Earthen plaster is a blend of clay, fine aggregate, and fiber. Other common additives include pigments, lime, casein, animal dung, and linseed oil. Earthen plaster is usually applied to masonry, cob, or straw bale interiors or exteriors as a wall finish. It provides protection to the structural and insulating building components as well as texture and color.
All plasters and stuccos have several common features: they all contain a structural component, a binding element, and some form of fiber. Usually the term plaster refers to a wall covering made from earth, lime or gypsum, while stucco uses a cement or synthetic binding element.
Clay: the binding agent
Clay is a soil component consisting of extremely fine particles. Most clays consist of hydrous aluminum silicates, though there is a considerable amount of chemical variation among the clays. Clay is cohesive and binds to the sand and the straw, holding the mixture together, as well as securing the mixture to the wall. Clay is also plastic when wet, which makes the plaster mixture workable.
Sand: structural strength
Sand provides structure, strength, and bulk to earthen plasters. Sand consists of tiny mineral granules of rock, its parent material. Predominately composed of silicon dioxide (quartz), sand is a non-reactive substance. Because sand occurs naturally in many subsoils, all of the sand necessary may be already found in the clay.
Fiber: tensile strength and reinforcement
Dry straw, hemp fiber, catails, coconut fiber, and animal hair are all suitable fiber choices for earthen plasters. Fiber forms a reinforcing meshwork in plasters, which helps to hold the plaster together. Fiber also provides some flexibility to a dried plaster. When Clay dries it shrinks and tends to crack, but this cracking can be countered by the fiber. The fiber used in plasters must be clean, dry, and mold-free.
Additives are usually blended with the clay, sand, and fiber to improve the workability and strength of a plaster. Sometimes additives are added to the finish coat and other times additives may be added to all coats. Some of the most common additives are wheat flour paste, manure, cactus juice, casein (milk protein) and various natural oils such as linseed. Other additives include, salt, sterate, tallow, tannin, leaves and bark of certain trees, xanthum gum, alum, natural glues, gum arabic, kelp, lime, powdered milk, or the blood of livestock.
Cooked flour paste is an a cheap natural glue that is easy to make from common ingredients. The water and flour slurry is cooked until the gluten binds the elements of the mixture, creating a durable glue. In plaster, the flour paste serves as a binding agent and a hardener.
Manure serves as a binding agent and gives plaster more body. Manure also contains small fibers that provide additional tensile strength as well as reduce cracking and water erosion. Different types of manure have different effects. Horse manure has a high microfiber content, but cow manure has more hardening enzymes. People have reported success with llama and alpaca dung. Manure should be as fresh as possible when mixed with plaster, as composted manure loses its enzymes and adhesive qualities. Manure should be sifted before use.
Prickly pear cactus juice
The liquid from prickly pear cactus is one of the most common additives. Prickly pear is one of the most prevalent species of cactus in the world.
Interior earthen plaster
Pure earthen plaster (plaster without lime, cement, or emulsified asphalt) is applied to interior surfaces more frequently than exteriors. Before the plaster can be applied, it must have a surface to bind to. Many types of wire mesh may be used, such as expanded-metal lath, woven wire lath, or welded wire lath. Reed mats are another option. If the plaster is being applied to a smooth surface such as drywall or plywood, a mixture of sand and wheat paste may be painted on to the surface to create the texture needed for plaster to bond.
The plaster is usually applied in three coats. The first layer of plaster is called a "scratch coat". A "comb" is used to scratch the surface horizontally or in a crisscross pattern to provide a key for the second layer. In the United States, the "comb" is commonly referred to as the "scratcher". The next layer is called the "brown coat" or leveling coat. It is leveled with tools called "Darbys" and "Rods", scraped smooth, and floated to provide a nice even surface onto which to apply the color coat. It is then allowed to dry (cure) for 7-10 days minimum to allow "checking" (shrinkage) to take place. The final layer is referred to as the "color coat" or "finishing coat", and is typically 3 mm (1/8") thick. It usually consists of clay with no fiber and little added sand. Coloring pigments are often added.
Advantages and disadvantages of earthen plaster
Earthen plasters are less toxic and energy intensive than many other wall coverings, which makes them appealing to the environmentally-conscious. Earthen Plasters are also easily repaired and inexpensive. Earth plasters resist water penetration but are permeable to water vapor. However, earthen plasters are often more labor-intensive than other forms of wall covering. If the mix is not correct in component proportions, then many other problems may occur, such as dusting and cracking. Earthen plasters are not be approved by all local building codes, as some require the use of cement stabilizers or asphalt emulsion. Earthen plaster is a rare wall covering in the developed world and there is a dearth of local experts for construction and repair.