes into a room he knows at once if the walls are earthen or cement. They feel different. Saying the walls breathe is not hyperbole.
Because clay is permeable to water vapor, moisture gets sucked into the wall and released continuously. It filters the air and acts to regulate temperature, keeping it more within the human comfort zone. Clay walls also have softer acoustics than those covered in cement stucco. Perhaps more immediately, they shine with a luminosity that is subtle yet undeniable. They invite touch.
There are other advantages to using clay rather than stucco. Repairs are easier and there is less cracking. Because there is no cement in the mix, the plaster is much gentler on the hands and is easier to clean up. Working with clay plasters feels like child's play. It has none of the skin-burning caustic qualities of cement-based stucco.
It's not essential to have a cactus patch before you can go natural. American Clay, a company based in Albuquerque, produces a variety of clay plasters for DIY home use. Some can be used over drywall. You could figure out a recipe for using nopal with American Clay, says Croft Elsaesser, president and co-founder, but it's not required.
"Clay is a historic building material that allows for texture and color to be applied at the same time," he says. "And you get a surface that you can relate to naturally."
To duplicate the earthy, slightly loamy finish of Rottler's recipe, a blend of American Clay's Marittimo and Porcelina clays should do the job, he says.
American Clay is available at Jill's Paint in Atwater Village (3534 Larga Ave.,  664-9067), among other locations. There is also an American Clay video channel on YouTube that offers instructions and advice.