Test Batches

Get to know the earth by playing with it. When we travel to conduct workshops, we have to start from scratch to discover the merits of a new batch of soil. The key is in experimentation. The fun-d-mental principle is this: adapt the ratios of the three magical ingredients until the plaster behaves the way you want it to. As a general rule of thumb, 30 percent clay to 70 percent well-graded sand is optimal with enough straw to give it body and eliminate cracking. Straw accounts for about 20 percent-50percent by volume according to personal appeal and the behavior of the mix.

14.4: A variety of well-graded fibers for making plaster: clockwise from upper left; paper cellulose, chopped straw, long straw (on ground), sun-bleached grass clippings, and horse manure.


• Concrete or garden hoe

• Wheelbarrow

• Tarpaulins

• Large screen (one-quarter to half-inch [0.625-1.25 cm] hardware cloth over a rigid frame)

• Semi-fine screen (kitchen wire mesh colander)

• Chipper/shredder or machete to chop straw

• Mortar or cement mixer (optional)

Application Tools

• Buckets and cans

• Fat paint brush, or garden sprayer, or hose with spray gun (for dampening cured plaster or cob)

• Assorted trowels - steel pool trowel, square-edge margin trowel, pointed trowel (for hard to reach corners), wood float, sponge float (or thick cellulose sponge)

• Dimpler (for texturing surface to provide key-in for successive plaster coats)

• Hawk (optional tool used by "the pros")

Other Equipment Handy for Making the Job Easier

• Plastic and paper (for protecting windows, doors, floors, and wood or metal trim)

• Tape — blue (quick release), masking, and duct (for attaching the plastic or paper to windows, doors, and trim)

• Scaffolding or planks over saw horses or straw bales

Optional Equipment

• Air compressor

• Drywall texture gun and/or plaster sprayer

Some or all of these tools and equipment can be used for any type of plaster project, be it earthen, lime, cement, or cob.

Use a section of the wall as a sample board. The area should be at least one-square-foot (0.1 sq. meter) in size for each test batch. Let it dry. If the plaster sample shrinks or cracks a lot, add more sand or straw or both. A few small hairline cracks are all right, as long as the plaster adheres to the wall without trying to curl away. The following layer will fill in these small cracks. If the plaster dries powdery and weak, the clay could be too expansive or the earth have too high a silt content. If clay is scarce or the soil in general is of a poor quality, we resort to adding a binder of cooked flour paste (see recipe under "Additives for Fat Plaster" in Chapter 15).

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