How to Prepare Exterior Earthen Plaster

After years of stomping mud with our feet, we bought a used cement mixer for mixing earthen plaster and cob. Small new and older used cement mixers are cheaper and easier to come by than mortar mixers. They can also handle a coarser earthen mix (less screening!) with an abundance of long straw.

Sift all the earthen ingredients through a quarter-inch (0.625 cm) screen or, to avoid having to pulverize and screen large, dry clay lumps, presoak the soil in a pit close to the mixing area. Allow the soil to percolate overnight or until the clay has softened. If using a cement mixer, start with about two gallons (7.5 liters) of water in the mixer. Throw in several fist-loads of long straw. Shovel in the pre-softened dirt until the consistency in the mixer is that of thick cake batter (adding the straw first helps keep the mud from splashing out). If you are using earth of two different qualities, like a clayey soil and reject sand, add the proper ratio determined from test batches (for example, seven shovels of reject to three shovels of clay, etc.). Add more water, and keep adding straw (and any other desired fibers) and soil until the mix starts to ball up. The soil will sometimes glob up and stick to the bottom of the mixer. If this happens, stop the mixer, pull the glob off the bottom and sides, shove a few handfuls of straw into the back, and give it another spin (Fig. 14.5).

14.5: Adding long straw to a batch of earthen plaster.

Foot mixing can be done in a shallow pit in the ground lined with a tarp, old bathtub, water trough, or a ring of straw bales with a tarp lining them (Fig. 14.6).

If you are dealing with dry soil, screen all the soil first to break up the clumps (tampers work great for pulverizing clay lumps). Fill the pit or container with

14.6: Presoak the soil in a pit, and add handfuls of straw while stomping around in the muck.

an appropriate amount of water and broadcast the screened soil into it with a shovel. Screening helps dissolve the soil quicker and is easier on the feet. Layer the mix with straw, and get in there and stomp around.

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