Finish Earthen Plaster: Part 1: Materials and Recipe

Applying finish earthen plaster may be one of the most satisfying parts of building a natural home. Smoothing over rough cob walls with a creamy earthen plaster mix is incredibly satisfying and brings about a truly dramatic transformation. What was once coarse is made buttery and gracefully smooth. For a successful earthen plaster job, it’s important to carefully prepare and mix your material. In this first finish earthen plaster post, I will explain the preparation and recipe I used for the interior cob walls of my home.

Finish earthen plaster recipe

First and foremost, it’s important to know what you’ll need for your earthen plaster mix. The following recipe includes the exact ratios that I used. However, as is the case with natural building, you may find slightly different quantities to perform better, depending on your materials. This should be a good guide, though:

1. 3 (5 gallon) buckets of 1/16 inch sifted sand
2. 1 bucket of clay
3. 1 bucket fresh cow manure (not dried)
4. 8 cups wheat waste
5. fluff of 3-4 cattails

Preparing your earthen plaster material

Unlike cob, which can be made pretty sloppily without much worry, earthen plaster requires a bit more delicacy.

Sifted sand

For a truly smooth finish, it is important to sift your sand to remove small stones and larger particles. I used a 1/16 inch window screen to sift all of my sand. This is a pretty tedious task, so it’s nice to have company around while you push sand through a tight screen. Also, it is a huge help to have completely dry sand, as it will go much, much quicker.

Clay

Many builders choose to sieve their clay to remove large particles, but I actually skipped this step. I was pretty confident that my clay did not include too much particulate matter (and it didn’t, for the most part), so I gladly passed up sieving my clay. If you choose to screen your clay, prepare for a similar process to sifting your sand, except that you will use a larger screen.

Wheat paste (and how to make it)

To make wheat paste, mix 1 cup of wheat flour (I used white flour, but I guess you can use whole wheat…? I doubt it matters that much) in 2 cups of cold water, while you bring another 6 cups of water to a boil. Once the water is boiling, whisk the flour and water in, and mix. Lower the heat, mix for another minute, and it should become slightly translucent and sticky.

Be sure to use the wheat paste right away! It’s generally recommended that you don’t let earthen plaster with wheat paste sit for too long – it can get stinky. (I think the smell is reminiscent of rotten mashed potatoes.) The wheat paste makes for a stickier, gluey plaster. (Although I’m sorta convinced that it’s not totally important – in fact, I think some of the mold spots in my plaster are from the wheat paste, although this is just a guess for now…)

Cow manure!

I am a strong adherent of cow manure. If you are squeamish, just get over it. Seriously. Manure has been used in plaster for eons – it is totally safe and beneficial, and yes, the smell will go away. Manure lends strength, durability, and texture to earthen plaster. The fibers make for great tensile strength, and I think that overall, manure helps make the plaster softer (at least to work with).

Try to get manure from cows that are strictly grass-fed. I am a little leery of manure with grain in it, although that is what I had to resort to using in my interior plaster. I don’t think that it is a huge deal, but 100% grass-fed is what to aim for. Fill up some 5 gallon buckets and keep them covered to prevent the poop from drying out. After a day or two, the manure will ferment a bit and expand and become quite airy. That’s the best time to use it.

Cattail fluff

Cattail is a common plant found around marshes, ponds, lakes, and wetland areas. In the fall, pick the mature punks and keep them in a dry place to use in your earthen plaster. The punks explode upon twisting them, resulting in a cloud of beautiful, soft fiber that is a perfect addition to finish plaster, adding tensile strength in place of straw. I typically use about three or four punks in each batch of plaster. I like to be able to see the fibers in each handful of material. (Split your handful in half to see the fibers sticking out.)

Some folks break the cattails ahead of time into a large barrel and then fluff it with a weed whacker, but I find this to be unnecessary. You can break the punks open right into your mix.

Mixing finish earthen plaster

When mixing finish earthen plaster, it helps to have a relatively clean tarp (or a tarp free of straw and large chunks of old material, at least). Dump your sand and make a well, add your clay and manure, and stomp away. Once you have a nice even mix, throw in your cattail fluff, and try to avoid getting chunks of fiber (or “cattail bombs”, as we say) in there. You can either add the wheat paste now, or to individual buckets of material. Your final product should be consistent, smooth, wet, and sticky! Add water during the stomping process if you have to, but it may already be wet enough between the clay and manure.

Next up I will describe the finish earthen plaster application process…