Fun with clay
I wanted to plaster the interior of my SIPs. The simplest method: make a plaster of clay, straw, and maybe sand, and apply it directly to the OSB. I tested some variations, found a mix I liked, then discovered that it didn't stick well enough. It fell off the wall in one chunk after a couple of months.
For the plaster to stick, I would have to put some sort of lath on the OSB, which I don't want to do. So I'm putting up drywall. However, if you want to try earth plasters, here are the results of my experiments, which taught me a bit about mold:
Skip to the test results (updated Nov. 18, 2003).
The ingredients
I used local fill "dirt," which seems about 98% clay. It comes in lumps, and forcing it through a screen just makes smaller lumps that refuse to play nicely with the other plaster ingredients. The answer:
- Cover the clay with maybe an inch of water and leave it to soak (for days if possible).
- Whip the soaked clay with a drywall compound mixing tool attached to a drill.
- Pour the resulting glop through a 1/4 screen into another container.
- Let the clay settle in that container, then pour off the excess water (can take days).
The result is a bucket of rich, oozy slip. Few can resist dipping a hand in the slip bucket. The slip shown here is still too wet.
For the experiments, I bought a bale of clean, bright straw at a hardware store. I shredded it by sucking it through the same electric leaf vacuum/mulcher I use to make compost. I sent the straw through three times to reduce it to approx. one-inch sections. One bale of straw will make a lot of plaster.
I also bought a bag of all-purpose sand at the hardware store. It was pretty coarse but all of it passed through a 1/4-inch screen.
I added borax to some plaster recipes because it prevents mold. The mold most likely comes from the straw. Even if the straw had no mold spores, I could have contaminated it by shredding it--the shredder undoubtedly had leaf mold in its bag.
Mixing and applying
I mixed the plaster in a wheelbarrow, using a hoe or my hands. Then I scooped it into a 5-gallon bucket, carried it to the test spot, grabbed a handful, and pushed it onto the wall. I spread it with my hands. It helped to have a bucket of water to dunk my hands into, to keep the plaster from sticking to them.
At first I used a trowel, but I quickly abandoned it. It became much more fun to feel the plaster and it was easy to achieve a more-or-less uniform coat. I could see troweling the finish coat, however.
1: clay, sand, straw, borax. 3 shovels of clay, 2 of straw, 2 of sand + 1/4 cup borax dissolved in some water; applied about 1/4 inch thick. Took two cool days to dry.
This first mix had too much clay in it, resulting in many small cracks. But it sure stuck to the wall--I had to use a chisel to pry off a chunk to check the OSB for signs of moisture damage (none). So it might be a good start on a rough coat recipe. It's also tougher than drywall. If you stab the patch with a screwdriver, you barely make a dent.
Four months later, this patch had grown small patches of evil black mold. Not enough borax?
2: straw, clay. Straw plus enough milkshake-consistency clay slip to hold it together; no sand. Applied about 1/2 inch thick on walls; thicker in a corner to round it. Took a week to dry and grew furry white mold.
This was easier to mix than #1 and even tougher--I couldn't pry it off with a chisel; I had to chop away at the edges. I loved this mix--it was easy to work with, had sculptural possibilities, went on thickly and quickly, and was very tough. But the mold had to go.
I killed the mold on the plaster by spritzing it with diluted bleach. No more mold grew, but the dead mold wasn't exactly appealing.
3: straw, clay, borax. Same as above, but I added 1/2 cup borax powder for every pail of straw. I dissolved the borax in the clay slip before adding it to the straw. Applied about 1/2 inch thick. Took a week to dry but grew NO MOLD! It also retained its bright color.
Fire test: I used a chisel to pry a 1/4-inch-thick chunk from the edge. I put the 2- by 3-inch chunk directly into a hot fire in my woodstove. After 15 minutes, it had charred but never flamed. Later I broke it open to find all the straw gone, leaving just the clay. It looks like this plaster would be weakened by fire but wouldn't feed the flames.
It fell off the wall about three months later. The edges pulled away from the wall, and all it took was a gentle tug to separate the whole thing. It came off in one big chunk that would have made a sturdy table top.
The other similar mix is still on the wall but I suspect I won't have much trouble getting it off.
If I were more determined to plaster, I would put up lath and use this mix. It's a pleasure to work with and easy to sculpt.