EARTHEN Plasters and Clay Paints

When picking a colour scheme for a house ,the long process of colour picking and matching can be an tedious one. The colour scheme will be with most people for a long time and each year the amazing colours produced vary and tend to be in and out of taste with critics. The application of the paint is then usually left to a professional painter to apply and there is always a gaseous volume of volatile organic compounds in the house following pint applications which can be very harmful and last for months.This is called “off gasing” which is not ideal for someone with ashtma for instance.

Creating pigments and mixing paints and earthen plasters is a natural building process which is relatively easy to do. To apply the earthen plasters first requires a little knowledge of the volumes to be mixed ,and the smooth pliable Japanese trowel,which is a joy to use and unlike conventional rigid trowels allows curves and cornices to be finished with ease.

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.

Strawbale, cob and lightclay are common natural building methods that are very compatible with natural plasters. One common mistake is using any type of wire or plastic mesh over strawbale walls with natural plasters. Stucco wire works great with cement stucco, but almost all of the natural builders/plasterers I`ve met advocate not using stucco wire with clay plaster. All of the failing clay plasters I`ve seen have stucco wire underneath. (However a final coat of interior clay plaster can be applied to an existing layer of stucco without problems.,says Cindy Walker of Natural Plasters Nelson BC. I met Cindy Walker recently at an earthen plasters and paints workshop whilst interning in a natural building school,and i have to say the use of clay sand silica and flour paste amongst other trade secrets including the seeds of the bulrush produce a fine paint to compliment the earthen plaster.

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 aluminiumsilicates, 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 silicondioxide(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 coconot 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 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.

Flour paste

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

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.

Earth plasters have all natural ingredients, such as clay, sand and straw, which are usually locally sourced. Earthen plasters are used as base coats to fill in the surface of strawbale, cob, lightclay & adobe walls. Clay (from the local ground), sand and chopped straw are used. Natural builders believe that natural plasters are more breathable than cement stucco and have a lower environmental impact, as well as being made with non-toxic materials...you don`t have to worry about getting your hands dirty. Guidance is recommended to figure out the recipe and how to process/mix ingredients. The better quality the base coat is...the easier the final coat will be.

Michael P Holmes
Mayo Thatch

Contact us
Michael and Angela Holmes
Daubworks
Culmore
Foxford
Co Mayo

www.heritagenaturalbuilding.eu