Note: This article is excerpted from Earth Construction Handbook (by Gernot Minke, WIT Press, Southhampton, Boston, 2000) which contains further information about weather protection, physical and mechanical properties of clayey soils, treatments and additives and modern earth construction techniques worldwide.
1) General. Earth plasters mainly consist of sand and silt with only as much clay as is necessary (usually between 5% to 12%) for developing their adhesive and binding forces. It is difficult to state what the proportions of an ideal earth plaster should be, because not only does the proportion of clay, silt and sand influence the properties, but also the grain size distribution of the sand fraction itself, the water content, the type of clay, the method of preparation and the additives. In order to test the appropriateness of earth plasters, samples with varied compositions should be tested. Earth plasters stick very well not only on earth surfaces, but also on brick, concrete and stone surfaces, if the surface is rough enough.
2) Preparation of substrate. As earth plaster does not chemically react with the substrate, the surface has to be sufficiently rough in order to develop a good physical bond. A good method of getting a strong bond is to wet it sufficiently until the surface is soft, and than scratch diagonally patterned grooves with a small rake or a nail trowel. In order to ensure that the plaster adheres better, it is also possible to use latching in the form of galvanised wire mesh, plastic mesh, reed mats, and such on the substrate before plastering.
3) Composition of earth plaster.
3.1 General. In order to get earth plaster free of shrinkage cracks, the following points must be kept in mind:
The earth should have enough coarse sand.
Animal or human hair, coconut or sisal fibres, cut straw or hay should be added (however, too much of these additives reduce the ability of the plaster to adhere to the substrate).
For interior plastering, sawdust, cellulose fibres, chaff of cereal grains or similar particles can also be used as additives.
In order to develop enough binding force, the adhesive forces of the clay minerals should be sufficiently activated by adequate water and movement.
When the plaster sticks to a sliding metal trowel held vertically, yet is easily flicked away, the correct consistency has been achieved.
astounding, this is the first authoritative view on plasters I have found outside of Michael Smiths work!
I am on od being, I was a production stoneware potter for 55 years before venturing into “COB” about 15 years ago, After Creating some innovations with my Friend Kirk Mobert,of Mendocino c. I launched into making Cob benches and Ovens with children for schools. IN this practice,and my experience working as a professional Mason,I have come across some value,and some Challenges with outdoor plastering techniques ,most of whom do not want to afford a roof.Michael Smith and Darryl actually said ,”we don’t bother anymore”
I have had my best success using a Primary plaster that is a tad clay rich,and contains finely chopped “straw” gleaned from tractor mower decks (full of seeds,definitely HAY) Great sculpting medium as well. I pin larger sculptures with nails(bamboo will rot,wood will expand and contract)
my secondary or finish plasters are containing a quart of oil, (any vegetable)and about one large COW DUNG per hundred lbs or about two 5 g. buckets.
My preferred FIBER is a nylon fiber sold at all Masonry supplies for cement.((Please spare me the bleeding heart hippie stuff about plastic!* see below)) I also use Natural fibers when not to laborious to procure,or crazy to mix such as human hair,goat,cat ,dog hair, etc etc ((The BATCH will ROT if left in buckets with these organic ingredients for more than 2 or 3 days.)) . I use mostly powdered pigments of the iron sources,although sometimes I include artist pigments ((Careful with children on this)) after the piece is finished and well dried,we apply the FIRST COAT of oil with a 1/4 mix of D Limonene (Kid safe thinner,you can use terps,or ghastly pet.thinner) second coat when dry full strength oil wipe excess from sitting areas.This creates a finish plaster that is (at about 3/16th) a very strong ,yet resilient skin that cannot be penetrated with the thumbnail,and sheds water well. The only problem with this is it MILDEWS when applied on a north face in moist climate.
I am currently experimenting with A) commercial mildewcide (works,probably toxic)B)Bay Laural infusion,Black walnut hull extract,Eucalyptus,Usnea,Tea Tree oil,Rosemary ext, (all of these depend on availability,price,and MUCH more testing.
PLASTIC FIBER why NOT? does not rot when damp,10 OZ is adequate strength for a cubic YARD (according to concrete manufacturers standards,adds significant strength(something like 25%) Your CORDLESS DRILL(probably not your first or last) contains about 10 times as much plastic,lets not even THINK about cars or computers) All organic fibers SPOIL in the mix ,losing strength,gaining stench,and if let rot long enough,destroying the color of the batch. with these plasters It is necessary to flatten and dry and make “cookies” of left over plaster for next session.
try this if you have not,and enjoy(personally,I dislike lime,if needed ,I will just go to a clay rich cement morter with same additives(-cowdung!)
a small amount of cement for art is a good thing (Lobby against a freeway overpass) when we win,that offsets all the cement in every cob house forever.
Love your article! OH and have you tried CACTUS (Prickly Pear “Nopal”) for interior plasters! wow at about 15 or 20% of the mix its like natural clay compatible Acrylic! REAL strong and flexible has some fiber (Boil the cactus for a while cool and strain,) bingo!
sincerely Nico Morris (fb)