Earth Based Finishes for Walls

Kurt demos proper hawk and trowel technique.

I just spent a blissful weekend in the California desert learning earth based finish techniques from adobe master Kurt Gardella. Some observations:

  • Earth plastering is not something you can learn from a book. It takes practice and hands-on experience. In this excellent workshop we got three full days of learning the techniques both for indoor and outdoor surfaces.

Clay, sand, wheat paste and prickly pear juice as an exterior plaster.

  • There’s no simple recipe for earth based finishes since clay and sand have different properties depending on where they come from. You figure out the right proportions of clay/sand/straw by doing test patches. Too much clay and the surface will crack. Too much sand and it wont stick to the wall when you try to trowel it on. Straw can be used to strike a balance. We did a second scratch coat on the first day that ended up cracking badly. We tried to add more sand but that made the plaster too hard to trowel on. The solution was to add more straw, which allowed us to keep a higher clay content in the mix, while preventing the coat from cracking when it dried.

Kurt shows how to work over the final mud plaster coat.

  • Earth based walls have an indescribable, almost metaphysical presence. I had been in the small desert cabin we plastered when it had wood paneling. Once the walls were filled with cob and the plaster coats applied it had a weightiness that’s difficult to describe in words. Some other advantages: it absorbs sound, regulates humidity and is a good insulator. Drywall seems flimsy in comparison. There’s something about traditional plastering techniques (even the lath and plaster of our 1920s house) that give a room a comforting feeling. Of course, plastering with a hawk, trowel and darby take a lot of skill and time which is why they are seldom done anymore. It’s a pity.

Image from japanesetrowels.com

  • The Japanese make the best trowels. And, yes, there is a japanesetrowels.com. The flexible ones are especially nice for finish coats. But they ain’t cheap.

Fermenting prickly pear.

  • Rotted prickly pear cactus juice, combined with some wheat paste, makes an amazing stabilizer when mixed with adobe. The mucilaginous texture of prickly pear gives adobe a stability that helps it resist water penetration. To extract the cactus juice you chop up prickly pear, put it in a bucket with some water, and let it ferment for a few weeks. We used a prickly pear stabilizer in a mix that covered the top of a cob oven.

If you’re interested in learning adobe techniques Kurt Gardella will be coming to LA to teach a class on oven building. More info here. Some plastering will be done in the course of the oven class.  There’s also a nice book on the subject: Using Natural Finishes: A Step-by-Step Guide.

One last thought: Working earth plaster by day and sleeping in a cozy desert cabin with a copy of the Odyssey is just about as perfect a life as I could hope for. Thank you Meredith and Doug for your hospitality!

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3 Comments

  1. For more information on earth plasters here are a couple of links to Carol Crews, an accomplished plasterer in Taos, NM-
    (1) http://www.thelaststraw.org/bonus-articles/earthplaster.html#carol
    (2) http://carolecrews.com/
    Also I did two posts on the plastering of the St. Francis de Asis Church in Ranchos de Taos-
    (1) http://altbuildblog.blogspot.com/2011/06/annual-mudding-of-st-francis-de-asis.html
    (2) http://altbuildblog.blogspot.com/2011/06/annual-mudding-of-st-francis-de-asis_30.html
    Thanks for the topic and I hope this helps.

  2. Most of our walls are done in American Clay, which we highly recommend. We learned as we went, so perfect they are not, but we love the “weightless” feel and natural tones. We have a few drywall walls and they are so soulless in comparison.

    American Clay make products to use the existing drywall if you are so inclined…

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