Stomping around in a mud pit may not seem like the most civilized activity in which a highly evolved hominid can participate. However, if the mud squishing between your toes is an earthen plaster destined to adorn the walls of your home, this pursuit may just turn out to be one of the most enlightened acts of civility you can engage in. Earthen plasters, lime and gypsum are kinder to the environment than “modern” products such as cement and synthetic stucco—and they provide unrivaled beauty and superior protection.
Earthen plasters date back thousands of years. For as long as humans have been building shelters out of mud and grass, they have likely been coating interior and exterior walls with earthen materials to safeguard against wind and weather. Although earthen plasters may seem like an anachronism in a world of sophisticated building materials, they are a highly appropriate choice for homes in many climates, from hot and arid to cold and rainy, provided walls are protected from rain. They provide durable, protective wall finishes far more breathable than modern substitutes.
Suitable for both interior and exterior walls, earthen or mud plasters typically consist of local clay-rich subsoil. (With the exception of desert climates, topsoil contains too much organic matter to be suitable for an earthen plaster.) The subsoil, containing a mixture of clay, silt and sand, is mixed with water. Straw or some other fiber is stirred into the mix, and then the plaster is applied to walls.
Earthen plasters are applied in two to three coats, depending on the type of wall. Straw bale walls typically require three coats. Adobe, cob, and rammed earth walls provide smoother working surfaces and therefore usually require only two coats. Many mudders apply the earthen plaster by hand, although trowels can also be used. Hand application often results in a better bonding of the plaster to the walls, and it’s an easier technique to master. Earthen plasters dry in a few days to a few weeks, depending on the climate. When finished, they produce a stunning finish that is both warm and inviting.
Pros and cons of earthen plasters
Earthen plasters offer many advantages over other wall finishes, especially cement and synthetic stucco. They are extremely environmentally friendly. Earthen plasters are typically harvested locally—often from the dirt excavated to build the foundation or the basement of a home. They require very little energy to extract and “manufacture.” Costs for making earthen plasters, such as excavating dirt, equipment, and additives, are minimal too. (Dirt must be screened to remove pebbles and debris; straw or some other fiber is often added, and you may need to add sand or clay or additives to improve the plaster mix’s performance and function.)
Earthen plasters are durable. When applied correctly, they can provide years of reliable protection. Maintenance and repair of earthen-plastered walls is minimal and simple.
And then, of course, there are the aesthetics. Earthen plaster can be exceedingly beautiful. The final coat, known as the finish coat or color coat, is often pigmented to produce a stunning finish.
By far the most important benefit of earthen plasters is their unique ability to protect walls from moisture. Earthen plasters resist water penetration and allow walls to “breathe,” so any moisture that accumulates in walls can escape. In contrast, cement stucco used on straw bale and some adobe homes wicks moisture into the interior of the walls, where it accumulates, dissolving away earthen materials and causing straw to mold and decompose.
Like any building material, earthen plaster has its downsides. Earthen plasters require a fair amount of work to make. Local building codes may prohibit them or may require that exterior earthen plasters be “stabilized” with materials such as asphalt emulsion—an oil-based product that outgasses potentially toxic chemicals. If local building codes require a stabilizer, don’t despair. You may be able to talk officials into letting you use a natural stabilizer such as flour paste or manure, especially if exterior walls are adequately protected from the weather. Cooked flour paste (a thickened mixture of flour and water that is similar to wallpaper paste) and manure improve the binding qualities of an earthen plaster. When the plaster dries, they create a hard, durable finish that glues the particles in the plaster more tightly together.
Earthen plaster and drywall
Lime plaster dates back to around 4000 b.c. in Turkey. Popular among the Romans, lime plaster has been used extensively in the cold, wet climates of northern Scotland and Wales to protect stone buildings and mortar from inclement weather. Many a cob building in rain-drenched parts of England has been plastered with lime as well.
Although it is considered a natural material, lime itself does not exist in nature. It is manufactured by first heating crushed limestone at more than 2,000 degrees Fahrenheit to produce quicklime. This material is then mixed with water and left to sit for long periods to slake or hydrate. The result, a putty-like material known as lime putty, is mixed with sand to produce lime plaster. Over time, the lime in the plaster converts back to limestone and produces a hard, durable finish.
Lime plaster is applied by hand or troweled onto interior and exterior walls. Some applicators use a special tool called a harling trowel to toss lime plaster onto walls. Lime plaster can be applied directly to straw bale or earthen walls, or it can be applied as a top coat over earthen plaster. This is an easier option because earthen plaster is a far cheaper and more agreeable material to mix and apply. Pigment can be added to final coats of lime plaster to provide color, or limewash, a dilute solution made by mixing lime putty, pigment, and water.
Pros and cons of lime plaster
Lime plaster produces a durable, long-lasting finish and is ideal for exterior wall surfaces, especially in rainy climates where an earthen plaster may not fare as well. Like earthen plasters, lime plaster is beautiful to behold and is vapor-permeable, so it’s ideal for straw bale and earthen-walled natural homes. Moreover, it is fairly resistant to cracking. Code approval should pose few, if any, problems.
Despite its benefits, lime is a fairly caustic material. It can burn the eyes, skin, and mucous membranes. Slaking lime to make lime putty produces a great deal of heat, which causes bubbling that may splash caustic lime on workers. Goggles, gloves, and full protective clothing, including a hat, are essential.
After lime plaster is applied to walls, it requires some pampering. In many climates, walls must be misted one to three times a day for one to two weeks after application to ensure a good set.
Because it is more dangerous and more finicky than earthen plaster, lime plaster is used sparingly these days in North America. Its excellent performance in rainy climates, though, makes it a significant asset to the natural builder.
Gypsum is a relative newcomer to the natural plaster family; it was first applied about 2500 b.c. to the interiors of Egypt’s magnificent pyramids at Giza. Unlike earthen and lime plaster, gypsum is used exclusively for interior walls. The material is too water soluble for exterior applications.
Made from naturally occurring gypsum widely found throughout the world, gypsum plaster is produced by heating the raw gypsum to drive off water, at temperatures of more than 400 degrees Fahrenheit. Gypsum plaster is typically purchased in bags of dry powder or in buckets mixed with water. To begin plastering, simply mix the powder with water, or scoop the wet mix out of the bucket and begin work.
Typically applied by trowel, gypsum adheres well to straw bale and earthen walls and can be applied directly or as a topcoat over earthen- or lime-plaster base coats. While gypsum is rarely used in new homes, it can be applied over drywall to give the flat surface more interesting texture. When it dries, gypsum plaster produces a hard, durable finish.
Pros and cons of gypsum plaster
Gypsum plaster is widely available and easy to prepare and apply. It adheres well and is smooth and creamy. (However, it is difficult for inexperienced plasterers to achieve a perfectly smooth finish). Gypsum plaster produces an excellent interior wall finish. It generally dries without cracking and requires very little maintenance. Like other natural plasters, gypsum plaster is permeable to water vapor. Finally, gypsum plaster is a familiar product to code officials, so obtaining approval should not pose a problem.
Gypsum plaster must be mined and heated and thus requires more energy to manufacture than earthen plaster but less than lime. In addition, this sticky, adhesive plaster sets up very quickly—in thirty minutes to two hours—a trait that can pose problems for novice applicators.
The most significant downside of this material is that commercially available gypsum plaster products contain a host of chemical additives. Chemical binding agents, fungicides, setting agents, and other chemicals are added to the material, especially to the wet mix. These chemicals may outgas from walls and impair indoor air quality.