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Hello Dejan-
There's no secret; I'm happy to share what I know.
I and many other natural builders in North America do not add any sand to our base coats of earthen plaster, especially when applying it over straw bale, straw-clay, and other low-density walls. (But also more generally than that; I use a sandless straw-clay plaster for daub on wattle walls and in many other applications.) To avoid excessive cracking you have to add A LOT of chopped straw to your plaster; at least as much volume of chopped straw as clay slip. The best length for the chopped straw is a range between 1 and 4 inches. The resulting mix is very sticky, ensuring a good bond with the substrate; it also dries incredibly strong and tough and provides a lot of strength to the wall. I have demolished a straw bale wall in which the bales were held together by nothing other than this straw-clay plaster (no mesh or pins of any sort), about an inch to an inch and a half thick. I was incredibly impressed at how strong the wall and how hard it was to separate the bales from each other.
You can also make earthen plaster by adding sand and less straw; many people do it that way also. You might try both styles and see which you like best. In my opinion the sandless straw-clay plaster is a bit harder to mix but the results are better. There are some tricks to the application, which is a bit different than a sandy plaster. You have to really push a thin coating of the plaster (called a "discovery coat" or smear coat) onto the bales with your hands - trowels don't work. Then you can immediately build the plaster out as thick as you need to protect the bales and straighten your wall. With a stiff straw-clay mix you can easily build out 3 or 4 inches thick in a single application without sagging or much cracking. You want to do this only in dry weather to avoid mold growth. I float the surface with a wooden float and will often leave that as an exterior finish. On interiors I come back later with a finish coat of sand-rich plaster.
good luck!
Michael
Hello Dejan-
The clay does not have to be dry. In fact, I nearly always soak the clay before using it. Soaking makes it stickier and makes the mixing easier. The only reason to sift clay for a base coat is if it has a lot of rocks in it. Rocks that are smaller than the thickness of the plaster you plan to apply are not a problem, unless there are so many of them that they interfere with mixing or application.
If I have access to one, I use a mortar mixer. A cement mixer does not work with a high-fiber mix; the straw just rolls around in a ball and doesn't get thoroughly combined with the clay. A mortar mixer has a paddle that moves independently of the drum, and allows you to mix a very high-straw mixture. With a mortar mixer, add the clay and water to the mixer first and let them mix until you get smooth mud the consistency of pancake batter or a smoothie.
If I don't have access to a mortar mixer, I use an electric drill with a mixing paddle to combine clay and water to the same consistency described above. Then mix in the straw - at least as much chopped straw as clay slip by volume. You can mix this by hand in a wheelbarrow or by foot on a tarp. One relatively easy way to do it is to place alternating layers of clay slip and chopped straw in a wheelbarrow, let it sit for 15 minutes, and then pull material from the bottom of the wheelbarrow. If you get the proportions right, it should almost mix itself.
good luck!
Michael
Tobias Ber wrote:this site says: first coat of plaster without sand. just clay and straw
http://strawclaywood.com/natural-building-techniques/clay-plasters/
what about that? they use sand for the finishing coat of plaster.
i was always thinking: clay + SAND + fibre ... but we re talking about plaster here, not about structural cob for walls n stuff. any thoughts/experiences?
Tobias Ber wrote:I read that portland and lime might hurt the straw, like degrading it through the alkalicity. Is that true?
dejan, did you do a test batch on a strawbale? Just dig out your local clay. Soak it. Knead/tread it until there are no lumps left. With adding water to desired consistency. and then just plaster a straw bale. you might do a few batches, where you mix your clay with sand. Write down the ratio, let it dry (maybe inside to speed things up) and test it.
Terry Ruth wrote:If you get a hold of some Kaoline it will add luster, shine (gloss or semi), workability to the finish but has little to no binder. Add some pumice, scoria, lava, rock aggregates, straw, hemp, two tones, looks nice to some. Remember just because a test seems to be holding does not mean it will last. Add some heat and moisture cycles depending how harsh the local is. If you get high humidity or wind driven rains on exteriors take a hose with the right nozzle to it. When it comes to adhesion try and get the highest scratch/brown as possible. The two properties to test for is tension and shear. I try and scrape the test mix off the wall with a drywall knife @ 45 degree. If on lathe it's more tension or a pulling or falling off from weight and vapor pressures.
BTW: If the soil is not binding well it can be stabilized with lime (type S, or NHL 3.5 or 5), ashes like fly as pozzalans, portland cement. If you look on American Clays website they have some good trowling techniques, primers, and sealers.
But yes there are so many variables at play here there is no cookie cutter technique for the most part.
80% clay is too high BTW: 10-30% binder should do it or it gets too expansive depending on type. A jar test gives an idea on clay content, a lab soil sieve size, etc, test is better.
Tobias Ber wrote:it depends on your local clay. you might mave to add sand and maybe chopped straw to the mix. it should cling very well to strawbale, when you massage it in.
you need to check if there are stones in your clay. people would let lumps of clay soak over night in water. strain it. add sand and straw. so there s no need to work with dry stuff. but dry stuff will be easier to mix in a cement-mixer like normal plaster.
finishes might be possible. you need to experiment. like making many, many different test batches of 1-2 sqft. little cracks can be filled in with another application of the same stuff. the look of the surface will depend on the size of your sand-grains and technique.
did you check for the cob builders handbook? it s for free download here: http://weblife.org/cob/pdf/cob_builders_handbook.pdf . in the preface of that book, they state that it s ok to share for non-commercial reasons, so i think it s ok.
i ll attach a photo of my experiment with dug-up clay, water, sand and short straw (around 1cm). the wall-surface on it is not perfect, it still has residues of acrylic paint on it. the cob mixture might have too much sand. but it seems to work well so far. i smeared it on the wall and flattened it with a small, wet trowel in circular motions. so the surface will look more rough than smoot/polished
Roberto pokachinni wrote:I have a friend who did his whole two story straw bale house with common grey clay/silt/soil from his yard (I think he said his soil was 80% clay where he was digging it). I helped do some mudding, and it was very effective mushing into the bales.