Clay Culture: Clay on the Wall Jessica Knapp

Appears in the April 2016 issue of Ceramics Monthly.

A recent green-building and renovation trend is to use unfired clay plasters (traditionally used in cob building and other natural building techniques) to finish interior walls. Find out why.

Several companies are now offering a clay plaster (also called earth plaster) that can be applied over primed walls in both residential and commercial settings. Although plaster means something pretty specific to ceramic artists, namely a gypsum powder or mold made of lime/gypsum, as a term used in the architectural and building world it can be more generic, and here means a wall coating that does not include gypsum. The surface has the look and feel of traditional adobe, helps to regulate temperature and humidity in a room, absorbs sound, and is both durable and environmentally friendly (recyclable, reusable, compostable). Since the clay plaster does not chemically set as it dries (there’s no plaster or cement in it, so it is essentially dried clay that can be reactivated by water), it is also easy to repair. Architects and designers are interested in this material not only for the aesthetic possibilities it offers beyond a painted surface but also for the LEED credits (Leadership in Energy & Environmental Design is a green building rating and certification system through the U.S. Green Building Council), it is eligible for, making them attractive for use in green buildings.

What’s in the Mix?

Clayworks (http://clay-works.com) located in Cornwall, England, was founded by Adam Weismann and Katy Bryce. After nine years of research and development, their plaster was launched in 2011. The company offers clay plasters made with raw materials sourced in Southwest England, long known for its clay deposits and production. Pigments to color the clay plaster and binders used to seal the surface are sourced from around the world. Clayworks marketing manager, Clare Whitney, explains that the “company won the Business Green Leaders Awards in 2015 for our ability to sustainably replicate many popular wall finishes that contain toxins and synthetics.”

American Clay (www.americanclay.com), located in Albuquerque, New Mexico, manufactures and distributes clay plasters that are made from 72% post-industrial recycled materials. Croft Elsaesser, the company’s founder explains that their clay plaster wall coatings consist of three types of clay along with calcite marble sand, all of which are sourced from North America. A regional clay was tested, but was not suitable due to surface friability. The clays now in use were chosen for green strength, trowelability, humidity capacity, and odor capacity. Color is added through mineral pigments including iron oxide, ultramarine, titanium dioxide, and manganese dioxide.

Whether you choose a company or work with your own clays, the plaster can be used as is with no sealer, or for high traffic areas or kitchens, where extra durability is needed, a sealer can be added. The binders used in the optional sealants offered by most companies include silicates, and polymers along with wax. Elsaesser recommends the untreated clay surface as the ideal because it is easy to fix if damaged and provides the environmental benefits of humidity control and temperature control.

Application Process

Elsaesser explains that 50 pounds of clay plaster will cover about 100 square feet. The thickness of traditional gypsum plaster wall board is about ¾ inch of added material. With clay plaster, the thickness is ¼ inch or less for smooth surface finishes and slightly more for textured surfaces.

The application process for pre-made clay plaster involves preparing the wall with a primer paint or sanding to create tooth, applying a first coat of plaster, then applying a second coat.

1 Prepare the wall by adding texture and applying a suitable primer.

2 Mix plaster with water (colorants may be added during this step or possibly included in the mix).

3 Apply the base coat to the entire wall using a trowel. The thickness and exact trowel suitable for the job varies based on the product being used. The thickness is between 1⁄16 and ¼ inch.

4 Mix the top coat with the optional binder and apply after the first coat fully dries. The thickness depends on the consistency  of the clay plaster and whether the final surface will be smooth or textured. Generally, the smoother the surface, the thinner the coat will be.

5 Compress the surface via troweling and sponging.

6 After the appropriate drying time, apply an optional sealer.

Conclusion

While we in the ceramics field might be able to source our own materials and make a serviceable clay plaster with testing to combine clays with good adhesion and those with green strength and minimal cracking and colorants that give the desired tone, it’s good to see that a traditional building material like this is reentering the larger market.