For Love Of Mud
Plasters & Strawbale
Earth Plasters and Aliz
The Straw Bale Earthen House
—Athena and Bill Steen
|For Love Of Mud|
by Keely Meagen
Mud is magic. Mud is fun. Mud can also be quarrelsome.
Don't get me wrong, I love earth plasters. I want all new strawbale homes to be wearing them. They make the most beautiful walls I have ever seen. The colors are gorgeous and incredibly varied. The walls feel soft and homey. Earth plasters can be rustic and undulate with the bales, or create smooth walls with even curves and a polished finish. The materials are cheap, easy to come by, and healthy for the environment. Earth-plastered walls breathe like living beings, protecting bales from moisture damage by exhaling moisture instead of locking it inside. Mud is a blast to work with, can be extremely durable, and is easy to patch and repair when necessary. (Durability sidebar.) I highly recommend it.
Sometimes earth plastering is so simple it is astonishing. Friends in Arizona and California have found that the earth beneath their strawbale cabins had the perfect proportion of clay and sand. All they had to do was add water, one also added a bit of chopped straw, and voila! Beautiful plasters that didn't crack.
So why do I quarrel with my walls? Well, I don't really (mud is always right). But I have had several long, drawn-out head-scratching sessions while questioning my walls ("just why are you doing that?") Working with mud keeps me on my toes and forever curious. (Whoops sidebar.)
The simple truth is, mud will never be a standardized material like cement. Each batch of dirt is different and has its own personality. (Clay sidebar.) Earth plasters have a slower drying time and must be built up more slowly than cement. There are very few people with big machines to spray it on (and even fewer who have enough experience to be beyond the experimental stage). If you want smooth walls that don't reveal the shapes of the bales, you are probably looking at a lot more labor and time. And then, with each new batch of dirt, there are the potential surprises. In this country, we are only beginning to recover the lost art of earthen plasters.
It used to be different
Information from this section comes from research conducted by Susan Barger and was published under the title "Investigations Into The Durability of Plasters, Part One: Initial Materials Characterizations and Correlation With Oral Tradition."
A hundred years ago in New Mexico, each village had its own location for the best plaster dirt. Adobes could be made from virtually any earth, but the plaster dirt was special and was often carted, if necessary, to the building sites. Its properties were known and loved, and the techniques for working with that particular dirt were passed down from mother to daughter in the tradition of the enjarradoras.
Unfortunately, much of that plastering knowledge was lost with the arrival of the upstart newcomer, cement. The source sites for the dirt are remembered by few. Carole Crews, Cornerstones (the folks who restore the old adobe churches in NM), and others have done a lot of research into the old ways. Susan Barger has some fascinating interviews with older folks who remember participating in the plastering as kids. She also analyzed old plaster samples, looking for clues to what makes a durable mix. Ms. Barger found the chemical reactions in mud plasters to be so complex that they had more questions than answers at the end of the research. Clearly more is needed.
But even if we understand how to work with one particular dirt, the fact remains that the soil of Santa Fe is different from the soil in Oregon. In fact, the dirt under my feet may be completely different from the dirt 30 yards away.
So What Does This Mean?
First of all, relax. Earth plastering is easy. Once in the mud, many of us feel we are remembering information our cells have tucked away for us long ago. Getting in the mud helps us access that knowledge. I had a similar experience in one of Robert Laporte's timber framing workshops. After struggling with the chisels for days, feeling clumsy and awkward, I experienced something clicking in, and all of a sudden I was handling that chisel like I'd been doing it for years.
But we shouldn't have to rely completely on intuition and memory of past lives! Or whatever it is that happens in those magical moments. So here are some tips avoiding potential problems and keeping the fun in the mud.
First, ask yourself these questions:
· How much durability do I want and need?
Enough Talking And Thinking... Let's Get Into The Mud!
Is it a meditation hut or a conference center? Do you have ten kids?
· How much time do I have?
(If you need it finished this week, call the local cement stucco crew. Earth plasters take longer. Plan it into your schedule. Prioritize the areas that need to be finished first. Prioritize the exterior if winter is coming on. Interiors can be done in cold weather, but see sidebar cautions about plastering in winter
· How much money can I put into this?
If you are on a tight budget, get your friends to help you and/or do workshops. The materials are dirt cheap and the labor will be free. If it is a big house, consider bringing in an experienced person to show you the ropes and help you figure out the mixes. If you want to hire a crew and want smooth, polished walls, expect to spend more than you would for cement stucco. If anyone out there knows how to make it less expensive, write an article and let us know!
· What do I want the finished walls to look like?
It takes more labor and materials to create smooth walls out of lumpy, undulating bale walls. If you like the natural curves, keep them!
· What materials do I feel comfortable using?
The mining of materials—even dirt—is harmful to the earth. Look first at what is around you, and what friends have available on their lands. Try to use materials that have the least impact to create the plasters you want and need. We will all make different compromises—make sure yours feel ok to you. And get creative if compromising doesn't work for you. Perhaps you'll discover something that will help us all out!
Start Simple. Use the dirt from digging your foundation. Test simple mixes (clay and sand, or clay and straw in varying proportions). If one of those does not give you the durability you need, or if they are "fussy" (hard to trowel) look at other clays, or start adding amendments one at a time. Don't assume you must use a particular amendment. I tried for months to get flour paste (aka wheat paste) into the mix at a Moab, UT cohousing community center, thinking I needed it for durability... but even tiny amounts made it ridiculously difficult to trowel. In the end, we settled for clay and straw. Really strong, easy to trowel, and didn't dust. (Thank you Kaki for pointing out the obvious).
Reject sand and crusher fines from the local gravel yard sometimes have clay and sand in perfect proportions for plaster. It's worth investigating.
Make test batches for each layer of plaster. If you need a really durable plaster, or you are doing a big house, do a lot of testing. The more familiar you are with your materials, and the larger you make your test patches, the fewer the surprises you'll find when plastering the walls for real.
Forgo chicken wire. It's labor-intensive, a pain to work with and prevents you from working the plasters deep into the bales. Cedar Rose developed the idea of using a drywall texture gun to spray a thin clay slip onto the bales before plastering in order to help the plasters really grab onto the straw. It Works. (You can also apply the slip with your hands, but it's really messy).
I now use the following system: patch holes first by stuffing them with straw dipped into a clay slip. Let the patches dry. Spray the walls once with a clay slip, and have others come behind working the scratch coat in with their hands before the slip dries. Don't build out the wall at all, just work that first coat deep into the bales. (The elements of this system all came from Cedar Rose.)
On smaller buildings where it doesn't make sense to rent a drywall texture gun, I combine the slip and scratch coats by making one sloppy, high clay-content plaster and mush that directly into the bales. It seems to bond to the wall almost as well and saves me from going over the wall twice by hand.
Pits work well for mixing large batches of plaster if you have a bunch of people. I think it is faster than using a mixer, and easier to let the plasters sit overnight. (Old folks in New Mexico say plaster that sits overnight is better—easier to work with and makes a more durable plaster.) To make a pit, put 4 or 5 straw bales together in a square or circle, and cover with a tarp big enough to go over the edges. Mix it by stomping around with bare feet. You can then roll the mix by grabbing the edge of the tarp and pulling towards you. (This helps get unmixed materials off the bottom). If there are only one or two people plastering, or it is getting cold outside, a mixer is faster and easier.
Use fresh, clean chopped straw so that you do not introduce mold spores into your plasters. Chop it quickly with a leaf mulcher (a sort of stationary weed whacker that shoots chopped straw out the bottom). Sears has a "Leafwhacker Plus" for about $110, and it is well worth the price for big plastering jobs.
Keying plasters into the previous layer helps create a strong wall. Things that will help: wet down the previous layer of plaster before adding a new layer, and leave the scratch and brown coats rough (if you use a trowel, come back with your hand or whisk broom or rake to give it some texture before it dries.)
To Each Their Own
In Building with Earth, John Norton says:
"It is important to recognize that the technology of earth building is extremely varied, not only in a technical sense—the soils available, the way they can be used, and the functions to which they are applied, but also in a social sense—from the user's viewpoint there is a great variation in what is regarded as an acceptable standard."
Mud plasters have come a long way in the last ten years, in part due to the significant contributions made by some amazing folks researching traditional earth plastering methods and developing new systems for plastering straw bales. My thanks to Bill and Athena Steen, Carole Crews, Cedar Rose, and Cornerstones for their dedication to the craft and for sharing what they have learned. My work has been made easier and more effective because of their teaching. Special thanks goes to dirtbag magicians Kaki Hunter and Doni Kiffmeyer for their innovative approach to earth building and plastering.
I could go on for years with suggestions and ideas, but these are some of the most basic helpful suggestions I have found from my own work with earth plasters.
Before I finish off, I have to mention two fabulous ideas I have come across:
Earth Plaster on Drywall. If you use wheat paste in your finish plaster mix, you can put it right on drywall. It's quick, easy, and beautiful. Just be sure to use a gypsum-based (non-synthetic, non-asbestos) joint compound. I had to scrape our test patches off drywall and it was tough. If you want it even tougher, paint the drywall first with a mix of 10 parts hydrated or homemade wheat paste, 1 part fine sand and 1 part clay. Let it dry and don't wet it down before you plaster it. I've sunk 16 penny nails into drywall finished this way without splitting, chipping or other damage. It surprised even me! Thanks to Cedar Rose for the adhesion coat recipe.
Easy Nichos.Troweling nichos into smooth curves with no trowel marks is near impossible, and can take even experienced plasterers huge chunks of time. A quick, easy way to get lovely shapes with no trowel marks is to run a strip of 4 mil plastic along the curved edge (perpendicular to it). It smooths out those edges in a snap. Thanks to Stephan Bell for this gem.
C'mon Y'all, Join Us In The Mud!
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Keely Meagan co-coordinates Artisan Earth, a women's roving natural building and earth plaster crew. Email her at email@example.com
Plasters & Strawbale
by Cedar Rose
Most people choose strawbale for environmental considerations, wanting to be kinder to the earth and to themselves. Straw bales have low embodied energy; they're easy and fun to use, while being light on the earth; and they can help create a nurturing home environment.
Earthen plasters share all of those characteristics with strawbale; they are completely compatible mates. The straw provides us with a natural material to keep our homes well-insulated and protected from harsh exterior elements... and similarly, earthen plasters protect our bales with natural techniques which embrace the ancient roots of cultures all over the world. We can achieve strong, durable finishes while being responsible for our personal impact on the planet and the health of our homes.
The current production level of cement and concrete causes serious environmental damage by creating significant amounts of air pollution, water pollution, and damage to the earth from mining operations. Current uses of cement and concrete introduce outgassing into home environments from additives, curing agents, radioactive materials, and possible radon exposure from aggregate used in the mix. Synthetic stuccos introduce a combination of harmful chemicals which also outgas. Concrete and synthetic stuccos compromises the health and well-being of our families and our planet.
While concrete in some applications (like foundations and slabs) can be sealed with non-toxic sealers to prevent out-gassing into a home, the use of these sealers for concrete stucco, or the use of synthetic stuccos, is not recommended because it reduces the breathability of your strawbale wall system, potentially creating significant moisture-related problems.
Whether we are addressing moisture from condensation, the weather outside, or build-up from the interior of our homes, the ability to dissipate it is important. Moisture can be a problem when it is trapped in the building components by a non-breathable surface.
Straw bales have the ability to breath and dry out, except under extreme conditions, without compromising the bale's integrity. Cement stuccos can cut down the breathability of the wall system; sealed cement stuccos more so. On the other hand, earthen plasters protect the bales by shedding rain while simultaneously creating a breathable surface which assists the drawing-out of moisture which inevitably enters the wall system, usually in the form of vapor.
Clay molecules are shaped like platelets. When wet, they swell naturally to seal the moistened surface preventing deeper penetration of moisture in towards the bales—unlike concrete, which wicks moisture in toward the bales. The clay platelets also act to draw moisture from within the bales out to the surface to dry. This is an important element of system durability, especially in harsher climates with higher moisture and/or freeze thaw cycles which can wreak havoc with concrete finishes. When time comes for repairs, earthen plasters are much simpler and easier than concrete.
We have found time and time again that, when done right, earthen plasters and finishes are far superior to concrete and synthetic stuccos in many ways. In comparing the complete picture of earthen systems against other stucco systems—including durability, longevity, materials costs (financial and environmental), ease of use and application, availability, long term maintenance, effects on the complete building system, and human health—we have found that hands-down (and hands-on), earthen is the way to go.
A long lasting, durable Earthen finish on a strawbale structure is accomplished by considering the following factors:
House Design. Where does the water go? Think about interior as well as exterior moisture concerns. Climate considerations: Snow? Rain? Winds? Hurricanes? Blizzards? Where do the storms come from? What time of year? How do roof systems come together? How do your walls come together? etc.
Construction details. With consideration of your plaster system, how have you connected your walls, foundation, roof? How do you tie in from one building component to the next? Have you eliminated non-natural and/or non-breathable surfaces that your plasters may come in contact with? Constantly keep in mind how you can "key" your plasters into your structure.
When using earthen plasters, always have your bales stacked with the cut sides to the interior and exterior; don't have your bales on their sides—this eliminates the best surface area for your plaster to key into.
Plaster Mixes. Your mixes are determined primarily by the type of soils you have available to you, which are unique to each project. Where are the materials from? Do you have an adequate supply to complete your project? What is the clay content? How does the clay respond to moisture? What type of mineral composition is your clay? How "workable" is your clay? How absorptive is it? What is the clay/silt/aggregate content of your dirt?
Additives are often used in earthen plasters: manure, cactus juice, flour paste, ox blood, etc. Fibers are variable as well, including straw, animal fur, cattail fibers, hemp, etc. Each has their own contribution based on their constitutions: proteins, fibers, enzymes, microorganisms, pH, minerals, etc. Your mixes are unique, and additives and combinations of materials are also dependent on the materials at hand. Do many experiments, determine the composition of your mix and determine which will serve each application the best that it can.
Application. It is extremely important to "key into" (that is, have a mechanical bond between) each layer of straw and plaster. Chicken wire, metal lathe, gaps in the bales and loose straw can all contribute to eventual failure of an earthen stucco finish. The air trapped behind the plaster causes its surface to be vulnerable and be one of the first areas to breakdown.
Joints and spaces between bales need to be packed with a damp clay/straw mixture, and kneaded into the bales themselves to prevent loose areas. Deep holes can first be packed with dry straw, and then packed on the surface with damp straw/clay. We like to do this during our first slip spray process. This packing gives the plaster a strong surface to key into and eliminates the problem of plaster from falling off of our wall for lack of mechanical bond.
A common mistake made with earthen plasters is to spray or spread a thicker plaster mix directly on the bales as a first coat; this eliminates a strong bond between the plaster surface and the straw bales. Instead, apply deeply-penetrated slip coat of water and clay first. You can use a stucco sprayer, drywall texture gun or a hand pump sprayer to apply a clay slip coat. Another option is to use a very damp straw/clay mix and "slop" it on the bales. The most effective and least time-consuming is the sprayer or texture gun technique, which provides a consistent coated surface for the plaster.
Let the slip coat dry, then mist the wall and go over it with a wet straw/high-clay/sand scratch coat mix, making sure you are keying into the spray coat well. Whether you apply this by hand or sprayer, use your hands to work it well into your surface coat.
Once you have applied this coat, let it dry. Mist it down again and apply a higher sand mixture that has more structural integrity to it. This layer is used to fill in and shape your walls. Once this coat has dried, moisten again and apply a finish coat.
Between each coat, except for your finish coat, leave your surfaces rough and mist them before applying your next coat. All the while, focus on "keying into" your previous coat, working your plaster layers in well. Cracks in your surfaces are good for keying your next coat into. Avoid cracks in your finish coat, of course. Although earthen plasters take work and time, the end result is incredibly satisfying, beautiful, durable, and contains your own unique personal touch.
The earthen finish coat can be left as is, or oiled or finished with an assortment of natural clay plasters, paints, aliz, or casein finishes with a huge variety of natural pigment colors.
To simplify a fairly complex topic, if you have a strawbale house well designed for the use of earthen plasters and natural finishes, if your mixes achieve a solid, durable, breathable surface, and your application techniques have keyed in and secured your finishes well, you will achieve a durable, long-lasting earthen finish for your strawbale structure. For a plaster system which is natural, easily available, fun, environmentally supportive, and a healthy building material, Earthen Is It!
With over 20 years of experience in natural and holistic building and design, Cedar Rose Guelberth has developed effective natural plastering techniques for strawbale for many climate conditions in a large variety of projects. She teaches and consults throughout the US, Australia, and Canada. She also owns and operates the Building For Health Materials Center, which offers non-toxic building supplies and finishes, appliances, and more. Email firstname.lastname@example.org
|Earth Plasters and Aliz|
by Carol Crews
A clay slip, known to some as an aliz, is used on an earth-plastered wall almost like paint is used on other surfaces. The purpose is to seal and beautify the surface. After it has become soiled or damaged, another coat may easily be applied to renew its fresh look. Traditionally, the Aliz was applied with a sheepskin in a very thin layer. This was done as a part of spring cleaning after a winter of woodsmoke, often using the skin of a spring lamb which had been killed for food. In Taos, the favorite material was a micaceous pearly-grey clay called Tierra Blanca found in a cave south of town. The finish work on adobe buildings was traditionally women's domain, and many vintage photos portray this activity.
The oldest example I've seen of a clay slip not used in pottery was in an Anasazi cave in the Tsankawi ruins. On the floor under two inches of dust was an intact layer of peachy-colored clay sealing the coarse graininess of the tufa cave. In Africa and India the use of clay slips for decorative work on houses is well documented. Earthen architecture is the most basic and common known to man in most of the world, but the accessibility of cement and cinder-blocks are causing people to value their traditional methods less. They want to try out what they think is a more permanent and therefore better means of construction. This is very apparent in Mexico where people associate mud dwellings with poverty, and prefer the status of concrete buildings despite their failure to moderate temperatures.
Modernization has definitely affected the use of clay finishes here in Taos. Even at the Pueblo, some people are buying latex paint for their interior walls instead of using the old techniques. Wherever there is a leak the paint peels off, revealing dark mud beneath. Paint keeps the wall from "breathing" properly, or more technically correct, its ability to transpire moisture without it being trapped against a barrier of a more impermeable substance. Clay slips on the interior and mud plasters on the exterior allow an earthen structure to absorb moisture and then dry out again without creating major problems.
A big lesson was learned at the famous St. Francis de Assisi Church in Ranchos de Taos when it was coated with cement plaster in 1967. The plaster cracked and allowed moisture to go deep into the adobes and stay there, unable to dry out again. Large sections of the buttresses had to be rebuilt, and the community has now gone back to annual renewal of the mud plaster—which not only keeps the church building in beautiful condition, but strengthens neighborhood ties as well.
Before sharing the details of clay slips with you, I will briefly explain the basics of plastering the wall with mud.
Clay is essential for stickiness, so first you must locate some. Do a shake test by placing your sifted earth in a jar of water with salt added and shaking it up thoroughly. When it settles, the clay will be the top layer, the silt is in the middle, and the heavier particles of sand will have sunk to the bottom. Plaster dirt should be at least 20% clay. Even at this percentage, you may wish to add manure or flour paste to make it stickier.
When plastering straw bales, I find it easiest to first spread a thin layer of mud with a high clay content and no additional sand or straw on the bales to lock into the straw and provide a surface upon which the next layer can adhere. If you use this technique on tight bales, you can avoid using stucco netting. Even when chicken wire or lathe are used, the smooth mud will penetrate the metal and leave no air spaces to cause future cracks. You don't have to wait for this to dry before adding the thicker layer of plaster with straw added.
On rough cob or adobe and for the next layer on straw bales, I like to use a plaster with high clay content and lots of straw mixed to a slippery, easily-spreadable consistency. This can be applied with the hands to a dampened wall and is very good for filling in depressions. (It gets rid of your own depression too, because it feels so good to sling that mud around.) The condition of your walls and how much shaping they need will determine whether to use long straw, chopped straw or a combination. The thicker the layer needs to be, the more long straw it should have. Don't trowel this layer down smooth, but get it as flat as you can with your hands and let it dry out thoroughly. It will make lots of little cracks and provide a perfect surface for the next layer to adhere to.
Always try out an earth plaster you are not familiar with by making a test patch of several square feet. Clays differ in their shrinkage rates, and if it cracks too much you need to add sand and more straw. I've seen some plasters dry into the sort of cracks you find on the bottom of a dry lake bed and fall off the wall. This happens more often to a trowelled surface because there is less surface area to release moisture than if it's left rough. If the dry plaster is very soft when you scratch it, it has too much silt or sand and needs more clay, fresh manure and/or flour paste, which makes long-straw plaster very good for the outside of a building.
In India, the Middle East, and elsewhere, it was discovered long ago that old stinky mud works better than fresh mud, and so it is mixed in a pit with animal manure and other wastes and left to "ripen." This causes the molecules of clay to line up as closely as they can, improving wet plasticity and dry toughness. The same principle is well known in pottery-making.
The next layer is the final coat before the Aliz, and if your adobe, cob, rammed earth, pumice-crete or straw/clay walls are even, as I know the last three surely will be, this is the only layer of plaster you may need. If you are using a found adobe-dirt clay, sift it through a quarter-inch or window-screen size mesh and add an equal part of medium- to fine-grained sand to it. Add about 1/4 to 1/3 the dry ingredients' volume of chopped straw. This should be easily workable, sticky enough to adhere well to the dampened wall, and wet enough to trowel on easily but not so wet that it's hard to pick up. Robert LaPorte recommends adding a small amount of cooking oil to the mixture to make it slide on more smoothly, and I find it does make a positive difference. Disks of plastic cut from the tops of coffee cans, yogurt containers or whatever, are a useful tool for going around curves and "bullnosing" around windows and doors. If you have gathered a more pure clay that is in lumps, it's best to soak it and make a slip by stirring it into water. A paint mixing tool put on the end of a drill simplifies this job as long as the lumps are not so big they would jar the tool. Add fine sand to the thick clay slip until it is the right consistency—up to 70%, depending on the clay—and then add the straw to it.
To apply this plaster in preparation for a clay slip, you need a trowel, a tile sponge, and a bucket of water. I prefer to work from a bucket of plaster rather than a hawk so that I can use both hands and don't wear out my arm holding the awkward hawk... though I know most professional plasterers would disagree. I wear rubber gloves, scoop some plaster onto the trowel and apply it to the dampened wall like icing on a cake. I cover about a square yard at a time without worrying about smooth perfection. Then I go on to the next yard. When I finish that, I go back to the first yard, which has had time to "set up" a bit, and smooth it out with the sponge. A floating tool could also be used, but this seems to leave the surface rougher. The sponge leaves a perfect, fine-grained "tooth" for the Aliz to bind with. If it's still too wet to sponge smooth, wait a little longer. If you want to leave the plaster without a clay slip, you can trowel it hard and smooth at this point, spraying it lightly with a squirt bottle as you go.
A very fine finish plaster can also be made without straw, to be applied in a thin layer to a wall which is quite flat already. If clay is hard for you to find, you can purchase it at a pottery supply store. White Kaolin, which we use for slips, can also be used for plasters. If you're less concerned about color, and planning to put a slip over it anyway, ball clay of a grey color is very good in plaster because it has greater plasticity and dries somewhat harder than Kaolin. Either mix the fine dry sand (70%) and clay (30%) together first and then add water, or mix the clay and water into a slip first and then add the sand. Don't forget the splash of oil for workability. Proportions may vary somewhat depending upon your sand.
And now for my favorite part, the final clay slip, or Aliz. I use white Kaolin and ground mica as my major ingredients. Often I add a small amount of fine sand, especially to the first coat to make it thicker and fill in any small irregularities in the plaster surface.
I use cooked flour paste as a binder in the proportion of 20% to 25% of the liquid. To cook the flour paste, set a 2/3-full pot of water to boil on the stove. In a mixing bowl, whisk together equal parts cheap white flour and cold water. When the pot of water comes to a rolling boil, pour in the flour and water mixture and stir it well with your whisk. It should thicken immediately and become somewhat translucent. Don't keep cooking it or it will scorch. The proportion of all the water to the flour is approximately four to one.
I understand that gum arabic also works well as a binder, but I haven't tried it yet. I have used Elmer's glue on occasion to give the first coat more strength over a weak plaster. Instant flour paste can be purchased as wallpaper paste. Milk products also act as a binder, buttermilk being best.
To mix the Aliz, you will need a container at least as large as a 5-gallon bucket, and a big whisk or a paint-mixing attachment on the end of a drill. Start with 3 parts water to 1 part cooked flour paste in the bucket to approximately 3/5 full. Use a saucepan, coffee can (or whatever) for a scoop, and start adding the ingredients proportionally. Recipes vary according to the surface and whether people love mica or not, but generally I use three scoops of white Kaolin, two scoops ground mica (or more), and one scoop (more or less) of fine sand.
Sand is mainly for the first coat, or if mica is not available. Be careful not to breathe the fine particles of dust and mica, or wear a dust mask while mixing. Keep adding these ingredients until the mixture is the same thickness as heavy unwhipped cream. You may have to add a bit more water to achieve this. Sodium Silicate is an ingredient used in slip-casting to keep the particles of clay afloat in the water, and is useful in this context as well. A very small amount is required. It will also thicken the mixture somewhat, as will powdered milk which also makes the final product a little tougher.
Colored clays or pigments may be added to create different colors. The colored clays would replace Kaolin in the recipe, and pigments should be soaked in water, if not actually ground, to prevent color spots from showing up. The earth pigments sold to color concrete are quite suitable for our purposes too.
If mold might be a problem, add a little dissolved borax powder.
I love to add larger chips of mica to the mix, but they are not commercially available as far as I know. (If anyone out there knows of a source, please let me know.) If you can find large flakes of mica with no rock attached and want them smaller, chop them up in the blender with plenty of water. Bits of chopped straw are a popular addition to the Aliz too.
Bags of ground mica are available through KMG Minerals (PO Box 130, Velarde NM 87582; phone 505 852-2727) at $10 per 50-lb. bag, but shipping costs are high. The V-115 is the largest grain currently available, and gives a visible sparkle to the surface. The 1-1-17 is the next largest grade and has a more subtle sheen. Mica-200 is finely powdered and offers no sparkle at all, but improves the texture and thickens the mix. If it's easily available, mica makes a lovely addition to finish plasters as well as clay slips, and adds to their workability. It is like a molecularly flat sand which is smooth instead of gritty. It's a major ingredient in joint compound and is also used as a lubricant in drilling oil wells. I have appreciated this quality of mica when sculpting with sticky mud. Just dip your hands in a pan of mica frequently to keep the mud from sticking to them. (KMG should hire me as a salesperson.)
You will need a few tools to apply the Aliz to your wall: a 3- or 4-inch-wide brush, a one-inch brush for edges (natural bristles are best), two small buckets, and a fine-grained tile sponge. Sheepskins work, but I find them slippery to hold on to and the coat of slip is not as evenly applied.
Do not moisten the wall first. Make sure it is completely dry, because damp plaster will leave water stains. Start brushing at the top of the wall so you don't mess up your fresh work with drips. Cover the floor with drop cloths. If the wood of your window edges and lintels is rough, tape it first to save yourself cleaning work. If it's smooth and painted, the slip can be wiped off easily while it's still wet. Use the small brush for edges, or another way is to mix some of the slip with extra sand in a small container and apply it to edges with a palette knife. In small curvy areas, sometimes I mix the Aliz a bit thicker and apply it with my hands, and often at the bottom of the wall I use the sponge to dip out some slip and slide it up the wall, starting at the base.
Most walls require two coats. Make sure the first one is completely dry before applying the second one. When the second coat starts to dry and look mottled, becoming "leather-hard," take your sponge and a bucket of warm water and polish the finish in circular strokes with the squeezed-out sponge. Rinse and squeeze out your sponge often so that it will cleanly polish off the mica flakes and bits of straw (if you have chosen to put them in). Polishing smooths out the brush strokes and gives the surface a finer texture. You can even dry-polish it again later with a rag to get off any last bits of sand and polish the mica to a greater sheen.
When you are finished with your job, save the leftovers by drying them into "cookies". These may be stored, then soaked and used to repair any small damages.
I can't emphasize enough the need for trials and experimentation in this type of work. There are many variables in Mother Nature's materials, so never take them for granted. I haven't gone into the various additions like psyllium husks, cactus juice and micro-fibers here, but they are things that can be used. Happy Mudding!
Carol Crews, Gourmet Adobe; HC 78 Box 9811, Ranchos de Taos NM 87557; ph (505) 758-7251
The Straw Bale Earthen House
by Athena and Bill Steen
There is an old saying, "an adobe without straw is like a person without a soul." That is not to say that it is impossible to make a good adobe without straw, but speaks of the long-standing relationship between the two materials that is as old as anyone can remember.
Earthen materials and natural fibers have a strong affinity for one another. They preserve and protect one another. In combination with earthen materials, natural fibers such as straw increase tensile strength and help control shrinkage and cracking. The addition of earth or clay to natural fibers brings about a significant increase in fire, water, and insect resistance.
In contrast, natural fibers decay when they come into contact with highly alkaline substances like concrete, especially in hot, humid environments. For that reason natural fibers are not used to reinforce concrete without additives or modifications that neutralize the negative effects of the alkalinity.
Prior to the publication of our book, The Straw Bale House, most of our work with straw consisted of building with it in its bundled and compressed form—i.e., bales. Over the past three years, the beneficial relationship between earth and straw has increasingly captured our imagination. For us, the above statement with a slight twist would be true: "A strawbale building without adobe is like a person without a soul." The two substances are so compatible that we find it increasingly difficult to conceive of using one material without the other, and in the process have discovered a world of seemingly unending possibilities in the combination of earth and natural fibers.
Our most recent strawbale structures have become earthen/strawbale hybrids. Once the strawbale walls have been stacked, we use traditional English Cob, a wet earthen mix with a high concentration of straw, to fill all the cavities and joints in the bale walls. Cob is like an adobe mix, but with as much straw as the mud mixture can accept before it fails to bind. We often draw on the same material to shape window and door openings, create moldings, or form extensions or shelves that grow from the walls. The same mix of earth and straw has allowed us to create highly curved sculptural interior walls, bancos for seating, as well as free-form fireplaces.
When there are larger spaces in a wall that need to be filled but are too small for a bale to fit, we often choose a mix of loose straw coated with a clay slip. Popularized in Germany over 400 years ago, this is known as Leichtlehm, or "light clay." This mixture of straw and clay is most commonly packed into slip forms, creating walls that are essentially "rammed straw." Unlike rammed earth however, it requires no heavy machinery, and less intensive form work. It can be used to create thin exterior or interior walls, insulation and ceiling panels between rafters, load-bearing components between floor joists with second stories, or insulation below adobe floors. The straw/clay provides less insulation than bales, but offers greater flexibility in constructing walls of different widths. Packed into adobe brick forms, straw/clay can also make lightweight blocks.
We often use straw/clay to fill smaller framed wall sections where bales would take up too much space—for example, in south walls with large expanses of glass, or in gabled ends. Above window and door frames is another place we often use this mixture, with one of the most recent examples being a beveled window opening with a rising arch built of split bamboo and filled above with straw/clay.
Our pursuit of more knowledge about this wonderful material brought us into contact with German earth builder Frank Andresen, who widened our world of clay and natural fiber combinations to include clay/wood-chips, clay/sawdust/straw, wattle-and-daub, clay/water-reed/burlap-panels, as well as marvelous natural clay plasters with natural fiber lathing and reinforcement systems.
These combinations of earth and straw have accompanied us over the past few years to Ciudad Obregon, Mexico, a modern agricultural city in the state of Sonora, which is annually overwhelmed with the burning of almost a million acres of straw. Together with Mexican co-workers and friends Teodoro and Emiliano Lopez, and members of a neighborhood called Aves de Castillo, we have been stretching these earth and fiber combinations to new limits for creating ultra-low income housing and shelter. The last structure we completed was a simple rectangular, load-bearing strawbale structure with parapets of straw/clay and a unique flat roof constructed of small wood beams, carrizo (an abundant, bamboo-like reed) and a straw/clay roof coating. This little room (with electricity, no plumbing) cost a respectable $1.75 a square foot when completed—clearly competitive with the cardboard, corrugated asphalt paper, and pallet-wood shacks of the surrounding area. Approximately half of the $1.75 per square foot went for the wood window and door frames. The straw/clay mix on the roof was coated with a mix of white cement, powdered marble (marmolina) and an acrylic waterproofing agent.
Since this building was finished, another small structure built entirely of load-bearing straw/clay bricks has been completed.
Adobe block used for interior walls and bancos (seating) has proven to be an excellent way of providing thermal mass for structures with strawbale walls. Straw bales can be used to create super-efficient passive solar buildings, but they need the addition of adequate thermal mass to store the heat gained during the day from the sun. Several years ago, we built small passive solar strawbale guest house that uses a combination adobe-interior-wall/banco for thermal mass. By using adobe, we were able to create a beautiful, sculptural, curved wall that was quickly constructed and perfectly fit the space while providing the mass we needed to make the building perform as intended.
Earthen plasters have been used extensively in many countries for many centuries. We have found earthen plasters to be an excellent covering for strawbale walls. An almost inseparable bond is formed between the plaster and the bales as the mud mix becomes completely integrated with the straw.
Earthen plasters are breathable, allowing whatever moisture may be in the bale walls to escape. There may be no other building material capable of regulating moisture levels as effectively as earth, which continually absorbs and releases moisture as necessary. Being a flexible, forgiving material, like straw bales, earthen plasters are easier to repair than harder surfaces like cement. They can be applied by hand or machine to the bales directly, without wire reinforcement. We do trim with a chainsaw the "folded" side of the bales to ensure a good bond between the earthen mix and the straw. By screening this loose straw through ¼" hardware screen, we also are able to make all the short-length straw we need to add to the plaster mix.
We typically stabilize the exterior plaster with the gel from the prickly pear cactus, commonly known as nopal in the Southwest; however other materials such as linseed oil, lime, or polymer-based products like concrete bonder can also be used. If more weather resistance is needed, we use one or two coats of lime plaster over the initial coats of mud, in much the same way that German straw/clay buildings are plastered.
Straw and earth also can make a soft, insulating, and beautiful floor which, if done carefully, can be sufficiently durable for most domestic applications. The use of straw/clay between floor joists for a second story floor base also allows the use of an earthen finish floor. Our own experience over the years, coupled with extensive interviews with other builders of earthen floors, has yielded methods of constructing earthen floors that we feel are suitable for most climates and conditions.
A lingering dream of using straw thatch roofs is also beginning to come to fruition. During the past year, we have worked with thatchers from the southern Mexican state of Tlaxcala (where thatching with rye is still in practice), and with English thatcher Aaron Greenwood. The first step in reviving this beautiful art has been securing old long-stemmed varieties of grain that are suitable for thatch roof construction. We are currently in the process of growing out crops of some of these special varieties for use on upcoming projects. Some of this straw will be used in Mexico, while the rest of it will be reserved for a five-week training program that Aaron will be conducting during the summer of 1997.
It's truly an amazing experience to watch these two materials—straw and clay—combine to create almost every facet of an entire building. By viewing building as a process of combining different-but-complementary materials rather than adhering to a particular building system, we have given ourselves the freedom to create structures that respond to a wide variety of contexts and circumstances. They can be elegant or simple, quick or detailed, inexpensive or costly, and probably most importantly, they can be built from predominantly local materials in whatever combination best matches the local climate. Like the rest of life, building can be much more fulfilling when founded on a good relationship. For us, viewing building as a process of combining earth with natural fibers has led to an unfolding of options and possibilities that would not be open to us if we were to remain simply straw bale builders.
The Canelo Project, HC1 Box 324, Elgin AZ 85611; ph (520) 455-5548, fax (520) 455-9360; email@example.com; http://www.caneloproject.com
|A Word About Durability|
In the cliff dwellings of the Southwest, there are intact mud plasters that have lasted well over 700 years, mostly without maintenance! Earth plasters can be extremely durable.
There are different approaches to the durability question. Some say if you want plasters to last longer, make them thicker. It's true, that works. In the Southwest, vertical earth plasters on exterior surfaces erode an average of 1 inch every 20 years (per Adobe & Rammed Earth Building Design and Construction by Paul Graham McHenry Jr.), so a few inches should last decades.
Cedar Rose has found that house design, the plaster mix, and application techniques (particularly the need to key plasters in to the straw and any previous coats) all affect durability. I agree. If you want your plaster to last, pay attention to all of these. (This subject is probably an entire article in itself).
More and more in my work I try to think of durability with sustainability. How hard is it going to be to repair? I try to keep my mixes as simple as possible, and leave the owners with several buckets of dry mix that can be wet down and slapped on a needy wall quickly and easily.
To check a plaster patch for durability, rub it hard with your thumb. Does it resist dusting and crumbling? Pinch it hard between thumb and finger. Is it hard to smush? Turn a hose on exterior plasters (after they have dried thoroughly). Does it eat into the plaster slowly or does it disintegrate quickly? These tests will help you determine what will hold up and what won't. (If you get blasted with horizontal rain and hail often in your area, consider adding lime to the exterior mix or putting a finish coat of lime over your plasters).
A Word About Clay
Clay is amazing. There are many different types and according to potter friends, they run in a spectrum from the bentonites (sticky and expansive/more prone to cracking) to the kaolinites (not so sticky and hardly expansive/less prone to cracking). So a soil with 30% bentonite will act differently than a soil with 30% kaolin. And there can be several kinds of clay present in the same soil.
Bevin Dunn blew my mind when she informed me that the clay particles are not responsible for clay's sticky nature. The mineral fragments linked to the clay are what make it sticky! The ramifications of this tidbit seem enormous. I just can't figure out where to go with this. Ideas, anyone?
Whoops, What is This?
These are a couple of the major problems I have run across in my work:
Mold. At a recent job we switched clays for the finish coat, put out a lot of work in one day to get it on the walls, and the next day we were surprised to see the wall covered with little white fuzzy spots. We hoped they would disappear as the wall dried, but instead they stained the wall, leaving the rich color with lots of lighter blotches. Wiping it with a borax solution or hydrogen peroxide changed the color and the texture... so we redid the wall, adding dissolved borax to the plaster. Had it been summer, when we could have had better ventilation and faster drying, this would not have been such a big problem—but that mold was fierce and I think borax would have been necessary anyway.
Mold spores can come in with the dirt or straw. Use clean chopped straw and try changing your dirt if you discover mold. Ventilate well. Adding borax or lime to the mix will kill molds. If you develop mold when plaster is on a wall, spray it with hydrogen peroxide to kill it before adding new plasters. Mold can be a significant health problem, especially if it is of the black slime variety. (I hear vinegar and lemon juice also work, but have never tried them.)
Effervescence. Excess minerals in dirt can rise to the plaster surface, blotching finish plasters. Use another clay or wash your clay (Call me for directions if you have to do this), or learn to enjoy the mottled affect. This was a serious headscratcher in Moab, UT and the solution (washing) was spirit-inspired, but slowed us down significantly. Some batches of clay were washed better than others, and consequently we still had problems with minerals rising to the surface by the time we reached the finish coat. You can tell this might be a problem if a white or yellowy film rises to the surface after mixing up a clay slip and letting it dry.
Freezing Plasters. Do not let your new plaster freeze when wet, before or after it go on the walls, until it cures. If they freeze hard enough before curing when on the walls, they will become quite crumbly and need to be removed. If you don't remove them, you may suffer blowouts on all of the remaining coats, as the plaster no longer grips whatever it was keyed into and can't support the weight of new plasters.
If you must do interior plasters in the winter, do the following first:
· Put at least one layer of plaster on the exterior if weather permits or tarp the walls.
· Caulk the windows.
· Insulate the ceiling.
· Keep the place heated.
Ventilation. When a lot of plaster is drying at once, you must have some ventilation (all that moisture has to go somewhere), but don't open windows over wet plaster. Speed the drying of thick scratch and brown coats by turning up the heat or pointing a fan at them; it is ok if undercoats crack as long as they don't pull away from the wall. But don't point a fan or heater at finish coats or they may crack severely. Keep an even, warm temperature once you begin the finish coat.
Sections of plaster that seem to take longer to dry and change color. Usually this is caused by a section of plaster underneath that was not fully dry before the next layer went on. Because it is still wet, the plaster on top takes longer to dry, and for some reason plaster that stays wet for a long time frequently changes color. This will most likely be a problem over holes that have been patched with a lot of plaster. Before you put on your finish coat, make sure your walls are bone dry!
Money. This is not a problem that comes from the mud, but it definitely affects the plastering. Cost overruns are common in building, and funds may be tight before the plastering even starts. The more of the labor provided by owner/friends and workshops, the cheaper the finishes will be. If it is a big house, a fancy finish, or must be exceptionally durable, getting the help of experienced plasterers for developing appropriate mixes and troubleshooting can save you time and trouble. The strawbale email list hosted by REPP-CREST is an excellent and inexpensive way to get answers to your questions from folks who have experience, or are willing to take wild guesses. (Laughs are guaranteed, the wild guesses are usually admitted as such, and there is amazing information and experience shared).
Prioritize what you need. Think about doing the plastering in stages (some this summer, some next summer). Get creative and let us know if you find any ways to speed up the process!