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Earthen Plasters

This project has afforded us the opportunity to master the art - and perhaps advance the science - of earth plastering. Our plasters are mixes of sand, clay-rich soil, lime putty and chopped straw fibers. Small quantities of added casein (a milk product) and linseed oil (from flax seed) significantly improve weathering and moisture penetration properties. Our recipes vary, depending on location (interior or exterior) and layer (first coat, brown coat or topcoat).

In addition to our own on-site field research, we're working with scientist Douglas Piltingsrud to understand more about durability and moisture performance of stabilized earthen plasters. We will be making our results and recipes available as soon as we feel we have sufficient reliable data.

Anthony prepares the wall for the first coat of earthen plaster. He uses a drywall texture sprayer to apply a bond coat of pure clay slip to the cured straw-clay. The sprayer drives the slip into all the small spaces and thoroughly coats the fibers.

Screening the slip avoids clogging the sprayer.
Click here for more on our techniques for mixing better slip.

The straw-clay surface, ready for plastering. Fiberglass mesh tape, 6 inches (15cm) wide, covers the framing and is embedded in the plaster to control cracking. We found the tape - made in Canada - available through True Value hardware stores and Home Depot. Our local True Value was more helpful in special-ordering the tape.

In the foreground, Anthony wields the "Tirolessa", a plaster sprayer which operates with air pressure from a conventional compressor. The sprayer is a plaster delivery system only, not a finishing tool, but it speeds the work along nevertheless. The Tirolessa, made in Mexico, is well-designed, sturdily constructed and reasonably priced. For more information, visit Paul Sarnstrom's site.

In the background, Bill compacts and trowels the plaster smooth. This first coat is a clay-rich mixture with plenty of chopped straw to help control cracking.

 

Click here for some helpful plastering articles on "The Last Straw" website.

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