Troweling a finish clay plaster on the straw bale walls of our house

Having recently finished plastering the walls of our straw bale house, I have a renewed sense of enthusiasm for using clay plaster. Not that I ever felt ‘meh’ about it or anything — it’s more that I feel so inspired to go further with it than ever before and learn some of the deeper nuances to the craft. Clay plaster is just that – a seemingly simple craft, but full of subtleties, and the difference between an okay plaster and excellent plaster are profound. While I’m satisfied with the plaster work we just accomplished, I feel hungry to go further with it.

There’s a few tricks to plastering walls, including the quality of the base coat, your plaster recipe, and the alignment of the stars. Okay, maybe not that last part… but you get the idea. It’s a lot more than slapping some mud on the straw bales.

Finish Plastering Straw Bale Walls

The quality of your finish coat depends on more than just the quality of your recipe and your trowel skills, I think. It has so much to do with what happened before you were ready to put finish on… meaning, what the quality of your substrate is. I think we did a pretty good job flattening our straw bales with a weed whacker, but in hindsight I would have spent more time creating a flatter plane with the base coat of plaster. I would have used much more chopped straw in the base recipe to eliminate any cracks in that coat, too.

The point is that much of what you do before it’s time for finish plaster is extremely important for the finish itself. I will be constantly telling myself this when it’s time to build our next house (oh boy). Always aim to make the next step easier (as is true in pretty much all building phases, right?)

Finish Clay Plaster Recipe

I used my standby finish clay plaster recipe for the straw bale walls. We were much more diligent with screening materials this time than in the past, which definitely improves the quality of the mix. It takes time, but it’s worth it. We ended up screening all of the clay through 1/8″ hardware cloth, and we even screened the cow manure through 1/4″ cloth to remove the long grass that had found its way in there.

From left to right: screened cow manure, screened site clay, sifted sand, and cattail fluff below

I debated about whether or not to include wheat paste. I ended up skipping it, but next time I think I’ll go ahead and use it. I’ve had negative experiences with it causing mold (during the drying process) in the past, but I realize now that had much more to do with the extreme humidity in the space preventing the plaster from drying appropriately… and less about the inclusion of the wheat paste alone. The foundation of my finish clay plaster recipe is as follows:

• 3 buckets sifted sand (sifted through 1/16″ window screen)
• 1 bucket screened site clay (screened through 1/8″ hardware cloth)
• 1 bucket screened cow manure (screened through 1/4″ hardware cloth, manure also allowed to sit for a few days prior to use)
• 3-4 packed handfuls of cattail fiber

If I did use wheat paste, I would have added about 5 cups. Wheat paste is useful for its gluey texture and the fact that it prevents dusting in the finished product.

Application: Plastering Walls with Clay

Finish clay plaster should go on quite thin… certainly less than 1/4″. Something more like an 1/8″ is appropriate. It’s tempting to build it up in uneven portions of the wall, but you really need to avoid the temptation. That’s why it’s critical to really nail the base coat application. There should be no fussing when it comes to applying the finish. This is not the time to correct ‘errors’ and try to bring the wall to an even plane. The devil is truly in the details, and everything should be completely ready for a thin finish coat when the time has come. Changes in the thickness are one of the causes for cracking once it dries.

The wall needs to be thoroughly wetted before the finish goes up, too. If it’s too dry the plaster will not adhere to the surface. See my post about this handy pressure sprayer for what I use at this phase of the game.

Here’s a look at some freshly polished plaster and some of the wall that has yet to receive its finish coat

I still feel like an intermediate when it comes to troweling, so I’m not going to get into that here. It’s a bit too difficult to try to put into words anyway. I like a fairly small Japanese flexible trowel for smoothing the finish. They’re lightweight, versatile, and a pleasure to use. I’ve got a post coming about where to source these fine tools coming down the pipes. In summary, the plaster gets laid up and smoothed out, and once it’s had a little bit of time to dry and harden up (which often doesn’t take too long), I’ll go back over it with a flexible trowel and polish it and remove trowel marks.

Clay Plaster: The Finished Product

I love how the wall looks when it’s been freshly plastered, but I especially love when the labor of plastering walls is completely done. It’s a tremendous amount of work, between preparing the materials, mixing, application, and clean up. Let’s not underestimate that.

Fully dried clay plaster… note the subtle shift in color away from the light source

Niches and window curves are particularly tricky and time-consuming

The true joy of clay plaster is its incredible softness, and the way light plays on the wall. Drywall cannot hold a candle to the incredible texture of a fine clay plaster finish. As the day goes by, and the light changes on the walls… the color and textures shifts and subtly changes, too. It’s wonderful. And to think that it’s not much more than some sand, clay, and cow poop? What could be better than that?

Some lovely afternoon glow around this north window… what a treat!