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Natural Plasters

Natural plastering is a truly ancient form of building, found in civilizations throughout history. Since some of the earliest habitations, humans have been covering their walls, for decoration and performance, first with earth, then with lime and gypsum. These three substances — clay, lime, and gypsum — are the basic materials known as natural plasters; these stand in contrast to cement-based stuccos, which are far more common in the modern industrialized world due to their speed of set and ease of uniform mass production. The term 'natural finishes' generally refers to a whole family of wall treatments, including renders (exterior treatments) and plasters (interior treatments), and paints (thinner treatments, brush/sponge/roller applied) and washes (thinner and less opaque than paint). It should be noted that generally the terms "render" and "plaster" are used interchangeably here in the United States. In natural building, plasters generally serve more than one purpose. In straw bale or other highly insulative wall systems, the plaster is often the air barrier for for the wall, keeping the wind from whistling in. Plaster is used as front-line defense for moisture intrusion into the structure, and is employed to help control moisture within wall assemblies. Plaster helps to protect walls from premature wear and abrasion, and plaster lends a unique look to walls, whether built of earth, straw, stone, wall board, or cement. In this section, we will look at an overview of natural plasters and finishes, how they are used throughout the building process, and why they are superior in many ways to conventional treatments.

Why Natural?
The benefits of natural plasters are many, compared to other conventional options. They are the "natural" choice for a finish of earthen and straw walls — the former generally built of the same materials, the latter requiring the plaster for baseline functionality — and are highly adaptable to be applied over a wide range of substrates in a wide range of applications. They add mass in the interior of a structure — generally a good thing for thermal performance — and are inherently ecologically friendly, both in terms of embodied energy, end-of-life and waste disposal, and applicater- and occupant-safety. Perhaps one of their greatest strengths lies in their beauty — a gorgeous palette of colors and textures that can enhance any room, accentuate architectural emersions, allow for inlays and bas reliefs, sculpting and molding, high refinement or rough organic texturing. The versatility of natural finishes in attaining a wide variety of finishes over most any substrate make them applicable in any home, and they are approachable by even the most inexperienced owner-builder.
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Anatomy of a Render
There are three basic components to a plaster: binder, structure, and fiber. The binder does what its name implies: glues the mix together. Plasters are generally defined by their binder; in the case of natural plasters, this would be either clay, lime, or gypsum; manure is also a very common binder (one of the most common worldwide), although less so in the west. Clay is well-known as a fundamental of natural building, found from structural wall systems to color washes. Often used in raw form taken directly from the earth (often as a component of the soil, mixed in with silt and/or sand), it can also be purchased in a refined form from pottery suppliers. Lime is sourced from limestone, or calcium carbonate. It is this same mineral that forms the feedstock for cement, as well. The limestone is processed through a series of heat- and water-treatments, to give us either lime putty, or hydrated lime powder (more common in the northeastern US). Gypsum, also a mined geologic product, is commonly found in various mixes and forms in conventional building, from wall boards, to joint compounds, to off-the-shelf finish plasters. Clay and lime also serve as the binder for many natural paints.

The structure forms the main body of the mix. In plasters, this is generally sand, which can range from sand-rich soils with larger particle sizes for rough base-coat plasters, to super-fine manufactured sand for finish plasters and paints. Other stable, hard, non-organic mineral products can be used for structure (in part or whole replacement for sand), including pulverized volcanic stone, glass, mica, or stone dust or powder (i.e. granite or marble). Different mixes and applications will call for different structural materials to serve their purpose.

The final basic component, fiber, provides tensile strength for the mix, allowing for a thicker application, or even moldable build-out, of the plaster, to bridge weak substrates or transitions in the substrate, and to help control cracking. For rough plastering, the most common fiber is chopped straw. Horse hair has been used historically for lime and gypsum plasters, as has horse manure. Cellulose (as in the material blown in for insulation) is commonly used for finish plasters, and some paints. The finer the application of a material, the less need for fiber - hence, fiber is generally not used in a wash, except for aesthetic purposes.

Other additives can be found in plasters, including manure, cactus juices, pigments, wheat paste, and many more, to help with binding, set time, appearance, or other purposes specific to the application.

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Choose Wisely
As we discussed, there are many kinds of plaster, and many contexts in a building in which they can be used. Each material has its own properties that lean more or less favorably to different applications. Part of the art of natural plastering - and a key to a successful application - is in selecting the right mix for the job; this comes from a good working relationship with and understanding of the materials, and that comes only from direct experience. Clay sports the highest levels of moisture absorption and vapor permeability, is the easiest to repair, and generally has the lowest embodied energy, but is also relatively soft compared to other finishes. Lime is very alkaline (high pH), and is therefore caustic, and therefore inherently anti-microbial. Lime is harder than clay, yet much softer than cement; it is commonly used to protect exterior walls. Gypsum plaster mixes are very easy to find, set up very quickly and are incredibly versatile, yet very sensitive to moisture and cannot be used in exterior or direct-moisture applications. By designing the appropriate mix for your wall, you will be well on your way to a satisfying and durable wall finish.
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Moisture Matters
Of all the forces at play on a building, as previously stated, moisture is our prime concern, particularly in the northeast. In the design section, we discuss strategies of protecting the walls from moisture; please read this section, as these strategies apply in almost any circumstance in which plaster is present. In brief summation, some basic elements of preserving the wall from moisture damage include: protecting the top of the wall with good overhangs and a solid roof; bringing the base of the walls a good 12-24" up off the ground to avoid "splashback" from rain, ensuring a good capillary break between the plaster and any moisture-wicking surface (such as the top of a concrete stem wall), and designing the house to avoid prolonged intense wind-driven moisture exposure. Each building is different, each site is unique, and you may (and often will) find yourself working in far-from-ideal circumstances. There is always a solution; however, sometimes that solution comes down to maintenance, which ultimately relies on human responsibility - a pretty uncontrolled variable, even for the most disciplined among us. The more you can do to protect your walls, the less they will wear. Consider the words of an accomplished building scientist: "The best moisture control strategies always involve designing problems OUT - not solving them after they have been needlessly designed into the enclosure."
- John Straube, Chapter 5.2, p. 144, Design of Straw Bale Buildings
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Proof in the Pudding
In the last 50-100 years, the use of cement stucco has become widely used, especially in areas in which plastered finishes are part of the historic vernacular, in an attempt to replicate authentic, old-world finishes in a modern-day timeframe and budget. Many historic, centuries-old, earthen structures originally finished in earthen or lime renders, however, have shown signs of failure within as little as ten years after being refinished in modern cement stuccos. Why? This is cause for some speculation and debate, but a basic understanding of the differences in performance between the two renders points to obvious conclusions. As mentioned in the design section, moisture plays a critical role in the longevity of a natural wall system (or any wall system, for that matter). Clay and lime are very attractive from a moisture standpoint for two big reasons: 1) their ability to hold large amounts of moisture, thus acting as a 'moisture sink', and; 2) their ability to allow for the transmission of vapor moisture, known as 'vapor permeability'. It is this latter strategy that gives name to the 'breathable' wall system — not for the passage of air through the wall, but for the migration of water vapor. Stucco, on the other hand, while porous and able to wick liquid moisture when in contact, is able to hold very little moisture — far less, in fact, than the straw or earthen walls they protect — and are relatively vapor impermeable, meaning that should water vapor find itself into the wall cavity, it will have a much harder time finding its way back out. Another benefit to the use of clay and lime finishes are their more flexible nature; stucco, in contrast, is more brittle, which can lead to more cracking, and therefore more points of entry for moisture into the wall. Finally, lime, and especially, clay's ease of repair makes it desirable from a long-term maintenance standpoint, once cracks do appear.

So what happened to those failing structures of old? Simply put, water damage. Whether from vapor moisture or liquid moisture, from infiltration through cracks or from condensation, the original, moisture-conditioning natural plasters proved, with years of experience and maintenance, to be a superior render in natural wall systems. Add to this the low embodied energy of less-processed materials, as well as the great savings that can come from performing basic processing on-site by hand or simple machine (for example, sifting excavation soil for use in base plastering), and the soft, gentle texture of hand-applied fine natural finishes, and the validity of their use in the natural home — or any home, for that matter — becomes quite convincing.

One more word on the use of cement: while the members of NBNe strongly advocate against its common use as a finish in natural homes in this climate, it should be noted that the use of cement stucco is not uncommon in many parts of the world, particularly dry, earthquake-prone areas such as the southwestern United States, where moisture issues are much less of a concern, and the structural properties of appropriately detailed cement stuccos are desirable. However, there are people in cold, wet, seismically stable areas, such as in the Ontario region of Canada, who promote the use of cement renders over straw bale walls. Ultimately, time will tell what materials and techniques prove successful in which areas; caution is given to the new and experienced builder alike to take as many precautions as possible against damage in whatever circumstances the building is to be constructed, and to evaluate the risks and benefits to the structure as a whole when evaluating the correct choice of plaster, wall system, roof, foundation, or any other element of the structure. The recommendations expressed here are based off of the collective experience of over two dozen building professionals in the northeastern US, and are not universal to the natural building world as a whole.
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