Here’s an overview of how we plastered the interior of our roundhouse with earthen plaster. After reviewing the various recipes and techniques, we decided to keep things as simple as possible. No doubt some of the extra plaster additives like manure and wheat paste can be helpful, but we wanted to see the results of using just clay and sand mixed together 2:1. We also were concerned about fibers such as straw molding in our humid climate, and we didn’t want the added work of cooking wheat paste, gathering and chopping fibers, scooping manure and so on. Plastering is a lot of work, and we wanted to simply the process and reduce time and labor.

So far we are very happy with the results. I’m not saying our techniques will work for everyone. We have access to excellent clay and our humid weather slows the drying process, both of which reduce cracking. Your mileage may vary.

Clay: Our clay soil is almost totally free of stones and organic matter. All we did for the base coats was a quick visual check, tossing out a few small stones here and there, and breaking up some clumps. For the finish coat, we screened the clay soil through fine fishing net. Dry soil sifts much faster than damp soil.

Sand: Usually you can buy washed sand from a gravel yard. In our case, we had to sift and wash it by hand using fishing net in a 55 gallon drum of water. (We needed washed sand for the concrete work that was going on at the same time, otherwise washing sand isn’t necessary.)

This is a good, stiff earthen plaster mix with the correct amount of water.

This is a good, stiff earthen plaster mix with the correct amount of water.

Mixing: We prefer making very small batches. This speeds the mixing process and makes it easy to move the plaster where it’s needed. First, we add about one quart of water in a dishwashing tub. (Actually, you’ll have to adjust the volume of water based on the moisture content of your clay and sand. Our sand was wet and so we needed very little extra water.) Then we added the clay and sand in layers to reduce mixing: one gallon of clay soil and then half a gallon of sand, then another gallon of clay soil and another half gallon of sand. Wait a few minutes for the clay to soak up the water, and then mix everything together. If you need more water, add just a little at a time. That’s all there is to it: 2 parts clay to 1 part sand, plus enough water to get a nice mix that sticks to the walls, but not too wet that it tends to crack. Make a test patch on your wall and adjust as necessary. If it cracks, then add more sand and/or use less water. If it doesn’t stick, add a little more clay and/or water.

Part 2 on Applying Earthen Plaster coming soon to your favorite earthbag blog.


Comments

Part 1: Earthen Plaster Summary — 7 Comments

  1. I stumbled across this fascinating video, so I’m dropping the link here.

    http://www.youtube.com/watch?v=aTVbt6wih8Y

    Am I the only one that finds this type of investigative attitude toward old structures fascinating? I think it’s very cool to see the process of digging into an old structure and discovering the details of its construction.

    In any case, I hope this gentleman keeps posting videos. I hope he gets enough traffic to feel is videos are worth posting. In my humble opinion, many old structures need to have this type of video documentation before many old techniques get completely lost among the commercialized industrial products. There always seems to be a few unique details inside any old structure just waiting to be rediscovered and brought back from the brink of extinction. Of course, earthen plaster is hardly unique. It’s used nearly everywhere if one bothers to look. However, the various specific details about how different structures have used it is worth documenting. In my humble opinion, this is the type of documentation deserves recognition and encouragement. I wish more people would take the time to share information like this.

    Check out Charles Robertson’s YouTube channel. Additional interesting videos can be found there.

    http://www.youtube.com/user/tomfool43/videos

  2. THANKS FOR THE INTERESTING DATA! I AM HOPING SOMEDAY TO BE ABLE TO GET ENOUGH CASH TOGTHER TO GET A FEW ACRES SOMEWHERE IN TN OR KY AND PUT SOMETHING IN THE WORKS!! UNFORTUNATELY ITS STILL IN THE DREAMING AND HOPING STAGE!!

  3. Daniel Chiras, who co-authored a book on natural plasters, has this to say about ingredients:

    Do not use topsoil. You screen out the large and small rocks add water and voila! you have a plaster. You only need a subsoil with 5 to 7 percent clay to get a good plaster in most locations. You’ll need to test your subsoil first, however, to be sure the mix is right.

    The recipe for earthen plaster that Janine Bjornson, the plaster expert at http://www.greenhomebuilding.com, recommends for a finish plaster is:

    2-2.5 fine sand
    1 pure clay
    1/4 flour paste
    The clay for this is bagged pottery clay (Kaolin). If you use wild clays the recipe will vary. You will need to do tests.
    I have applied this to flat smooth walls and have it stay on for years, no problem.

    Both of these recipes use much less clay than Owen is recommending, so obviously there is a very wide range of soil mixes that will work.

    • One thing that maybe wasn’t totally clear is I used clay soil not 100% pure clay. The clay soil we used is also used to produce high quality compressed earth blocks (CEBs). This helped convince me the clay was stable and would be excellent plaster.

      Also, I double checked and the Steen’s at CanelorProject.org use clay soil to sand in the same ratio of 2:1. So like you say, there’s a lot of variation and people will have to experiment a bit to find the right mix.

  4. Hi Owen
    Was Googling folks I know last night and found your site. This morning as I am digging in pure clay, fashioning clay animals as a break from the heavy-duty work of redesigning a garden, I took a break to review your site. found this lovely piece on earthen plaster.
    Your work is exciting, relevant and beautiful and mostly a huge contribution to this consciousness shift in our environment.
    Congratulations and thanks for all you do.
    Wendy

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