Earthen Plaster (Base Coat)

Sand, Clay and Straw, the cornerstones of a well balanced natural building endeavor. We’ve had many conversations with people that go something like this:

“What are you using for insulation?”
“Straw light clay.”
“What’s that?”
“Well, it’s straw covered in clay with some sand. We’ll also use cob in some places.”
“What’s that?
“It’s sand, clay and some straw.”
“And what are you going to cover the walls with?
“We’ll use a natural plaster.”
“What goes into that?”
“Sand, clay and straw.”

The difference between these substances: straw light clay, cob and plaster has to do with proportions of the materials sand, clay and straw, screened size of said materials, how much water is included in the mix and which food they most closely resemble

  • Straw light clay primarily unadulterated straw coated in clay slip. Clay slip is clay(and typically some sand) mixed with water to the consistency of a milk shake. The straw is tossed with the clay slip until all the straw is lightly coated akin to a Caesar salad.
  • Cob is 2-3 parts sand, 1 part clay and 1 part straw. Some screening is typically applied to remove any large clumps of dirt and rocks from the materials. Cob uses the least amount of water and should be fairly stiff and able to hold it’s shape. Think of fudge.
  • Earthen plaster uses the same ratio of materials as cob but the screening is more rigorous. We use 1/4″ hardware cloth to screen the dirt we’re using for the base coat. To screen the sand for the finish we’re using window screen. The straw is also cut up and screened. Plaster uses more water than cob taking on the consistency of icing.

We’re planning to apply two coats of plaster to the straw light clay walls, a base coat and a final finish coat. With the base coat we’re using cob to fill in gaps and voids in the straw light clay walls. Then we’ll use plaster to smooth out and level the wall in preparation for the final coat.

Here’s a video sifting, mixing and application process:

And here are some stills with more details:

Mixing cob/plaster

Sweet, sweet mud and straw

Plastering the loft

For the record, regardless of what the video implied, Bethel knew what she was getting into when she came over

Everyone looks pretty jolly so far be we’re just getting started

Misting the walls

“We need more mud!” was a common oath shouted many times throughout the day

“We need more mud!”

The art of the smear

You can’re press the plaster on the wall, you have to smear it on.

So, this is retired life

Jean smoothing out the curve at the bottom of the window that we later decided to cover with a window sill

Cassandra smoothing out the transition into the window opening curve

We initially cut cross hatches into the base coat to give the next layer something to grip on but found subsequent layers adhere without much problem. If I was going to finish with a lime plaster I would cross hatch. 

Dmitri laying on plaster like a pro

Cutting up straw with a string trimmer

After lunch and not working with cutting implements or at heights this is condoned

Finishing out the east wall

Cookies and beer, delicious!

Surveying the work