What Is Earthen Plaster?

Eugene P.

Earthen plaster is a type of building material made of sand, clay a fiber such as straw and, usually, an additive to increase the strength of the mixture. The plaster can be applied to a surface to act as both a finish and as insulation. It can be used on exterior walls and interior walls, although it is more commonly used on interior walls. Using earthen plaster to finish a home has many benefits, including being environmentally friendly. It does have some limitations, however, including the fact that it cannot tolerate extended direct contact with water or high moisture levels.

Many old buildings are made of Earthen plaster.
Many old buildings are made of Earthen plaster.

The creation of earthen plaster is relatively easy as far as building materials go. The main ingredients are clay and sand, both of which can often be found on or near the site where the plaster is to be applied. This is mixed with water and some kind of fibrous material. The fiber is usually straw but can be nearly anything that includes fibers from plants or trees that have been shredded into short strands. Adding the fiber to the plaster mix helps to prevent cracking once it is applied and adds to the overall strength of the material.

Cracks in earthen plaster are usually easy to fix.
Cracks in earthen plaster are usually easy to fix.

Many earthen plaster mixes contain some type of additive. This is something that, when added, increases the strength of the plaster. Baked flour — a paste of water and flour that has been cooked — is a popular addition, because it helps to increase the resistance to water and humidity. Manure is another popular addition, because it is inexpensive and contains additional fibrous material that can add even more strength to the plaster. There also are manufactured chemical additives that can help to add strength or durability to the clay.

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The use of earthen plaster in construction brings many benefits. It is an effective form of insulation when applied to both interior and exterior walls. It also absorbs water vapor very easily, which will keep the building materials between the plaster dry. The plaster has some flexibility if mixed correctly, meaning that minute shifts in the position of the home or minor expansion and contraction from weather can be tolerated without damage. Additionally, the color of the finished plaster can be controlled by adding pigments to the mixture, providing a unique finish that does not have to be painted.

There also are drawbacks to using earthen plaster. The plaster can crack over time, although the cracks can be easily fixed in most situations. When tinting the color of the plaster, certain pigments might contain harmful volatile organic compounds (VOCs). Finally, while the plaster resists water, it is not waterproof, meaning water damage can occur, especially in areas where there is consistently high humidity.

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