Mud is magic. Mud is fun. Mud can also be quarrelsome.
Don't get me wrong, I love earth plasters. I want all new straw-bale 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. 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 straw bale 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.
The simple truth is, mud will never be a standardized material like cement. Each batch of dirt is different and has its own personality. 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 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 for avoiding potential problems and keeping the fun in the mud.
First, ask yourself these questions:
1. How much durability do I want and need? Is it a meditation hut or a conference center? Do you have ten kids?
2. 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.
3. 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!
4. 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!
5. 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!
Enough Talking and Thinking... Let's Get Into the Mud!
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.
Forego 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 it 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 His 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 go to dirtbag magicians Kaki Hunter and Doni Kiffmeyer for their innovative approach to earth building and plastering.
Join Us in the Mud!
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 wheatpaste in your finished 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 the 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.
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.
Courtesy of Natural Building Resources