«The Cob Builders Handbook You Can Hand-Sculpt Your Own Home Table Of Contents: Acknowledgements and Warning INTRODUCTION What is cob? Why build ...»
A general rule of natural building is: like sticks to like. An earth-based plaster is perfect for a cob house. Lime has also been used around the world to make a breathing plaster or a paint to protect and beautify earth walls.
Cement-based plasters or stuccoes are usually destructive to earth buildings. The hard cement coatings almost always crack because their rigidity makes them brittle. The cracks let moisture in, wetting and often eroding away the earthen structure under the crack. The cement coatings breathe less and slow down the drying process of the cob beneath them. Many old earthen buildings that were 'restored' with cement based coatings are now having to be redone. All the cement has to be removed, the damage repaired, and the building replastered with more compatible, vapor permeable plasters.
Mixing Plaster (render) Plaster can be mixed on a tarp just like you mix cob. If you want to make really big mixes, you can make them in a pit dug in the ground. (See page 75 for straw bale pit ideas.) Smaller batches can be made in a wheelbarrow or in a bucket or tub. Cement mixers also work well for mixing earth plaster, if you can stand the noise!
you can use a little trowel for mixing the plaster, for applying the plaster and /or for smoothing it A swimming pool trowel works well for applying the plaster on large areas and is great for putting down a cob floor Basic earth Plaster Now that you've become familiar with your soil and with mixing cob, making an earth plaster will be easy. A basic plaster is simply a finely sifted, wetter version of cob.
It is made of the same ingredients in the same proportions as cob: sand, clay, and straw (or some other fiber). Use the purest clay you have for the plaster. Put everything through a fine screen and add more water. That's it! This plaster mix can be used for a lovely, durable finish on cob walls.
Sifting the ingredients
If you have large particles, pebbles, or a lot of organic debris in your soil and/or sand, you will need to sift it. A sifting screen can be made by stapling a screen onto a wooden frame, or by nailing strips of wood over a screen onto its frame. You may want to make more than one screen, using different sizes of mesh. If you are making more than one layer of plaster, the under layers can have larger sized particles (1/8 inch mesh). Only the outer coat of plaster will be seen. If you want a fine finish, you'll want to sift all ingredients through an ordinary window screen or an even finer screen.
You can lay the screen over a wheelbarrow or bucket and sift into that, or lean the screen up against a straw bale or wall and sift onto a tarp. The soil can be sifted dry after crushing up the lumps with your shoes or with a tamper if necessary. Or you can mix the ingredients with water, let it soak a day or two, and then push the goo through the screen by rubbing it with a smooth rock or your hands. Dust is kept to a minimum if you do the sifting when the soil is wet.
Shredding the straw
Straw for the plaster also needs to be sifted or shredded. Straw dust can irritate lungs and eyes. Wear a mask and eye protection when breaking it up. It's much easier to shred when it's very dry (not damp). You might want to leave the straw in the sun during the hottest part of the day and then shred it.
155 The straw can be shredded by placing a 1/8 inch mesh screen on top of a 1/4 inch mesh screen, laying them over a wheelbarrow or bucket, and 'grating' handfuls of straw through the mesh. The finer you want the straw, the finer mesh you'll need.
Straw can also be shredded with a lawn mower by running over and over it. Or you
can use a weed eater in a metal drum to break up the straw. Other methods include:
old blender, chain saw, electric knife, drill with wire brush, scissors, and chopping with sharp knife on a wooden board.
For the initial plaster coats (sometimes called rough coats), the ones that will be covered later, you can use the straw lying on the floor of the cobbing area, that has already been shredded by feet and tarps being dragged over it. I purposely throw extra straw down to be shredded for the plaster mixes. You can use a roughly shredded straw in the final coat of plaster if you like the look of the textured finish and the bright, straw pieces reflecting light. Make a test batch so you can see whether you like it.
For a very fine outer coat of plaster, you can replace the straw with fresh or finelygrated dry manure. The digestive systems of grass eating animals have already done the job of shredding the fibers for you. The dry manure will need to be 'grated' through a sieve to break it up. Fresh manure can be added in as is.
Plaster additions Purposes of plaster additions Additions to the basic earthare not necessary, the basic fine cob works well. Additions
serve one or more of the following purposes:
Making the plaster easier to apply.
• Making it more water resistant, discouraging erosion.
• Increasing its ability to stick together and to the wall.
• Enhancing thAdding strength to the plaster.e plaster's beauty.
• Minimizing cracking.
• Adding magic things adds magic.
• Make lots of different test batches. Experiment with various additions. You can add one or more to your basic cob plaster mix. Put the different mixes on the walls. (See page 157 for more on applying the plasters.) Thin layers of plaster are best - at most 3/8 of an inch thick. You can label them by writing the ingredients right into the plaster. These test spots can easily be plastered over later. Let them dry and have a look at them. Which ones were easiest to apply? Which ones look the best to you? Which ones cracked? As with cob, if your plaster cracks, add more fiber or sand.
Possible Additions plant fluff: is a strengthening fiber, sort of like felting. It can be used in addition to or in place of the straw, and discourages erosion, adds magic, and minimizes cracking.
This is one of my favorite things to add to the basic earth plaster. The tiny fibers add a lot of strength to the plaster mix. Another version of fluff can be found in the lint screen of your dryer.
hair: strengthens sort of like felting, discourages erosion, minimizes cracking, adds magic - especially if it is cut from human or animal friends. It can be used in addition to or in place of straw.
Hair is a very strong fiber. Animal hair or people hair cut into short (1/2 inch or less) pieces works well. It is hard to get it to mix into the plaster and it gives the finish a slightly hairy look. Try a little just for fun. This will make life interesting for the archaeologists of the future. Hair is an excellent fiber to add to lime plaster, because it is so resistant to lime's caustic effects.
manure from grass eating animals: strengthens, discourages erosion, makes it stick, minimizes cracking, increases water resistance. Manure definitely adds something magic.
This is an excellent addition to earth plaster. Some plasters are more than half manure.
The manure makes the plaster extra strong and easy to spread. The fresher the manure, the better it is for the plaster. This must be balanced with your ability to handle the stink. It's not as bad as you imagine. The smell will completely disappear as the plaster dries. If you are using dry manure, it is hard to break it down. It's already turned into a hard plaster. You may have to grate it through a 1/2 inch mesh screen before you stir it into your plaster mix.
157 I think cow manure is better than horse manure, but horse will do if that's all you have. Llama or alpaca manure is less stinky than cow and easy to collect because these animals have one special toilet area. I've heard that pig shit is a favorite in Switzerland. Deer or kangaroo poo are probably good plaster additions too.
A friend told me a story he heard from a Namibian man about how his tribe plasters their earthen domes. They collect fresh cow manure, roll it into balls the size of one's head and leave these balls of manure in the sun to 'cure' for exactly seven days. Then they are broken open (Peew!!!) and used to plaster the dome. Supposedly this is a very waterproof plaster.
An African recipe for wall plaster 1 part manure 1 part clay - you may choose to use a pretty colored clay Add water until the plaster is the consistency you want.
flour paste: (Thank you Carol Crews for passing on this great idea!) Adding flour paste makes the interior plaster a lot sturdier and less dusty. It makes it sticky and easy to apply. It can be used in the plaster and/or in natural paint or alis.
Mix up a flour paste. In ajar, shake one part cold water and one part flour, until it's well mixed with no lumps. Add this to boiling water, heating and stirring until it thickens.
You can use cheap white flour or whatever you have. Do some test batches using various ratios of paste to plaster. Start with one part paste and five parts basic earth plaster. Experiment from there adding more or less paste. I have only used flour on the interior plaster. I'm not sure how it will hold up outside.
Once I was doing a demonstration at a workshop and we ran out of flour. We did have some leftover cooked oatmeal from breakfast so we blended it up and added it to the plaster. It worked beautifully.
cactus juice: makes it stickier, increases water resistance.
This is commonly used where cactus grows. Every region has some plant that has mucilaginous juice that may serve the same purpose as the cactus juice. Check which varieties are used in your local region, how they are prepared, and what proportion of juice is added to the plaster. The cactus is usually peeled, de-spined, cut into pieces, boiled, and then strained. The goo is added to the plaster in place of some of the water. Experiment with amounts. Start with 1/2 a big bucket to a wheelbarrow of plaster.
ground psyllium seed husk: makes the plaster a joy to put on, holds the plaster together well, increases water resistance, and discourages erosion.
This can be bought in small quantities at health food stores. It is ordinarily used to clean out the colon. You'll only need a little bit. Start by adding a quarter of a cup to a 5 gallon bucket of plaster. Adding psyllium seed husk to the mix is kind of like
Elmers glue or other manufactured polymers: adds water resistance, makes it stickier.
This is a great addition for a plaster that will have tiles or other heavy decorations embedded into it. Try 1/2 cup (or more) of glue per big five gallon bucket.
Some folks have been doing experiments using mixtures of different types of glues and natural ingredients. There's still some experimenting to do before we'll know which kinds to add to the plasters, how much, how well they work, and whether they breathe enough to keep a cob wall healthy. They seem very tough and water resistant.
coloring: for looks.
You may want to collect special colored clays for the last layer of plaster or for a paint. A colored final layer of plaster can take the place of paint. You can use potter's clay as a colorant for the outer layer of plaster.
If you are adding colored dry pigments, mix them well with water before you add them. (See the section on paint, page 163-167 for more coloring ideas.) Termite mound dirt: If you live where the termites build dirt houses, you can find an abandoned mound, crush it up, and use it as part of your plaster. I met a few people who had tried this in Australia. They reported that it made a very weather resistant plaster and highly recommended it.
papier mache: ground-up paper, adds a small amount of insulation, helps the plaster stick together, adds tiny fibers that increase the strength and cut down the erosion.
Make some test batches with various amounts of ground-up paper. In one of our experiments, we used only ground up telephone book yellow pages mixed with water for an exterior plaster, and were amazed at how well it held up. Adding some clay and/or flour paste would make it even better.
mica or glitter: looks pretty, catches the light. Mica or glitter added to the final layer of plaster makes beautiful shiny bits that glint in the light. These ingredients can be sprinkled onto the plaster after it's on the wall and gently rubbed into the surface.
Mica is a naturally shiny, glass-like scaly stone available in the southwestern United States, and possibly elsewhere. It is like platelets of sand and may increase the strength of the plaster.
vermiculite, pumice, or extra straw: It may be possible to add insulative value to the plaster by the addition of materials that create pockets of air in the plaster. This concept needs testing. If you want to experiment, start by adding 1 part to 10 parts of plaster. See how much you can add before the plaster mix gets too crumbly. You may want to use a little extra of the sticky additions mentioned above to help hold the plaster together so it will hold more insulation.
Applying the plaster or render When to plaster Interior plaster usually goes on after the roof and ceiling are completed, and before the floor is made. (Unless, of course, you are replastering long after the floor is done.
In which case you'll want to protect the floor, furniture, etc., from plaster drips.) The exterior plastering can be done after you've moved into the house or you can leave the walls as they are.
Don't plaster when it's below freezing, or when there's a chance that the plaster will freeze before it's dry. If the sun is shining directly on the wall you will be plastering, set up a shade for it. It's best not to plaster when there's a dry wind blowing. Ideally, the plaster will dry slowly. This is especially important if you've used lime in your plaster. (See page 162 for more about lime plasters.) How many coats of plaster? Three is a typical number of coats on earthen structures. I've never done more than two on a cob building. Maybe it is because I am very careful as I build the walls so there isn't much filling in or adjusting to do.
Some folks like the look of unplastered cob. It can be whitewashed or a layer of alis can be applied directly to the rough wall, if you want a very textured surface. For a smoother wall, chop the loose straw ends off before whitewashing.