Learn about Clay Plaster

Clay plaster is a mixture of sand, fiber and clay used to provide a beautiful looking wall finish. Sand typically makes up the bulk of the material while clay acts as a binding agent and straw or similar fibers act as the strengthening agent. While commonly used on straw bale or cob buildings, natural earth plaster can be applied in any conventional home.

Clay plaster is often made onsite and can be applied over most standard building materials such as drywall and concrete. However, the surface should be sufficiently rough to develop a good physical bond.

The plaster itself can be colored or tinted by mixing in natural pigments and ochers. Alternatively, it can also be painted to maintain the humidity buffering benefits of clay plaster, be sure to use a paint that allows moisture to penetrate. Natural paints such as milk or clay-based paints can also be made onsite with local materials.

The recipe for earth plasters may change from region to region depending on the local availability of materials. Look for an experienced person in your area to better understand your local options.

Many techniques exist to make plaster that will behave in a particular fashion. For example, adding rye flour can protect the surface against abrasion, and adding lime creates a reaction that increases water resistance.

Earth plaster is made from natural materials that can often be sourced close to the building site, free of VOCs!

Clay also has a high capacity to absorb and hold moisture. In a home with high humidity levels or humid climates, clay plaster finishes help prevent mold and mildew and improve indoor air quality. A 3 cm (1 in.) thick coating can buffer daily fluctuations in humidity.

Clay plasters are one of the oldest wall finishing materials. They were even used to coat the walls of early Egyptian tombs!