Plasters, or renders, are the skin coverings or durable coatings to a wall or structure and in many cases can be applied on the interior and exterior. The job or function of a plaster is to help a wall or structure resist weathering, decay, and everyday wear and tear. Along with providing protection, they can also provide texture, wrap-around continuity, and aesthetic appeal for a wall or ceiling.
Natural plasters can be quite beautiful, with finishes having different colored pigments and texture while also providing a soft, inviting look to a home with its organic and natural feel. Natural plastering also invites artistic expression as plasterers can carve and sculpt patterns and caricatures on a wall’s surface. Some home owners may find natural plasters more inherently beautiful as the walls age and display their increasing amounts of subtle and innocent imperfections of occupant activity; an experience similar to an old-world aesthetic or when visiting regions of the world with buildings that are centuries old.
From the range of plasters or stuccos builders can use, natural plasters have a distinction in which they consist of materials directly from nature and include ingredients that are minimally processed and free from synthetic or petrochemical products. Natural plasters can be classified by their binder, with the three basic types being earthen (clay rich soil), lime, and gypsum. Builders can use earthen and lime plasters for both interior and exterior applications, but will primarily use lime plasters for exterior walls as it offers better protection against weather elements involving rain or moisture. Because gypsum is a rather delicate and water-soluble material, it should be reserved for interior applications. The ingredients of natural plasters are often an integrated extension of the natural components of a structure made from earth (e.g., adobe, cob) and from organic fibrous material (e.g., light-straw-clay, hempcrete). But for load-bearing straw bale systems, natural plasters will be the distinct coating or the applied skin type to the structure.
Earthen and lime plasters are the most common forms of natural plasters used in North America for natural buildings as the coverings are permeable and will allow walls with soil and natural fibers to continue in their capacity to capture and release vapor; a feature that enables humidity and moisture control for interior surroundings. In addition, many building codes will require permeable plasters for natural homes as a method to prevent condensation from occurring within the inner portion of the walls. If condensation were to occur inside the wall cavities of natural homes, it could potentially deteriorate earthen materials, such as adobe and cob, and promote mold or mildew that can decay straw, hempcrete, or wood components. Conversely, while earthen and lime plasters will allow vapor movement, their properties can also impair or hinder liquid movement that may attempt to go past the coating’s surface.
Although cement stucco can consist of natural materials, it does fall outside the category of natural plasters as the covering tends to crack by wicking moisture into earthen and straw wall cavities, while also having the potential to trap or prevent moisture from escaping. The situation can then lead to the deterioration of underlying earth or straw bale materials, which ultimately causes structural damage.
Humans have devised various plastering techniques throughout history based upon what was available locally or what may be in abundance. Early builders used earthen plasters, the oldest type of wall coating, primarily because they found sand, clay, and fiber readily available in many parts of the world. In central parts of Africa, builders have traditionally mixed local clay and cow dung as a covering for log and palm frond homes as the enzymes from the cow dung revealed they can assist in the hardening of a plaster mix while also preventing cracking.
The first recorded use of lime as an exterior plaster was in Turkey around 4,000 BC. In the northern desert parts of Africa, Berbers, during the eleventh century, came across a unique limestone deposit in the mountains near Marrakech and developed the ancient technique of tadelakt, a polished lime plaster that is usually treated with natural soap made of olive oil. The Berbers favored the material for wet areas such as bathhouses.
Examples of plastering extant include the Egyptian pyramids with archeological evidence showing the Egyptians applied plaster at least 4,000 years ago. The pyramids also show that the Egyptians used calcined gypsum, a material identical to plaster of paris, when doing finer work. Historical evidence of plastering also shows up in the Greek temples as past artisans there applied white lime stucco to walls as early as 1,400 BC. England also shows historical uses of plastering with the ornamental plastered ceilings that occurred during the reigns of Henry VIII, Elizabeth I, and James I.
INGREDIENTS AND APPLICATIONS
Most earthen plasters consist of clay, fine sand, and straw that is mixed with water to make the ingredients workable and malleable. To improve the quality of earthen plaster or to supplement what is lacking from raw materials, plasterers may also use additives. Examples of natural additives include cactus juice, flour paste, oils, lime, kelp, animal manure, cattails, and newspapers. There are no set recipes for earthen plasters as the proportions of the ingredients will vary and will depend on the quality of binding ingredients and the sand.
To make lime plaster, builders must first soak or hydrate quicklime (a powdered form of processed lime) over a period of at least two to three months. The soaking of quicklime is a chemical process known as slaking. During slaking, the calcium oxide and magnesium oxide portions of quicklime react with water, forming calcium hydroxide and magnesium hydroxide. This results in a gooey substance called lime putty. Experts recommend extensive setting periods for slaking quicklime (two years or more) so that the materials become fully hydrated and prevent any potential damage caused by remaining clusters of un-hydrated materials. If un-hydrated materials were to become trapped within an applied plaster, it could lead to explosions creating tiny pits in the wall’s surface. To shorten the slaking process, plasterers can use hydrated lime, which is a pre-slaked product available in bags. After plasterers complete a suitable slaking process, they can then mix the lime putty with sand to create the lime plaster and then apply the mix to walls or ceilings. Just as plasterers apply coatings of lime plaster, another chemical reaction slowly takes place where the calcium hydroxide in the mix reacts to the carbon dioxide in the air, a process known as carbonation. Carbonation is a slow curing process that can occur over many decades and overall prompts the plaster to revert to limestone. This feature then allows lime plasters to become very durable while also offsetting some carbon dioxide levels that are harming the planet.
To apply natural plasters to a wall or ceiling, plasterers will utilize a layer or coat system. This piecemeal approach helps prevent cracking and slumping, while also enabling quality control and the dialing in of other suitable ingredients and ratios. For straw bale walls, plasterers will apply three coats with the first (1) coat being the base, scratch, or discovery coat, while the second (2) coat will be a brown or infill coat, and the last or third (3) a finish or color coat. For earthen walls, plasterers will often apply just two coats, the base and finish coats, and for lime coverings, plasterers can apply the mixes directly to walls using three coats, with each layer having varying ratios of sand and lime. Plasterers can also add fiber to the inner layers for tensile strength. Typically, the finish coat has a higher ratio of lime to sand to maximize protective elements against wetness. Builders can also top off the inner coats of earthen plaster assemblies by applying two layers of lime plaster as the finish coat. The last coat for any wall system can include pigments, decorative elements, or a natural paint to give a wall color.
Due to their high mass and limited thickness, plasters do not generally contribute to a building’s insulation or R-value, but they can provide valuable air sealing properties that can contribute to energy efficiency. An added benefit for clay plasters is that they can absorb humidity from the air when levels are high and then release the humidity back into the structure when levels drop, helping to regulate indoor humidity and air temperatures.
Proponents of natural plasters assert that lime and natural plaster coatings have fire resistance properties, but studies are not yet available examining natural plasters using ASTM 119 fire ratings or techniques. The International Residential Code (IRC) for straw bale construction lists fire resistance requirements for a one-hour fire resistant/clay-plastered non-load-bearing wall, but has no requirements for load-bearing walls. Among the listed requirements for the one-hour rating are clay-plaster on both sides of the wall, a plaster thickness at a minimum of one inch, and a composition of three parts clay, two parts chopped straw, and six parts sand. Other parts of the International Residential Code for cob and light-straw-clay currently do not list any fire resistance standards applicable to natural plaster coverings.
Durability and Strength:
Exterior walls with natural plasters can have a long and lasting lifespan if the building is designed and built properly, as well as correctly preparing and applying natural plasters. Although erosion can occur during extreme weather events or under poor design and installation conditions, damaged natural plasters are usually easier to repair than cementitious or sythetic stuccos. For walls that have lime wash, periodic maintenance may need to occur with homeowners applying a fresh coat every two to ten years depending on climate conditions.
For interior walls, natural plasters are durable to an extent, but are of course subject to the same wear and tear that can occur for conventional walls that are dry-walled and painted. In high-traffic areas of a home or building, a more durable lime plaster may be a better choice than a softer clay plaster. A very durable and water-resistant type of lime finish, called tadelakt, can also be used in high traffic areas and in kitchens and bathrooms.
As to the technical strength of natural plasters, the International Residential Code for straw bale construction does list a minimum compressive strength of plasters for structural walls. For clay, it is 100 psi and for lime, it is 600 psi.
Indoor Air Quality:
According to natural building experts, natural plasters should have no negative impacts to indoor air quality if properly formulated for no dust. For builders who purchase plaster products, experts suggest looking closely for acrylics or fortifiers that may not be natural, healthy, or free from volatile organic compounds. Natural plasters can also have a positive effect on indoor air quality by having attributes that are less favorable for mold growth. This is because lime has high alkalinity and clay can mitigate high indoor humidity levels by absorbing and storing water vapor. According to the Canadian sustainable building non-profit Endeavour, some testing shows that clay plasters can take impurities out of the air and store and transfer them. The institution also says that clay plasters can be a beneficial source of negative ions in a home, which may help reduce odors and make the air seem fresher, a sensation similar to what the air feels like after a rainstorm.
Carbon Footprint of Lime:
Compared to earthen plasters, lime plasters will be more energy intensive due to the heat required to process the raw material. However, lime can significantly offset its own initial carbon footprint after installation by naturally re-carbonizing in the air.
Stucco is part of the plaster spectrum, but its distinction in the United States is that it contains Portland cement. Stucco in other parts of the world primarily means “to protect” and may not necessarily include a cement component.
Under careful preparation, plasterers can apply natural plasters to most any surface, including new or painted drywall, tile, metal, wood, brick, and cinder blocks. Before plastering begins for some surfaces, plasterers may need to apply an adhesion coat or cover the surface with a solid substrate or lath.
Other than possible free on-site subsoils, owner-builders can obtain individual ingredients for natural plasters from pottery stores, local quarries, masonry supply shops, aggregate/landscape suppliers, hardware stores, and straw from local farms or farm supply outfits. The pricing of the materials can vary by region and by the quality of the ingredients produced by the supplier. Material costs for lime-based plasters may be slightly higher or overwhelmingly higher than clay-based. Most materials for homemade mixes will have a lower cost as compared to companies supplying ready-made products and pigments.
Although earthen or clay plasters are more susceptible to erosion when there is driving rain hitting their surfaces, natural building experts say the material is suitable for most, or if not all, climates. But a caveat that is often stated by experts – is only if the builder designs and constructs the home well – a statement that often applies to most any building material and method. To withstand the stressors of wetness, natural building experts recommend that earthen homes have suitable roofs with decent overhangs while also having well graded or trenched foundations that can intercept the wetness of standing snow and water. To make a home more weather resistant, some natural builders will incorporate natural stabilizers such as cactus juice, flour paste, or lime to plaster mixes. In addition, some builders will also apply a lime coat as a finish coat to the inner layers of earthen plaster. To protect transition zones near ground level, some owner-builders will also use flashing and angular stone facings.
The last coat to a plastered assembly is the finish coat, which can include some or many of the same ingredients that were used for prior layers. The finish coat is when a plasterer will often introduce color, texture, and artistry to the assembly, as well as expressing an endless range of artistic ability, as finishes can include various materials and pigments. Examples of natural finishes are: 1) Alises – thin decorative natural clay finishes with several variations that are applied with brushes, sponges, or sheepskin; 2) Litema – a mixture of manure and colored clays applied by hand; 3) Decorative Clay – a thicker and more pliable coat, similar to alises, but applied by trowel; 4) Silica Paint; 5) Casein or Milk Paints; 6) Lime Washes; and 7) natural paints made from egg yolks, egg whites, beer, and natural oils.
Natural plaster experts suggest owner-builders test and experiment with pigments and carefully note recipes and results before introducing pigments to a wide-scale plaster application. Experts also caution that both natural and synthetic pigments can be toxic and advise that owner-builders should inquire about toxicity prior to purchasing any pigment. The inquiry should also include knowing about possible emissions of volatile organic compounds (VOCs). Experts also advise that owner-builders should practice safe handling procedures by wearing protective gear, working in well-ventilated spaces, and keeping pigments away from children.
Essential Natural Plasters: A Guide to Materials, Recipes, and Use by Michael Henry and Tina Therrien, New Society Publishers, 2018.
The Natural Plaster Book: Earth, Lime, and Gypsum Plasters For Natural Homes by Cedar Rose Guelberth & Dan Chiras, New Society Publishers, 2003.
Using Natural Finishes: Lime and Earth Based Plasters, Renders & Paints: A Step-By-Step Guide by Adam Weismann & Katy Bryce, Green Books Ltd, 2008.
The International Code Council – Model Building Codes
Applicable International Residential Code (IRC) Appendixes: AR-Light Straw Clay Construction, AS-Straw Bale Construction, AU-Cob Construction, and the pending AY Hemp-Lime (Hempcrete) Construction
Ecological Building Network – San Rafael, CA
Canelo Project – Elgin, AZ
Community Rebuilds – Moab, UT
Yestermorrow Design/Build School – Waitsfield, VT
Endeavour Sustainable Building School – Peterborough, OT