Working your walls with earth plaster: play in the mud with earth building expert Mollie Curry

New Life Journal, April-May, 2005 by Mollie Curry

If you think smearing mud all over your living room walls is either an act of insanity or vandalism, think again! Without having to build a straw bale house or a mud hut, you can experience the beauties and benefits of earthen (clay-based) plasters by doing a bit of eco-remodeling in the house you live in right now. You can totally change the personality of a room that is tired or sterile with a coat of mud on the walls.

Many colors are possible, mostly in muted hues. Earthen plasters, with their slight--or major if you choose--variations in surface texture, reflectivity, and color bring a sense of life to a room or a whole house. They lend a handmade feel, often in a classic Old World sense. Some finishes look almost like leather or marble, but there is a lot of room for creativity. You can smooth and round corners and transform boring flat sheetrock by adding a bit of sensuous undulation or trowel or hand marks. Most people feel more comfortable in rooms that have some variation in wall surface, shape, texture, and color, perhaps because we humans have been housed for millennia in caves, and houses of wood, stone, mud and thatch--not in flat-planed boxes!

If you want to start with a small project, try accent touches around doorways or windows, over a brick fireplace, or in an alcove. You can even do relief sculptures on the wall, sometimes with the help of simple armature like screws. Mosaic with tile or mirror is another option, as well as a form of high-relief stenciling. With the addition of oil and wax, people have even made sinks and bathtubs out of mud! Plastering the exterior of an existing home is possible in some cases, but has different issues to address than interior work.

LOW ENVIRONMENTAL IMPACT

Clay-based plasters represent less environmental cost than cement or gypsum-based products because they are not heated to high temperatures, which take a huge amount of energy, with its resulting pollution and greenhouse gasses. They are not as resistant to impacts or abrasion as the other common stucco/plaster options, but with reasonable forethought about where to apply them, as well as attention to proper mixing and application, they should do fine in many situations.

Unless certain pigments are added for color, earthen plasters (and paints) are very low on the toxicity scale--they are the original "no VOC" (volatile organic compound) wall coatings. Iron oxides and ochres are the safest pigments. Do be careful not to breathe the dust of clay or sand (or anything) while mixing, as this can result in serious lung problems.

WHAT ABOUT WATER AND HUMIDITY?

Earthen plasters can get wet on occasion, but repeated driving rains, constant wetness, or excessive humidity are likely to result in some degree of damage. Luckily, clay-based plasters are easier to fix than concrete stuccos. What about our famously humid climate? It's all about choosing where to put your plasters. They do best in places that receive light and air circulation, not in dark, damp places that already grow mildew. However, because of clay's ability to molecularly absorb water without getting wet (to a point, of course), then to let it evaporate easily when drier air wafts past, earthen plasters with the right ingredients can be surprisingly resistant to mold. Some museums have even earth-plastered their walls to help moderate swings in humidity, which can damage valuable works of art.

Mold doesn't actually grow on clay or sand, but the "organic matter" ingredients of plaster could feed it. However, experience has shown that it has to be a pretty bad situation for a mold problem to develop (little sun or air circulation). Clay has been found to protect straw used as insulation in Tudor-style timber frames in Germany for hundreds of years. Hydrated lime-based plasters (which can also have clay in them) are even more mold-resistant due to their alkalinity. Unlike garden lime, hydrated lime (builder's lime) is heated to a high temperature (similar to cement) to change its molecular structure. If treated correctly, it slowly turns back to limestone as it cures on your wall.

OF RECIPES AND EXPERIMENTS

Plasters are usually made up of three main components: binder, aggregate, and fiber. In this case, clay is the binder, sand is the aggregate, and straw or manure from grass-eating animals serves as the fiber (the manure completely loses its odor when dry). There are also many potential additives that improve the workability, durability, or water-resistance of the finished product. I have found that ingredients that are sticky when wet generally become hard when dry, thus adding to the durability of this relatively "soft" coating. Some common additives include milk products, wheat paste (like old-fashioned wallpaper paste), oil, paper pulp, and cactus juice where lots of prickly pear cacti grow. Borax has sometimes been added to retard mold, especially in earthen paint, which is sometimes done over a plaster or "regular" wall for color or other decorative effect.