Plaster: the beginning

Yesterday’s work went very well. Dad, Jon, and I began by tying external pins to the rest of the straw bale wall. With three people, this job went a lot faster than the previous day. Again, pinning the walls really strengthened them by locking the bales together as a single unit rather than individual pieces stacked up.

After finishing the job of pinning, we stuffed voids with loose straw. There were places between bales where you could pretty much reach through the wall. This is is caused by the shape of the bales on the end – they aren’t quite square. You can stuff loose straw in until you get near the edge where it just won’t stay. We mixed up some heavy clay-straw and put in some of the resulting depressions, though.

Rather than continue filling all of the depressions in the wall this way, we decided to go ahead and mix up some plaster. In The Natural Plaster Book Cedar Rose Guelberth and Dan Chiras refer to the first coat of plaster on the bales as the discovery coat. It reveals the topography of the bales, showing the depressions and high places. It’s a thin layer of plaster, about 1/8” to 1/4” thick.

We screened some of the clay dirt I had stockpiled for the plaster using the 1/2” and 1/4” hardware cloth screens I had previously made. In the concrete mixer, we mixed together 5 gallons of clay with 3 gallons of sand. Then we added water to achieve a nice consistency (not too wet, not too dry – we figured it out as we went along). The final ingredient was one gallon of chopped straw. We wanted the plaster for the discovery coat to be fairly wet/loose and clay-rich.

Once we had a batch of plaster mixed up, we took it over to the wall and started applying it to the straw bales. I had already trimmed the loose straws with a small electric weed eater/trimmer a few minutes before. Trimming the bales this way leaves a nice surface for the plaster to be pushed into so that it can adhere well.

We applied the plaster by hand, working it into the ends of the straw on the bales. The intention is for this coat to meld with the straw on the outside edge of the bale so that the plaster will be keyed in very well. Even though the first coat is thin, it is pushed an inch or more into the surface of the bale, intimately bonding it with the ends of the straw.

Clay and straw are an ancient building material. Each one benefits the other. The straw helps hold the clay together, providing tensile strength. The clay protects the straw from the damaging effects of water as it absorbs and releases moisture and seals the straw away from the weather. The clay is absorbed by and fills the voids within the straw fibers themselves as the two meld together into a single entity.

We mixed up several batches and worked our way along the wall. We plastered about 20 feet of wall in three hours or so. Later, we’ll apply a second coat which will be thicker and will fill in voids and smooth out the surface. I will probably fill some of the deeper voids before we apply the actual second coat. I’m only planning on two good coats of plaster on the exterior before the siding is put on. The plaster will seal in and protect the bales from the weather, and the siding will be an additional layer of protection in order to keep water off of and out of the bale walls.