The fiber we use most commonly in earthen plaster is straw. Straw is derived from the stalk of grains such as oat, wheat, barley, rice, etc. Short, chopped straw is most often used for plaster, but the longer the straw, the coarser and stronger the mix. Straw provides tensile strength, much like chicken wire does for cement stucco. Straw performs like a mesh that weaves the surface together into a monolithic blanket. It helps resist erosion by exposing a matrix of little diversion routes that distribute rainwater evenly down the surface of a wall or the roof of a dome. Exterior plasters with an ample amount of long straw provide the most resistance to erosion.
We have seen how a beautiful earthen plaster prepared with short straw disintegrated into a fluffy golden bird's nest around the bottom perimeter walls after being pummeled by a violent thundershower. Exterior plaster made with primarily long straw as its fiber stays put. Other fibers we may use, in addition to long straw (depending on location, availability, and desired effect), include sun-bleached grass clippings, sawdust, slurried cow or horse manure, shredded paper, fibrous tree bark, hemp, sisal, coconut husk, cattail catkins, coarse, short animal hair, etc. A variety of fibers in combination gives the desired textured finish while enhancing tensile strength and resistance to erosion (Fig. 14.4).
EXTERIOR PLASTERS 173
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