Homemade Earthen Plaster

"Homemade Earthen Plaster" by Lisa Pezzino & staff, posted December 2004

Making an earthen plaster is just as simple as baking a batch of cookies, all that you need are the proper ingredients, a device for mixing, and a recipe to follow. A mixing tool might be the most difficult component to come by. At D Acres we use a homemade steel beater (created by attaching a welded rebar mixer (looks like an egg beater) to a power drill), but it is also possible to purchase a paint paddle at your local hardware store. The type of plaster we have used for our earthen building is particular for structures made of cob or adobe. If you are plastering a different type of natural building, other plasters will be more suitable. The following recipe is for a base plaster, which is ordinarily used as the first coat of plaster on an earthen building.

Ingredients:

Clay (presoaked overnight in water)

Sand

Flour Paste

Straw (cut into 1 inch pieces) or cattail fuzz

Materials Sources

Clay comes from the earth. All you need to do is locate it. A neighbor has quite a large clay deposit exposed on his property where we dig it up. It's fairly "pure" clay with a deep gray color and is quite elastic when handled. To ease in the mixing process, it helps tremendously to submerge the clay in water at least 12 hours prior to mixing, but ours soaks more or less all the time.

Sand can also come from the earth. Our soil in Northern New Hampshire is a sandy loam, which contains a considerable amount of sand. We sifted this loam through a ¼ inch wire mesh but found that it didn't have the strength of our other sand source. A truck from Rike Industries delivered a load of beach-like sand that yields a much stronger plaster. I guess the small amount of organic materials decreased the strength. Some books say that beach sand isn't the best because the individual particles have been eternally rounded by the ocean and that non-beach sand with jagged particles will lock together better. But, for the purpose of a plaster, I think beach sand works just fine.

Straw can be attained at any feed store and cut with some thick scissors.

Every source of clay and sand has varying components that yield different colors as well as finished plaster consistencies. Colors also vary depending on the source; to keep a consistent color, use the same sources for your materials throughout your project.

Step 1: Prepare Flour Paste
White flour is better than whole wheat, for your plaster but not your body. It's also cheaper and can be found in any grocery store.

Bring half a pot (about a 10 quart pot) of water to a boil. Pour a quarter of a pot of cold water into a mixing bowl and add white all-purpose flour until it thickens to the consistency of maple syrup (a pound is a fairly large batch). Now add this mixture to the boiling water slowly, stirring until the paste is thick and translucent. At this point, remove the pot from the heat and let the paste cool.

Step 2: Mix your Plaster
The previously noted ingredients should be mixed together in the following proportions:

4 Parts Clay (presoaked overnight in water)

4 Parts Sand

1 Part Flour Paste

2.5 Parts chopped Straw or Cattail Fuzz

Depending on the purpose of your plaster, you should choose either straw or cattail fuzz as the binding agent in your mix. The straw will be stronger when a thicker application is needed and dry rough, while the cattails will be not as strong but yield a smoother finish. Also keep in mind that cattails can only be harvested in mid to late fall. Beat the ingredients (adding the least amount of water as possible) until there are no lumps in the mix. At this point it is possible to add color to your plaster.

The coloring that we chose for our earthen building is iron oxide, which is non-toxic and turns the plaster a shade of red. Mica flakes can also be added to add a different, almost sparkly effect. If you do choose to use a powdered coloring alternative, it is smart to mix it by hand with a little bit of water before adding it to the plaster.

The consistency that we strive for is that of a stiff cake batter. If it's too thin, the plaster won't adhere to the wall.

Step 3: Application
When applying to an earthen surface such as cob or adobe, you will need to remoisten the surface with a mister. This opens up the pores of the wall so it will bond with the plaster.

The easiest way to apply this plaster is by hand. Use fluid motions and try to even out the surface of your earthen wall as much as possible. After the initial coat has dried two to three hours, you can smooth it out with a trowel or an appropriate substitute (we used recycled yogurt tops, and they worked great). We recommend testing your plaster on a small section of your wall before applying it to the entire structure.

After it has dried for a few days, check the wall for cracks. Serious cracking is usually indicative of too much clay in your mix, which can be adjusted by increasing the amount of sand or reducing the amount of clay in your recipe. Excessively quick drying will also cause cracking. This is not a problem in New Hampshire with our relatively high humidity, but in California it can be a concern. One way to slow the drying is to protect the surface from the wind with a tarp. You could also periodically mist the surface or try a combination of the two.

That's all there is to it! Do some experiments of your own if you like, nothing is set in stone. Good luck getting dirty!