Earthen plaster

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 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 soil.

Dry straw, hemp fiber, cattails, 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.

Cooked flour paste is 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 natural 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 fresh or fermented when mixed with plaster, as composted manure loses its enzymes and adhesive qualities. Manure should be sifted before use.

The liquid from prickly pear cactus used to be one of the most common additives in the Americas.[2][3]

The juice from the prickly pear cactus leaf pads will serve many functions. According to some sources, it helps the plaster set and increases its stickiness or adhesion. Cactus juice also serves as a stabilizer in that it helps make earthen plasters more water-resistant and more durable. It also prevents dusting.

Cactus juice can increase plaster's workability and its ability to be formed into the desired shape. Workability depends on the water content, the shape and size distribution of its aggregate (such as rock, sand, natural fiber, etc.), the age of the plaster, and the amount of other natural binder(s) (such as lime, wheatpaste, cactus juice, hardening vegetable oil, casein and other proteins, etc.) Altering the water content, changing the aggregate mix, soaking the clay, or changing the binders will increase or decrease the plaster's workability. Excessive water will lead to increased bleeding (surface water) and/or segregation of aggregates (when the natural binder and aggregates start to separate), with the resulting plaster having reduced quality. The use of an aggregate with an undesirable gradation can result in a very harsh mix design with a very low workability, which cannot be readily made more workable by addition of reasonable amounts of water or binder.

Cactus juice works well because it contains pectin, a water-soluble long-chain carbohydrate that acts as the binding agent to increase the adhesion of an earthen plaster. Pectin is also responsible for increasing the water resistance of an earthen plaster and has been used to augment lime plasters in both Mexico and the southwestern United States for hundreds of years.

Cactus juice is extracted by immersing cut leaves in water for as long as two weeks.[4]