Earthen plaster (adobe plaster, dagga) is a blend of clay, fine aggregate, and fiber. Other common additives include

pigments A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compoun ...

, lime,

casein Casein ( , from Latin ''caseus'' "cheese") is a family of related phosphoproteins ( αS1, aS2, β, κ) that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of the proteins in hum ...

, prickly pear cactus juice (''

Opuntia ''Opuntia'', commonly called prickly pear or pear cactus, is a genus of flowering plants in the cactus family Cactaceae. Prickly pears are also known as ''tuna'' (fruit), ''sabra'', '' nopal'' (paddle, plural ''nopales'') from the Nahuatl word ...

''),

manure Manure is organic matter that is used as organic fertilizer in agriculture. Most manure consists of animal feces; other sources include compost and green manure. Manures contribute to the Soil fertility, fertility of soil by adding organic ma ...

, and

linseed oil Linseed oil, also known as flaxseed oil or flax oil (in its edible form), is a colourless to yellowish oil obtained from the dried, ripened seeds of the flax plant (''Linum usitatissimum''). The oil is obtained by pressing, sometimes followed by ...

. Earthen plaster is usually applied to

masonry Masonry is the building of structures from individual units, which are often laid in and bound together by mortar; the term ''masonry'' can also refer to the units themselves. The common materials of masonry construction are bricks, building ...

, cob, or

straw bale Straw is an agricultural byproduct consisting of the dry stalks of cereal plants after the grain and chaff have been removed. It makes up about half of the yield of cereal crops such as barley, oats, rice, rye and wheat. It has a number ...

interiors or exteriors as a

wall A wall is a structure and a surface that defines an area; carries a load; provides security, shelter, or soundproofing; or, is decorative. There are many kinds of walls, including: * Walls in buildings that form a fundamental part of the s ...

finish. It provides protection to the structural and insulating building components as well as texture and color.

Physical composition

All

plaster Plaster is a building material used for the protective or decorative coating of walls and ceilings and for moulding and casting decorative elements. In English, "plaster" usually means a material used for the interiors of buildings, while "re ...

s and

stuccos Stucco or render is a construction material made of aggregates, a binder, and water. Stucco is applied wet and hardens to a very dense solid. It is used as a decorative coating for walls and ceilings, exterior walls, and as a sculptural and ...

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 Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

, lime or

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywa ...

, while stucco uses a

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixe ...

or synthetic binding element.

Clay: the binding agent

Clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...

is a

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

component consisting of extremely fine particles. Most clays consist of hydrous

aluminum silicates Aluminium silicate (or aluminum silicate) is a name commonly applied to chemical compounds which are derived from aluminium oxide, Al2O3 and silicon dioxide, SiO2 which may be anhydrous or hydrated, naturally occurring as minerals or synthetic. ...

, though there is a considerable amount of chemical variation among the clays. Clay is cohesive and binds to the

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class ...

and the

straw Straw is an agricultural byproduct consisting of the dry stalks of cereal plants after the grain and chaff have been removed. It makes up about half of the yield of cereal crops such as barley, oats, rice, rye and wheat. It has a number ...

, 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 Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

(

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...

), sand is a non-reactive substance. Because sand occurs naturally in many

subsoil Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil, it is composed of a variable mixture of small particles such as sand, silt and clay, but with a much lower percentage of organic matter and humus, and ...

s, all of the sand necessary may be already found in the soil.

Fiber: tensile strength and reinforcement

Dry straw,

hemp Hemp, or industrial hemp, is a botanical class of '' Cannabis sativa'' cultivars grown specifically for industrial or medicinal use. It can be used to make a wide range of products. Along with bamboo, hemp is among the fastest growing plants ...

fiber,

cattail ''Typha'' is a genus of about 30 species of monocotyledonous flowering plants in the family Typhaceae. These plants have a variety of common names, in British English as bulrush or reedmace, in American English as reed, cattail, or punks, in ...

coconut The coconut tree (''Cocos nucifera'') is a member of the palm tree family (Arecaceae) and the only living species of the genus ''Cocos''. The term "coconut" (or the archaic "cocoanut") can refer to the whole coconut palm, the seed, or the f ...

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

Additives are usually blended with the clay, sand, and

natural fiber Natural fibers or natural fibres (see spelling differences) are fibers that are produced by geological processes, or from the bodies of plants or animals. They can be used as a component of composite materials, where the orientation of fi ...

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

. Other additives include,

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

, stearate,

tallow Tallow is a rendered form of beef or mutton fat, primarily made up of triglycerides. In industry, tallow is not strictly defined as beef or mutton fat. In this context, tallow is animal fat that conforms to certain technical criteria, includ ...

tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' (from Anglo-Norman ''tanner'' ...

leaves A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, st ...

and

bark Bark may refer to: * Bark (botany), an outer layer of a woody plant such as a tree or stick * Bark (sound), a vocalization of some animals (which is commonly the dog) Places * Bark, Germany * Bark, Warmian-Masurian Voivodeship, Poland Arts, e ...

of certain trees, xanthan gum,

alum An alum () is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula , where is a monovalent cation such as potassium or ammonium. By itself, "alum" often refers to potassium alum, with the ...

, natural glues,

gum arabic Gum arabic, also known as gum sudani, acacia gum, Arabic gum, gum acacia, acacia, Senegal gum, Indian gum, and by other names, is a natural gum originally consisting of the hardened sap of two species of the ''Acacia'' tree, '' Senegalia se ...

kelp Kelps are large brown algae seaweeds that make up the order Laminariales. There are about 30 different genera. Despite its appearance, kelp is not a plant - it is a heterokont, a completely unrelated group of organisms. Kelp grows in "underwa ...

, lime,

powdered milk Powdered milk, also called milk powder, dried milk, or dry milk, is a manufactured dairy product made by evaporating milk to dryness. One purpose of drying milk is to preserve it; milk powder has a far longer shelf life than liquid milk and do ...

, or the

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...

of livestock.

Flour paste

Cooked flour paste is a cheap natural

glue Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advant ...

that is easy to make from common ingredients. The water and

flour Flour is a powder made by grinding raw grains, roots, beans, nuts, or seeds. Flours are used to make many different foods. Cereal flour, particularly wheat flour, is the main ingredient of bread, which is a staple food for many cul ...

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

Manure serves as a binding agent and gives plaster more body. Manure also contains small natural fibers that provide additional

tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials ...

as well as reduce cracking and

water erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dist ...

. 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 The llama (; ) (''Lama glama'') is a domesticated South American camelid, widely used as a meat and pack animal by Andean cultures since the Pre-Columbian era. Llamas are social animals and live with others as a herd. Their wool is soft ...

and

alpaca The alpaca (''Lama pacos'') is a species of South American camelid mammal. It is similar to, and often confused with, the llama. However, alpacas are often noticeably smaller than llamas. The two animals are closely related and can success ...

dung. Manure should be fresh or fermented when mixed with plaster, as composted manure loses its

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

s and adhesive qualities. Manure should be sifted before use.

Prickly pear cactus juice

The liquid from prickly pear cactus used to be one of the most common additives in

the Americas The Americas, which are sometimes collectively called America, are a landmass comprising the totality of North and South America. The Americas make up most of the land in Earth's Western Hemisphere and comprise the New World. Along with th ...

. 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 Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can ...

. 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 Aggregate or aggregates may refer to: Computing and mathematics * collection of objects that are bound together by a root entity, otherwise known as an aggregate root. The aggregate root guarantees the consistency of changes being made within the ...

(such as rock, sand, natural fiber, etc.), the age of the plaster, and the amount of other natural binder(s) (such as lime,

wheatpaste Wheat paste (also known as flour and water paste, flour paste, or simply paste) is a gel or liquid adhesive made from wheat flour or starch and water. It has been used since antiquity for various arts and crafts such as book binding, découp ...

, cactus juice, hardening

vegetable oil Vegetable oils, or vegetable fats, are oils extracted from seeds or from other parts of fruits. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed oils, or f ...

and other

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s, 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 Pectin ( grc, πηκτικός ': "congealed" and "curdled") is a heteropolysaccharide, a structural acid contained in the primary lamella, in the middle lamella, and in the cell walls of terrestrial plants. The principal, chemical component o ...

, a water-soluble long-chain

carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...

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 plaster Lime plaster is a type of plaster composed of sand, water, and lime, usually non-hydraulic hydrated lime (also known as slaked lime, high calcium lime or air lime). Ancient lime plaster often contained horse hair for reinforcement and pozzolan ...

s in both

Mexico Mexico (Spanish language, Spanish: México), officially the United Mexican States, is a List of sovereign states, country in the southern portion of North America. It is borders of Mexico, bordered to the north by the United States; to the so ...

and the southwestern

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

for hundreds of years. Cactus juice is extracted by immersing cut leaves in water for as long as two weeks.

Effect on indoor climate

In principle, all wall coverings have an effect on the room climate: vapour permeable coatings designed to be capillary conductive, allow the wall layers behind them to absorb moisture and release it again. Due to the property of clay plasters to absorb a lot of moisture (up to nine times more than gypsum), a climatic buffer is created on the wall, which absorbs moisture and releases it again when the air humidity is low. These properties distinguish clay plasters from so-called "film-forming" surfaces, such as emulsion and latex paints, which have little or no permeability to moisture in the layers below. The area of the plastered wall has the greatest influence on the ability of the clay plaster to act as a climatic buffer. The thickness of the clay layer is of secondary importance in indoor use, as more than 80% of the moisture is initially bound in the top two millimetres of the clay wall. Just 10 mm is important for the "climate buffer effect" in "normal living conditions" because thick plaster layers (> 20 mm) are too slow to react to the constantly changing room air humidity. Clay stores heat (depending on the amount installed), and due to its high specific heat capacity, clay walls are able to compensate for temperature fluctuations. Thermal conductivity 0.47...0.93 W/(m-K).

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 sand added. The color coat material was found in certain areas, red, green, white with mica and other colors, the material was gathered, made into a uniform consistency, water added, applied to the walls in multi layers until the desired surface was acceptable. The process in Spanish was known as, 'alisando'.

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. They resist water penetration but are permeable to

water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous p ...

. However, earthen plasters are often more labor-intensive than other forms of wall covering. If the mix does not have the correct component proportions, then many other problems may occur, such as dusting and cracking. Earthen plasters are not approved by all local

building codes A building code (also building control or building regulations) is a set of rules that specify the standards for constructed objects such as buildings and non-building structures. Buildings must conform to the code to obtain planning permiss ...

, and some require the use of cement stabilizers or

asphalt Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term ...

emulsion. Earthen plaster is a rare wall covering in the developed world and there is a dearth of local experts for construction and repair.

Notes

Sources

*McHenry, Paul Graham Jr. ''Adobe: Build it Yourself''. The University of Arizona Press: Tucson. 1974. *Norton, John. ''Building with Earth: A Handbook''. Intermediate Technology Publications Limited: London 1997. {{DEFAULTSORT:Earthen Plaster Soil-based building materials Plastering